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  • 1. ◆ LTE and HSPA؉: Revolutionary and Evolutionary Solutions for Global Mobile Broadband Anil M. Rao, Andreas Weber, Sridhar Gollamudi, and Robert Soni Universal Mobile Telecommunications System (UMTS) with its high speed packet access (HSPA) enhancements is currently being deployed as the primary mobile broadband solution by operators worldwide. To ensure continued competitiveness of the Global System for Mobile Communications (GSM) family of technologies in the world market, the 3rd Generation Partnership Project (3GPP) is rapidly standardizing the long term evolution (LTE) of UMTS, with significant performance improvement targets compared to HSPA. The fact that LTE is not backwards compatible with HSPA spurned the introduction of the HSPA evolution (HSPAϩ) effort in 3GPP to protect current operator investments in HSPA. HSPAϩ provides a framework for HSPA enhancements with the goal of providing performance similar to LTE in a 5MHz carrier, while at the same time offering the advantage of backwards compatibility with earlier releases. In this paper, we identify the key technology features of LTE which allow it to meet the desired performance improvements compared to HSPA, and then describe the key features of HSPAϩ which allow it to remain competitive with LTE. We examine features at the physical layer, medium access control (MAC) layer, and network architecture layer, as well as provide detailed air interface performance studies. © 2009 Alcatel-Lucent. packet access (HSDPA) in March 2002, which marked a significant evolution of UMTS into the so-called 3.5G space, providing not only significant improvements in spectral efficiencies, but greatly enhancing the end- user experience. HSDPA has already become widely available with over 180 operators in service in almost 80 countries [12]. Operators deploying UMTS today are typically doing so with HSDPA. To complement the downlink improvements offered by HSDPA, the Introduction Building on the tremendous success of Global System for Mobile Communications* (GSM*) for sec- ond generation (2G) deployments, 3rd Generation Partnership Project (3GPP*) Release 99 introduced Universal Mobile Telecommunications System (UMTS) in March 2000, and it has become the dominant third generation (3G) technology in the world with over 200 operators in service in almost 90 countries [12]. 3GPP Release 5 introduced high speed downlink Bell Labs Technical Journal 13(4), 7–34 (2009) © 2009 Alcatel-Lucent. Published by Wiley Periodicals, Inc. Published online in Wiley InterScience (www.interscience.wiley.com) • DOI: 10.1002/bltj.20334
  • 2. 8 Bell Labs Technical Journal DOI: 10.1002/bltj Panel 1. Abbreviations, Acronyms, and Terms 2G—Second generation 3G—Third generation 3GPP—3rd Generation Partnership Project 4G—Fourth generation ACK—Acknowledgement AMR—Adaptive multi rate BPSK—Binary phase shift keying CDF—Cumulative distribution function CDMA—Code division multiple access CLTD—Closed-loop transmit diversity CP—Cyclic prefix CPC—Continuous packet connectivity CQI—Channel quality indicator CS-RS—Channel sounding reference signal dB—Decibel DCH—Dedicated channel DFT—Discrete Fourier transform DL—Downlink DM-RS—Demodulation reference signal DPCCH—Dedicated physical control channel DSCH—Downlink shared channel DRX—Discontinuous receive EDGE—Enhanced data rates for GSM evolution E-DPCCH—Enhanced DPCCH eNB—Enhanced node B EPC—Evolved packet core EPS—Evolved packet system E-UTRAN—Evolved UTRAN EV-DO—Evolution data optimized FACH—Forward access channel FDD—Frequency division duplex FDMA—Frequency division multiple access FFR—Fractional frequency reuse FTP—File Transfer Protocol GERAN—GSM/EDGE radio access network GGSN—Gateway GPRS support node GPRS—General packet radio service GSM—Global System for Mobile Communications GW—Gateway HARQ—Hybrid automatic repeat request hs—High speed HSDPA—High speed downlink packet access HSPA—High speed packet access HSPAϩ —HSPA evolution HSUPA—High speed uplink packet access ID—Identification IDFT—Inverse discrete Fourier transform IFFT—Inverse fast Fourier transform IoT—Interference over thermal IP—Internet Protocol IPv4—IP version 4 ISI—Inter-symbol interference IST—Information Society Technologies km/h—Kilometers per hour LMMSE—Linear minimum mean square error LTE—Long Term Evolution MAC—Medium access control MC—Multi-carrier MCS—Modulation and coding scheme MIMO—Multiple input-multiple output MME—Mobility management entity MMSE—Minimum mean square error MRC—Maximum ratio combining ms—Milliseconds NACK—Negative acknowledgement NGMN—Next-generation mobile network OFDM—Orthogonal frequency division multiplex OFDMA—Orthogonal frequency division multiple access PAPR—Peak to average power ratio PARC—Per antenna rate control PCH—Paging channel PDN—Packet data network PMI—Precoding matrix indicator PS—Packet switched PSD—Power spectral density PSTN—Public switched telephone network PUCCH—Physical uplink control channel QAM—Quadrature amplitude modulation QoS—Quality of service QPSK—Quadrature phase shift keying RACH—Random access channel RAN—Radio access network RLC—Release complete RNC—Radio network controller RoHC—Robust header compression RoT—Rise over thermal RRC—Radio resource control RTP—Real Time Transport Protocol Rx—Receive SA—System architecture SAE—System architecture evolution SC—Single carrier SCCH—Shared control channel S-CCPCH—Secondary common control physical channel SDMA—Spatial division multiple access SFBC—Space frequency block coding SGSN—Serving GPRS support node SIC—Successive interference cancellation SINR—Signal-to-interference-plus-noise ratio SMS—Short message service SRS—Sounding reference signal TA—Timing advance TS—Technical specification TTI—Transport time interval TU—Typical urban Tx—Transmit UDP—User Datagram Protocol UE—User equipment UL—Uplink UMTS—Universal Mobile Telecommunications System UTRAN—UMTS terrestrial radio access network URA—UTRAN registration area VoIP—Voice over Internet Protocol WINNER—Wireless World Initiative New Radio
  • 3. DOI: 10.1002/bltj Bell Labs Technical Journal 9 high speed uplink packet access (HSUPA) technology was introduced in 3GPP Release 6 in March 2005. The combination of HSDPA and HSUPA is called simply high speed packet access (HSPA), and is strongly posi- tioned to become the dominant high speed wireless data technology for many years. Even before the standardization of HSUPA had been completed, in December 2004, 3GPP initiated a feasibility study regarding the long term evolution of UMTS, in order to ensure that the GSM family of technologies maintained a competitive position in the world market. The introduction of LTE was seen as a way to provide a smooth migration to the yet-to-be- defined fourth generation (4G), and to take advantage of new spectrum allocations with wider bandwidths that would become available (e.g., in the 2.6GHz 3G extension band). During the LTE feasibly study, an aggressive set of performance targets and require- ments were agreed upon to form the basis for LTE standardization work. To justify the introduction of a new technology, LTE would be required to provide very large performance gains compared to HSPA in 3GPP Release 6 and to fully take advantage of new spectrum allocations as wide as 20MHz. In order to satisfy these requirements, it was clear that LTE would have to be built from the ground up, and could not offer backwards compatibility with UMTS/HSPA. The fact that LTE would not be backwards com- patible with HSPA was not necessarily received well by the large number of operators who had already made significant investments in UMTS/HSPA and had not yet begun to realize the benefits of those invest- ments. This spurned the introduction of the HSPA evolution (HSPAϩ) effort in 3GPP in March 2006. While 3GPP had already started working on Release 7 enhancements as early as 2005, there was no general framework to these enhancements. HSPAϩ formally defined a broad framework and a set of requirements for the evolution of HSPA; the primary goal being to provide performance similar to LTE in a 5MHz carrier, while offering backwards compatibility with Release 99 through Release 6. HSPAϩ would then provide a compelling alternative to LTE for operators who were already deploying UMTS/HSPA, allowing them the flexibility to introduce LTE in new spectrum while enjoying enhancements which would protect their existing investments in UMTS/HSPA. In this paper, we will describe the requirements set forth in standardization for both LTE and HSPAϩ, and then give an overview of the key features of each tech- nology which allow them to achieve their performance requirements. We will see that many of the features introduced in HSPAϩ closely parallel the innovations developed in LTE. Where applicable, we provide detailed system performance studies which illustrate how close the technologies come to meeting the desired performance targets. The remainder of the paper is organized as follows: we start with the performance requirements set forth in standards, give an overview of the system architecture enhancements, describe the key features in the downlink, describe the key features in the uplink, describe the features in LTE and HSPAϩ that enable efficient transmission of Voice over Internet Protocol (VoIP), and offer our conclusions. Requirements and Performance Targets During the initial study item phase for both LTE and HSPAϩ, 3GPP agreed upon a set of requirements and performance targets to form the basis of the stan- dardization work, and to determine what key features or enhancements should be included as part of the new technology. In this section, we discuss the requirements and performance targets for both LTE and HSPAϩ. LTE Requirements and Performance Targets While 3GPP understood in early 2005 that the HSPA technology—the uplink component of which had just been standardized—would provide a highly competitive mobile broadband solution for several years, potential threats from other technologies cre- ated a desire to ensure competitiveness in an even longer time frame (i.e., for the next 10 years and beyond). This formed the justification for opening the LTE study item in 3GPP very quickly. Important considerations for the long term evo- lution of 3GPP included reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for operators. In order to achieve this, it was seen that both an evolution of the air interface as well as the network architecture would need to be taken into account. Looking to the future,
  • 4. 10 Bell Labs Technical Journal DOI: 10.1002/bltj the desire for even higher data rates also needed to factor-in future additional 3G spectrum allocations, and hence, LTE would need to include support for transmission bandwidths greater than 5MHz. At the same time, support for transmission bandwidths of 5MHz and less than 5MHz would be needed to allow for flexibility in whichever frequency band the system might be deployed in. The requirements and per- formance targets for LTE were agreed upon in [2], and it should be noted that these performance targets were decided when HSPA Release 6 was still being finalized. Hence, the targeted improvements are in many cases set relative to HSPA Release 6. The points relevant to this paper are summarized here: • Packet-only technology. LTE would support only the packet switched (PS) domain of the core network, and would be optimized to provide a high data rate, low-latency, packet-optimized radio access technology. • Scaleable bandwidth. Support for bandwidths of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Originally, the smallest bandwidth allo- cation was going to be 1.25 MHz to fit existing CDMA2000*/evolution data optimized (EV-DO) spectrum, but this was later changed to 1.4 MHz which would fit an integral multiple of GSM carriers. • Improved peak rates. 100 Mbps peak rate in the downlink in 20 MHz (5 bps/Hz) and 50 Mbps peak rate in the uplink in 20MHz (2. 5bps/Hz). Peak rates should scale linearly with the spectrum allo- cation. • Improved spectrum efficiency targets. – Three to four times improvement in the downlink spectral efficiency compared to Release 6 HSDPA. This assumes a 1ϫ2 antenna configuration for HSDPA but a 2ϫ2 antenna configuration for LTE. – Two to three times improvement in the uplink spectral efficiency compared to HSUPA Release 6. This improvement assumes a 1ϫ2 antenna configuration for both HSUPA Release 6 and LTE. • Improved user throughput. Target improvements are placed on both average user throughput as well as user throughput at the edge of the cell. The cell edge user throughput is defined as the fifth per- centile of the user throughput cumulative distri- bution function (CDF); this quantity is important to ensure broadband rates can be achieved through- out most of the cell coverage area. The target improvements below use the same assumptions described above for the improved spectrum effi- ciency: – Three to four times improved average user throughput per MHz in the downlink and two to three times improved user throughput per MHz in the uplink compared to Release 6. – Two to three times improved cell edge user throughput per MHz compared to Release 6. • Improved latency. LTE targets significantly improved control plane and user plane latency, including a – Control plane supporting transition time of less than 100 milliseconds (ms) from a camped state (i.e., idle) to an active state (i.e., CELL_DCH), and a transition time of less than 50ms from a dormant state (i.e., URA/CELL_PCH) to an active state. – User plane supporting latency of less than 5ms should be possible on the user plane in an unloaded condition. The user plane latency is defined as the one way transit time between the user equipment (UE) and the radio access network (RAN) edge node. • Co-existence and inter-working with UMTS and GSM. Given that LTE would co-exist with both UMTS/HSPA terrestrial radio access network (UTRAN) and GSM/EDGE radio access network (GERAN), requirements were placed on inter- working with these legacy systems. An interrup- tion time of less than 500 ms is targeted for a handover of a non-real time service between LTE and either UTRAN or GERAN, while an interrup- tion time of less than 300ms is targeted for real- time services. • Given the scope of these requirements for evolu- tion work, 3GPP agreed upon a work split—the evolution of the radio access network would take place in the 3GPP RAN working groups, and in parallel, work on an evolved packet core (EPC)
  • 5. DOI: 10.1002/bltj Bell Labs Technical Journal 11 would take place in the system architecture (SA) working groups. At this point, it is useful to clarify some terminology: the radio access network enhancements are referred to as either evolved UMTS terrestrial radio access network (E-UTRAN) or LTE; the names are used interchangeably. The evolution work for the EPC is referred to as sys- tem architecture evolution (SAE). For some time, the combination of these enhancements was referred to as LTE/SAE, but more recently it has become known as the evolved packet system (EPS). HSPA؉ Requirements and Performance Targets In order to protect operator investments in HSPA and provide a smooth evolution path towards LTE, which would not offer backwards compatibility with earlier releases of UMTS/HSPA, a study item on HSPA evolution was opened in 3GPP in March 2006. While development of HSPA Release 7 enhancements was already underway in 2005 with open work items regarding HSDPA multiple input-multiple output (MIMO), continuous packet connectivity (CPC), and the “one tunnel” solution for optimization of packet data traffic, there was no general framework in place to guide the evolution of HSPA. The HSPAϩ effort provided a broad framework for HSPA evolution with a clear set of requirements and performance targets, with the intent of identifying what performance bene- fits could be achieved with the existing Release 7 work items and what gaps still existed. The goal of HSPAϩ is not to replace LTE, but rather to enhance HSPA by providing an incremental evolution path for both the RAN and core network which will enhance performance while leveraging existing infrastructure. In addition, HSPAϩ aims to enable co-existence with the EPS since it will be part of future 3G systems. As described in [3, 8], the guid- ing principles behind HSPAϩ are as follows: • HSPA spectrum efficiency, peak data rates, and latency should be comparable to LTE in a 5MHz bandwidth. • The inter-working between HSPAϩ and LTE should be as smooth as possible and facilitate joint technology operation; the possibility of reusing the evolved packet core defined as part of the sys- tem architecture evolution should be analyzed. • HSPAϩ should be able to operate as a packet-only network, based on the utilization of shared chan- nels only (i.e., HSDPA and HSUPA). • HSPAϩ shall be backwards compatible in the sense that legacy terminals compatible with Release 99 through Release 6 are able to share the same carrier with terminals implementing the latest HSPAϩ features, without any performance degra- dation. • Ideally, existing infrastructure should only need a simple upgrade to support the features defined as part of HSPAϩ. As we will see in later sections, the framework provided by the HSPAϩ effort initiated the develop- ment of several new HSPA enhancements beyond what was already being considered in early 2006, in Release 7. Network Architecture Improvements Given the requirements and performance targets described in the previous section, it was clear that not only were enhancements to the radio interface required, but in addition, the network architecture itself needed enhancements. In the next section, we describe the network architecture enhancements for both LTE and HSPAϩ. Evolved Packet System Architecture for LTE The goal of the system architecture evolution effort in 3GPP is not just to define an efficient packet core network and RAN architecture for LTE to meet the requirements described in [2], but rather to develop a framework for an evolution and migration of current systems to a high data rate, low latency, packet-optimized system that supports mobility and service continuity across heterogeneous access net- works, since it is envisioned that Internet Protocol (IP)-based services would be provided through vari- ous access technologies. In its simplest form, the EPS architecture consists of two basic nodes in the user plane: a single node called the enhanced node B (eNB) comprises all radio access functions and a single node called the EPS gateway comprises the entire bearer plane (i.e., user
  • 6. 12 Bell Labs Technical Journal DOI: 10.1002/bltj plane) in the core network. In the control plane, the mobility management entity (MME) node is logically separated from the user plane EPS gateway with an open interface between them. Figure 1 provides a comparison of the EPS network architecture and the UMTS network architecture. EPS offers a flatter net- work architecture than UMTS, especially as far as the user plane is concerned, which reduces latency. The clean separation of the user plane and control plane is a key feature of the EPS architecture, as it allows for independent scaling of control plane functionality and user plane functionality. This is very important from a technical viewpoint because the scaling of the two depends on different factors: the capacity of the con- trol plane functionality typically depends on the num- ber of mobile devices and their mobility patterns, whereas the capacity of the user plane depends on aggregate data throughput required to be supported. Drawing a parallel between the EPS architecture and UTRAN, the enhanced node B absorbs all radio access functions that were contained in the node B and radio network controller (RNC) elements in UTRAN. Note that the eNBs are directly connected to each other via an interface called X2; this facilitates seamless mobility and interference management. Figure 2 presents a more detailed view of the EPS architecture, with the interfaces that exist to support mobility between 3GPP and non-3GPP networks. The EPS gateway may be split into two separate logical nodes with the optional S5 interface: the serving gate- way (GW), and the packet data network (PDN) gateway. The serving GW terminates the core network interface towards 3GPP radio access networks and serves as the local mobility anchor point for inter-eNB handover within the EPS, as well as mobility anchor- ing for inter-3GPP mobility (i.e., between EPS and UTRAN/GERAN). Note the direct control plane inter- face (S3) and user plane interface (S4) between the EPS network and the serving GPRS support node (SGSN) in the UMTS/GSM networks; such an inter- face allows for a packet session to be maintained in a way that is seamless to the user of a multimode ter- minal that migrates across LTE, UMTS/HSPA, and GSM/EDGE coverage areas. This meets the requirements Node BNode B Node BNode B Packet core network Radio access network eNode B eNode B Internet Internet UMTS EPSU-plane C-planeGGSN SGSN RNCRNC MME EPS gateway EPS—Evolved packet system GGSN—Gateway GPRS support node GPRS—General packet radio service HSPA—High speed packet access MME—Mobility management entity RNC—Radio network controller SGSN—Serving GPRS support node UMTS—Universal Mobile Telecommunications System Figure 1. Comparison of UMTS/HSPA network architecture and EPS network architecture.
  • 7. DOI: 10.1002/bltj Bell Labs Technical Journal 13 for co-existence/inter-working and gives the operator the flexibility to roll out LTE gradually, starting with the areas of highest demand first. The PDN gateway provides access to the packet data network through the control of IP data services and allocation of IP addresses; it also serves as an anchor for mobility between 3GPP and non-3GPP access systems, which is sometimes referred to as the SAE anchor function. Note that the specification of logical nodes does not mandate a mapping to physical entities. For example, the serving GW, PDN GW, and MME may be imple- mented in the same physical entity, or the MME may be integrated into the eNB. The mapping of logical nodes to physical entities may follow a highly integrated approach or a more distributed approach, based on ven- dor implementations and deployment scenarios. HSPA؉ Network Architecture For the HSPAϩ network architecture, we begin with a description of the one tunnel enhancement (also referred to as “direct tunnel”) that was already being discussed as part of Release 7 prior to the HSPAϩ initiative. 3GPP recognized that the amount of user plane data would significantly increase in the near future because of the introduction of HSPA. With the existing system illustrated in Figure 1, packet data traffic must traverse both the gateway GPRS support node (GGSN) and serving GPRS support node in the UMTS core network (through the use of two tunnels), independently of how the data traverses the UTRAN. A more scalable architecture is possible with the one tunnel solution, which permits direct tunneling of the user plane data between the GGSN and the RNC, as illustrated in Figure 3. In this way, a cleaner separa- tion between the control plane and the user plane is achieved, the advantages of which were discussed previously for LTE. The new SGSN controller performs all the control functions of the SGSN, and the enhanced GGSN takes over all data transport func- tionality that resided in the previous GGSN and SGSN. To further flatten the network architecture, HSPAϩ introduced the option of integrating the RNC serving GW PDN GW EPS gateway E-UTRAN UTRAN GERAN Non-3GPP† access S1-MME S1-U S11 S4S3 S5 Internet SGSN MME 3GPP—3rd Generation Partnership Project EDGE—Enhanced data rates for GSM†† Evolution EPS—Evolved packet system E-UTRAN—Evolved UTRAN GERAN—GSM/EDGE radio access network GPRS—General packet radio service GSM—Global System for Mobile Communications†† †Trademark of the European Telecommunications Standards Institute. ††Registered trademarks of the GSM Association. GW—Gateway MME—Mobility management entity PDN—Packet data network SGSN—Serving GPRS support node UMTS—Universal Mobile Telecommunications System UTRAN—UMTS terrestrial radio access network Figure 2. Detailed view of the EPS network architecture, with interfaces to support mobility across 3GPP and non-3GPP access.
  • 8. 14 Bell Labs Technical Journal DOI: 10.1002/bltj and node B functionality into a single node for packet switched services (denoted here as node Bϩ ); this is also shown combined with the one tunnel solution in Figure 3. The flatter architecture reduces latency and could be useful in deployments in which a high level of integration is desirable (i.e., HSPA femtocells). From Figure 3, it quickly becomes apparent how simi- lar the HSPAϩ network architecture is to the EPS net- work architecture shown in Figure 1, which allows easy integration of HSPAϩ and EPS networks. Key Features in the Downlink of LTE and HSPA؉ The downlink (DL) of a RAN bears a higher amount of data traffic compared to the uplink (UL) due to the increasing demand for unbalanced data services like, for example, FTP download or video streaming. A number of features have been intro- duced in order to support increasing data rates. LTE Downlink Key Features In the following subsections, we will discuss key features of the LTE DL such as OFDM transmission, MIMO, the possibility of using higher order modulation schemes, time and frequency selective scheduling, and fractional frequency reuse. Details about the LTE DL channel structure can be found in [7] and [12]. Orthogonal Frequency Division Multiplex The LTE DL air interface is based on orthogonal frequency division multiplexing (OFDM) which is a technique that avoids inter-symbol interference, exploits the scarce frequency resource nearly opti- mally, combines the advantages of broadband and narrowband transmission, and, at the same time, avoids their disadvantages. OFDM is further described below. In conventional high bit rate air interfaces, the data symbols are transmitted sequentially over the air interface. According to the Nyquist theorem, the mini- mum required bandwidth B is related to the symbol duration Tsym with B ϭ 1/Tsym. In real systems, guard bands are required at both ends of the used spectrum due to the application of non-ideal filters. In multipath environments, broadband channels show Node-B Packet core network Radio access network Internet HSPA؉ one tunnel HSPA؉ one tunnel with integrated RNC/node B GGSN SGSN RNC Internet GGSN SGSN Node-Bϩ GGSN—Gateway GPRS support node GPRS—General packet radio service HSPAϩ—High speed packet access evolution U-plane C-plane RNC—Radio network controller SGSN—Serving GPRS support node Figure 3. HSPA؉ network architecture.
  • 9. DOI: 10.1002/bltj Bell Labs Technical Journal 15 a frequency selective behavior with several deep fades in the frequency domain. In the time domain, this behavior corresponds to an overlapping of symbols, which causes the so called inter-symbol interference (ISI), illustrated in Figure 4. The smaller the symbol duration, i.e., the higher the symbol rate, the more symbols experience ISI. In broadband transmissions, an inversion of the channel transmission function is required which corresponds to an equalization of the received signal in order to cope with inter-symbol interference caused by the relatively short symbol duration (compared to the delay spread of the chan- nel echoes). One means to reduce inter-symbol interference is to extend the symbol duration so that it is longer than the difference between the delays of the earliest and latest channel echo. A further improvement is to extend the symbol duration by a guard time, during which transmission of the new symbol has already started but which is discarded by the receiver. This feature is called cyclic prefix (CP). The total symbol duration in this case is the sum of the original sym- bol plus the CP duration, which should be longer than the difference between the delays of earliest and lat- est echo in the multipath channel. The reason for the low ISI in the time domain is because of a flat channel in the frequency domain. In a multipath environment, this corresponds to a nar- row bandwidth used for the transmission of the sym- bol. Many parallel narrowband transmissions are required to obtain a high bit rate channel. OFDM avoids the guard band between the so-called subcar- riers by a modulation of these subcarriers with rec- tangular pulses, using the rect(t) function. In the frequency domain, the spectrum of a pulse with dura- tion Tsym corresponds to the sinc(x) ϭ sin(x)/x func- tion with zero crossings at k/Tsym, k ϭ . . . Ϫ 2, Ϫ 1, 1, 2, . . . . Consequently, if these pulses modulate a number of subcarriers, the inter-subcarrier interfer- ence is zero with a subcarrier spacing of 1/Tsym which is, according to the Nyquist theorem, the optimal value, as shown in Figure 5. Furthermore, this opti- mal value is reached without any filter. The modulation of the equally-spaced subcarri- ers with rect pulses corresponds to an inverse discrete Fourier transform (IDFT) in the time domain. At the receiver, the original symbols are reconstructed using the opposite function, namely a discrete Fourier trans- form (DFT). Figure 6 shows a schematic view of the OFDM transmission chain. Transmitted signal Echo 1 Echo 2 Echo 3 Received signal Symbol n Symbol nϩ1ISI ISI—Inter-symbol interference Figure 4. Inter-symbol interference caused by channel echoes.
  • 10. 16 Bell Labs Technical Journal DOI: 10.1002/bltj In OFDM the bandwidth can be easily adapted to the needs of the network operator. LTE FDD, for example, offers bandwidths of 1.4 MHz, 3 MHz , 5MHz , 10MHz , 15MHz , and 20MHz with a sub- carrier spacing of 15 kHz. The total bandwidth includes guard bands at both ends of the spectrum, so that 72, 180, 300, 600, 900, and 1200 subcarriers are conveyed in the respective bandwidth, as outlined in [10]. The inverse fast Fourier transform (IFFT) and corresponding FFT enable a very efficient calcula- tion of the transmitted signal and the correspon- ding reconstruction of the symbols. FFT and IFFT require that the number of subcarriers N is N ϭ 2n with an integer value of n, although not all of these subcarriers have to be transmitted. The granularity in which the transmitter has to calculate the IFFT and in which the receiver has to sample the 1 0.8 0.4 0.6 0 0.2 Ϫ0.2 Amplitude/maximumamplitude Ϫ0.4 Ϫ10 Ϫ5 0 5 10 Frequency (1/Tsym) OFDM—Orthogonal frequency division multiplexing Figure 5. Frequency domain of seven subcarriers of an OFDM signal. Mapper IDFT DFT DemapperChannel Bits Symbols s(t) s’(t) Symbols Bits DFT—Discrete Fourier transform IDFT—Inverse discrete Fourier transform OFDM—Orthogonal frequency division multiplexing Figure 6. OFDM transmission chain.
  • 11. DOI: 10.1002/bltj Bell Labs Technical Journal 17 channel is Tsym/N and, consequently, depends on the FFT size. Multiple Antenna Algorithms MIMO, a synonym for a technique that uses at least two antennas at the transmitter and at least two antennas at the receiver for the transmission of signals over the air interface, is depicted in Figure 7. The antennas can be used to obtain an array gain, i.e., a diversity gain, to reduce co-channel interference, or to enable multiplexing of several data streams to the same or to different receivers. Consequently, MIMO is able to increase quality of service (QoS), coverage, spectral efficiency, and peak data rate. One or several data streams are, after channel coding and modulation, multiplied with a precoding vector and mapped on the different transmission antennas. The precoding vector describes the phase shifts of the data symbols and the mapping of these data symbols on the antenna ports. The transmit (Tx) and receive (Rx) antennas, respectively, are either widely-spaced (or alternatively cross polarized) or closely spaced in a linear array. In the first case, the channel state at the antennas is uncorrelated. For widely-spaced antennas, the antenna pattern gener- ated is frequency dependent and has an irregular shape. In the case of a linear array, the channel state at the antennas is correlated; the generated antenna pattern is frequency independent over the used band- width and shows a regular shape with main and side lobes. A special case for the uplink is virtual MIMO, shown in Figure 8. In this case, two or more widely- spaced mobile terminals are multiplexed on the same resources. In contrast to the base stations, these devices do not require multiple antennas. In general, the maximum number of data streams corresponds to the minimum of Tx and Rx antennas. In the case of virtual uplink MIMO, the number of antennas in the base station is the limiting factor. In a closed loop transmission procedure, the pre- coding vector is chosen so that a constructive super- position of the signals is obtained at the receiver, or that different data streams conveyed over the same resources can be easily separated. In the LTE closed loop mode, the receiver chooses the precoding vector out of a limited set of possible precoding vectors in order to reduce feedback signaling load. In this case, the precoding feedback is reduced to an index in a table of predefined precoding vectors, known as the precoding matrix indicator (PMI). In the open loop mode of LTE DL, space frequency block coding (SFBC) is used under bad to medium channel conditions, while per antenna rate control (PARC) is used under good channel conditions and enables two stream transmissions. Feedback from the receiver is still required in order to signal the supportable rank, i.e., the number of supportable streams and the channel quality. The LTE DL works with orthogonal pilots for the different transmission antennas in order to enable the receiver to calculate the channel transmission function, i.e., the transmission function from every Coding Modulation Weighting Mapping Channel Data stream(s) Weighting Demapping Demodulation Decoding Data stream(s) MIMO—Multiple input-multiple output Figure 7. MIMO transmission and reception.
  • 12. 18 Bell Labs Technical Journal DOI: 10.1002/bltj transmission to every receive antenna. The following antenna algorithms can be applied with multiple antenna systems: • Transmit and receive diversity (with widely- spaced or cross polarized antennas), • Beamforming, or beamswitching (with closely- spaced antennas), • Spatial multiplexing (with widely-spaced or cross polarized antennas), and • Combinations of the previous algorithms. Transmit diversity generates a rich number of channel echoes at the receiver, which arrive from vari- ous directions. By leveraging maximum ratio com- bining (MRC), the receiver may use this receive diversity in order to combine the signal of the differ- ent receive antennas so that the received signal level is optimized. For beamforming and beamswitching, the anten- nas have to be closely spaced, with a spacing of half the wavelength of the carrier frequency. In an open loop algorithm, the antenna spacing has to be cali- brated. For a closed loop algorithm with channel feed- back from the receiver, calibration is not mandatory since the receiver chooses the optimal beam. Several data streams can be conveyed over the same resources to different users, if the beams have sufficient angular separation, possible via spatial division multiple access (SDMA). Spatial multiplexing is an antenna algorithm based on widely-spaced or cross polarized antennas. Several data streams are mapped on the transmit antennas. The receiver, e.g., a minimum mean square error (MMSE) receiver, repeats to combine the sig- nals of the different receive antennas so that the signal-to-noise-ratio of one data stream is optimized while the other data streams are suppressed, until all data streams are reconstructed. Successive interfer- ence cancellation (SIC) is an enhanced receiver algo- rithm that subtracts the interference of successfully received data streams from other data streams with low signal-to-interference-plus-noise ratio (SINR). A combination of spatial multiplexing and beam forming or beam switching is enabled by a number of closely spaced, cross polarized antennas at the transmitter. This antenna allows the transmission of up to two data streams and, at the same time, it allows a beam to form in order to optimize the signal level at the receiver. Coding Modulation Weighting Mapping Channel Data stream(s) Weighting Demapping Demodulation Decoding Data stream(s) Data stream(s) Weighting Demapping Demodulation Decoding MIMO—Multiple input-multiple output Figure 8. Virtual MIMO in the uplink.
  • 13. DOI: 10.1002/bltj Bell Labs Technical Journal 19 Other Performance Enhancing Techniques Link adaptation is a state of the art technique that allows the adjustment of channel protection to chan- nel quality by choosing the best suited modulation and coding scheme (MCS). LTE allows coding rates, i.e., the ratio of data bits and transmitted bits, close to 1 for excellent channel quality. Quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (QAM), and 64 QAM are possible modulation schemes and can be combined with any code rate. QPSK conveys 2 bits in every data symbol (resource element), 4 bits in 16 QAM, and 6 bits in 64 QAM. Consequently, in good channel conditions, three times more bits can be conveyed using 64 QAM than under bad channel conditions when QPSK is applied. However, only those MCS that have the best throughput performance for a given channel quality will be applied, i.e., those which are part of the hull curve of all MCS as shown in Figure 9. Frequency selective scheduling improves spectral efficiency and cell border throughput for OFDM by choosing only the best set of physical resource blocks for a transmission. In a frequency division duplex (FDD) system, for frequency selective scheduling, the mobile terminal has to feed back the channel quality for either all resources or for a subset of resources with the best channel qualities, i.e., channel quality indicator (CQI). The scheduler evaluates the individ- ual sets of CQI values in combination with the indi- vidual throughput of the mobile devices and SISO AWGN, 630 resource elements per transport block 0 1 2 3 4 5 6 Ϫ10 Ϫ5 0 5 10 15 20 25 SNR (dB) Throughput(bitspersymbol) MCS 1 MCS 2 MCS 3 MCS 4 MCS 5 MCS 6 MCS 7 MCS 8 MCS 9 MCS 10 MCS 11 MCS 12 MCS 13 MCS 14 MCS 15 MCS 16 MCS 17 MCS 18 MCS 19 MCS 20 MCS 21 MCS 22 MCS 23 MCS 24 MCS 25 MCS 26 MCS 27 AWGN—Additive white Guassian noise dB—Decibel MCS—Modulation and coding scheme OFDM—Orthogonal frequency division multiplexing SISO—Single input-single output SNR—Signal-to-noise ratio Figure 9. Throughput versus SNR for different MCS for the OFDM downlink.
  • 14. 20 Bell Labs Technical Journal DOI: 10.1002/bltj calculates a priority for every resource of every device. The target of the scheduler is to work as close as pos- sible at a predefined operating point which corre- sponds to a compromise between fairness and sector throughput. Measured over a certain period of time, a proportional fair scheduler gives every mobile device approximately the same number of resources. These are the best resources possible compared to an average channel quality in frequency and in time. However, the scheduling strategy can also be modified gradually towards higher cell throughput or fair mobile throughput. Fractional frequency reuse (FFR) is a technique that uses most of the downlink resources for every sector without any restriction. A small portion of these resources, e.g., one third or one seventh, are either never used, or are used with reduced power by the base station. Network planning is necessary in order to assign a different set of these resources to different base stations in such a way that every pair of neighbor sectors either do not use, or reduce the power of a dif- ferent set of resources. In the latter case, due to power reduction, these resources can still be used close to the base station, i.e., by mobile terminals which have good channel conditions. For a device at the cell border towards a neighbor sector, some resources are avail- able that have an artificially increased quality, namely resources that are either not used or are used with reduced power by the neighbor sector. HSPA؉ Downlink Key Features The code division multiple access (CDMA)-based HSDPA in 3GPP Release 5 and Release 6 already pro- vided an efficient high speed downlink air interface through the use of a short subframe length (2 ms), hybrid automatic repeat request (HARQ), and fast, channel-sensitive scheduling on a shared channel, facilitated by the use of channel quality feedback and the addition of a new advanced scheduling entity, MAC high speed (MAC-hs) located in the base sta- tion. HSDPA, in Release 5 and Release 6, supports QPSK and 16 QAM modulation, and offers a peak of 14. 4Mbps. Several enhancements have been intro- duced for HSDPA in Release 7 as part of HSPAϩ in order to improve spectral efficiency and cell border throughput [9]. Higher Order Modulation HSPAϩ allows up to 64 QAM modulation in the downlink, which conveys 6 bits per symbol instead of 4 bits in the case of 16 QAM and consequently increases the peak data rate by 50 percent to 21.6Mbps. 64 QAM can be applied under good channel conditions. Due to this fact, the possibility of using 64 QAM will enhance spectral efficiency but will not have a high impact on cell border throughput. For backward compatibility, new terminal types have been defined that support 64 QAM. MIMO HSPAϩ allows closed loop 2x2 MIMO with two transmit antennas and two receive antennas. Under good channel conditions, dual stream transmissions are possible that can double the peak bit rate to 28.8Mbps. As already described for LTE MIMO, the mobile terminal chooses the best precoding vector out of a set of predefined precoding vectors together with CQI values for one or two streams. In order to enable the device to measure the signal quality separately for both antennas, the antennas carry orthogonal pilot sig- nals. In case of dual stream transmission, both streams can have different modulation and coding schemes according to their channel quality. In case of low chan- nel quality, the scheduler can decide to switch back to single stream transmission, which then takes place on the two antennas via closed-loop transmit diversity (CLTD). The MIMO scheme, precoding vector, and MCS signal the mobile device via the high speed shared control channel (HS-SCCH). Dual stream MIMO in HSPAϩ supports improved system capacity rather than improved cell border throughput. However, the fallback mode of CLTD for single stream transmission will increase cell border throughput com- pared to the case of transmitting with a single antenna only. The combination of MIMO with 64 QAM is not foreseen for Release 7, but will be part of Release 8 of HSPAϩ, increasing the peak rate to 43. 2Mbps. Enhanced Receiver Types One common means to increase downlink sys- tem capacity and cell border throughput is to enhance the requirements for the mobile receiver. For Release 5, requirements are based on a single antenna rake receiver. Release 6 defined requirements based on a
  • 15. DOI: 10.1002/bltj Bell Labs Technical Journal 21 rake receiver with dual antenna receive diversity (enhanced receiver type 1) and on a single antenna receiver with equalization, e.g., an MMSE receiver (enhanced receiver type 2). Release 7 defines require- ments for the combination of dual antenna receive diversity and equalization (enhanced receiver type 3). The introduction of so-called interference aware receivers further improves performance. Using this feature, the receiver reduces the interference from neighbor cells, which works to enhance cell border throughput. This feature is used in conjunction with equalization for single antenna (enhanced receiver type 2i) or dual antenna Rx diversity (enhanced receiver type 3i). Downlink Performance Comparison Table I and Table II show the performance com- parison of HSDPA Release 6 with 5MHz bandwidth and LTE DL with 5 MHz and 10 MHz bandwidth, respectively. For the basic assumptions we used the HSDPA Improvement Release 6 LTE LTE compared (5 UEs/cell (5 UEs/cell (10 UEs/cell to HSDPA in 5MHz) in 5MHz) in 10MHz) 1 ؋ 2 Case 1 (1 ϫ 2) 0.47 1.33 1.52 2.8 – 3.2x Case 1 (2 ϫ 2) NA 1.47 1.60 3.1 – 3.4x Case 1 (4 ϫ 2) NA 1.73 1.85 3.7 – 3.9x Case 3 (1 ϫ 2) 0.44 1.24 1.40 2.8 – 3.2x Case 3 (2 ϫ 2) NA 1.37 1.50 3.1 – 3.5x Case 3 (4 ϫ 2) NA 1.60 1.70 3.6 – 3.9x Table I. Average downlink spectral efficiency (bps/Hz/cell) with NGMN assumptions. HSDPA—High speed downlink packet access LTE—Long term evolution NA—Not applicable NGMN—Next-generation mobile network UE—User equipment HSDPA Release 6 LTE LTE Improvement (5 UEs/cell in (5 UEs/cell in (10 UEs/cell in compared to 5MHz) 5MHz) 10MHz) HSDPA 1 ؋ 2 Case 1 (1 ϫ 2) 195 223 321 1.1 – 1.6x Case 1 (2 ϫ 2) NA 257 345 1.3 – 1.8x Case 1 (4 ϫ 2) NA 337 462 1.7 – 2.4x Case 3 (1 ϫ 2) 170 140 209 0.8 – 1.2x Case 3 (2 ϫ 2) NA 186 262 1.1 – 1.5x Case 3 (4 ϫ 2) NA 257 323 1.5 – 1.9x Table II. Five percent CDF downlink user throughput (kbps) with NGMN assumptions. CDF—Cumulative distribution function HSDPA—High speed downlink packet access LTE—Long term evolution NA—Not applicable NGMN—Next-generation mobile network UE—User equipment
  • 16. 22 Bell Labs Technical Journal DOI: 10.1002/bltj 3GPP performance verification framework [5], which is based on TS 25.814 [4]. Contrary to this frame- work, we scaled the average number of users per sec- tor according to the bandwidth in order to have a fair comparison across the different systems. For HSDPA mobile terminals we used enhanced receiver type 1, i.e., a rake receiver with two antenna receive diver- sity. For LTE, we used a maximum ratio combining receiver in the case of single stream transmission and a linear minimum mean square error (LMMSE) receiver in the case of dual stream transmission. Switching between single and dual stream in the case of 2ϫ2 and 4ϫ2 transmission was performed accord- ing to the precoding matrices presented in 3GPP TS 36.211 [7]. For all DL cases, we used the spatial channel model WiM C2 “macro urban” from the Information Society Technologies Wireless World Initiative New Radio (IST WINNER) project. We performed the simulations for case 1 with an inter site distance of 500 meters (m) and for case 3 with an inter site dis- tance of 1732 m; both cases use a penetration loss of 20 decibels (dB) and are at a carrier frequency of 2GHz. The 3GPP performance verification framework requests an improvement factor between 3 and 4 for DL spectral efficiency and between 2 and 3 for DL cell border throughput, which is defined as the fifth per- centile of the mobile terminal’s cumulative through- put distribution function. However, for the 3GPP performance verification framework, HSDPA with 5 MHz bandwidth is compared to LTE with 10 MHz bandwidth, and with 10 users in an average per sec- tor for both systems, which penalizes the average mobile device and cell border throughput of HSPA by a factor of two. Consequently, in our comparison, as we scale the number of users with the bandwidth, the required improvement factor for the cell border throughput of 3GPP has to be divided by a factor of 2 and, hence, shall be between 1 and 1.5. The required improvement factor for spectral efficiency remains the same. For all simulations, a standard proportional fair scheduler has been used which, in the long term, assigns approximately the same number of resources to the mobile devices. For HSDPA, the proportional fair algorithm has been performed in time, for LTE DL in time and frequency. It is interesting that for case 3, i.e., for an inter-site distance of 1732 m, the most comparable case, namely 1ϫ2 with 5MHz bandwidth, the LTE cell border throughput is 18 percent smaller but the spectral efficiency is 180 percent higher compared to HSDPA. However, the LTE DL propor- tional fair scheduler could be easily tuned towards higher cell border throughput on the cost of spectral efficiency. For case 1, a 500 meter inter-site distance, cell border throughput is superior to HSDPA Release 6 while we still obtain a 180 percent gain in spectral efficiency. Due to a higher channel diversity, spectral efficiency and cell border throughput increases with increasing bandwidth. With increasing numbers of transmit antennas, we also obtain a gain in both spec- tral efficiency and cell border throughput due to better exploitation of channel diversity. As a consequence, if we combine both effects, the spectral efficiency gains for increasing bandwidth decreases with increasing numbers of transmit antennas due to the fact that both exploit channel diversity. Further improvements are possible for HSDPA and LTE. HSDPA, according to Release 7, introduces 64 QAM and MIMO in the DL as well as new enhanced receiver types with equalizers and with intra-cell inter- ference cancellation. 64 QAM will lead to an improve- ment of spectral efficiency. Dual stream MIMO will not lead to an improvement of cell border through- put. However, transmit diversity, the fallback mode for 2ϫ2, may enhance cell border throughput as well as the new enhanced receiver types. All these features will reduce the gap between HSDPA and LTE DL per- formance. However, all results presented for LTE are without interference rejection combining, which will reduce neighbor cell interference and, consequently, will increase LTE DL cell border throughput. Key Features in the Uplink of LTE and HSPA؉ The uplink is most often the limiting link of mobile broadband technologies, due in large part to the limited transmit power available at the terminal as well as the complexity and battery life constraints imposed by practical handheld and portable devices. In this section, we describe the key features of the LTE uplink followed by the uplink enhancements in HSPAϩ.
  • 17. DOI: 10.1002/bltj Bell Labs Technical Journal 23 LTE Uplink Key Features The key features of the LTE uplink include a sin- gle carrier multiple access technique which provides in-cell orthogonality, the ability to obtain scheduling gains based on both the time and frequency varia- tions of the radio channel, and the ability to provide the always-on connectivity experience for the end user. Multiple Access Technique Unlike OFDM used for the LTE downlink, which is a multi-carrier OFDM transmission technique, single carrier frequency division multiple access (SC-FDMA) was chosen for the LTE uplink due to its more favorable peak to average power ratio (PAPR) characteristics. Consideration of PAPR is crucial in the uplink where power efficient amplifiers are required in the mobile device. In 2005, there was significant discussion in 3GPP on whether single carrier fre- quency division multiple access (SC-FDMA), orthogo- nal frequency division multiple access (OFDMA), and even multi-carrier CDMA (MC-CDMA) should be chosen as the uplink multiple access method. In the end, a majority of companies took the position that SC-FDMA was the best uplink multiple access method for LTE [1]. While SC-FDMA does have bet- ter PAPR than OFDMA, unlike OFDMA the SC-FDMA method suffers from inter-symbol inter- ference when the assigned bandwidth is comparable to or larger than the coherence bandwidth of the channel (i.e., when the channel is frequency selec- tive within the assigned bandwidth); OFDMA does not have this drawback. Therefore, equalization is required in the SC-FDMA receiver. To facilitate a sim- ple one-tap frequency domain equalizer, the chosen SC-FDMA technique uses a cyclic prefix as done in the OFDMA downlink. Further, the basic numerol- ogy in the uplink and downlink is the same; they share the same resource block size of 180kHz (12 sub- carriers), the maximum bandwidth utilization is the same, and the SC-FDMA symbol time is the same as the OFDMA symbol time for the downlink. In fact, one way to implement the SC-FDMA transmitter is to simply first take a DFT of the modulation symbols prior to mapping the IFFT in a conventional OFDMA transmitter. The only restriction is that the subcarriers which are utilized for a particular user must be contiguous in order to maintain the single carrier property. While the single carrier property can also be obtained using a distributed allocation with uni- formly spaced subcarriers and inserting zeros in between, this option was rejected by 3GPP due to poor channel estimation performance and the increased susceptibility to small frequency offsets from the individual mobile devices. In-Cell Orthogonality One important characteristic of the LTE uplink is that the users remain orthogonal even in the pres- ence of multipath, which is very different from the HSUPA uplink based on asynchronous CDMA. Orthogonality in the LTE uplink is maintained in two ways: first, by time synchronizing the users to within a small fraction of the CP through the use of timing advance (TA) signaling; and second by the base station scheduler ensuring that different users are assigned different subcarriers. Loss of orthogonality between users only occurs due to non-idealities in the system such as a residual frequency offset between users or in the case of very high Doppler where there is appre- ciable variability in the channel during the SC-FDMA symbol time. The fact that the LTE uplink is orthogo- nal means that, ideally, users do not see intra-cell (i.e., same cell) interference as in the case of CDMA, and hence the rise over thermal (RoT) is no longer the factor which determines performance, rather it is the interference over thermal (IoT), which is defined as the total interference power (other cell interfer- ence plus thermal noise) divided by the thermal noise. Techniques to manage and control the IoT are still being discussed in 3GPP, and require communication between eNBs, which is made possible through the X2 interface; this is sometimes also referred to as inter- cell power control. As intra-cell interference is signifi- cantly reduced in the LTE uplink, the use of fast intra-cell power control is no longer necessary as in the case of a CDMA uplink; rather, slow power con- trol to compensate the path loss and shadowing is the baseline power control technique for the LTE uplink. Practically, such power control is needed to ensure that the received power level from different mobile terminals stays within a prescribed range due to the non-idealities
  • 18. 24 Bell Labs Technical Journal DOI: 10.1002/bltj which lead to some degree of intra-cell interference, as well as dynamic range and bit-width considera- tions in the base station receiver. Frequency Selective Scheduling Another important feature of the LTE uplink which differentiates it from HSUPA is the availability of a channel sounding reference signal (SRS). The SRS is a known sequence which is transmitted by the mobile device, possibly over a wide bandwidth, in order to allow the base station scheduler to obtain channel state information. In this way, channel sen- sitive scheduling in both time and frequency, referred to generally as frequency selective scheduling, becomes possible. This enhances the spectral effi- ciency of the LTE uplink compared to HSUPA, espe- cially for latency insensitive traffic, i.e., best-effort traffic such as File Transfer Protocol (FTP) uploads or e-mail attachments, in low Doppler conditions. In order to capitalize on the improved SINR offered by the orthogonal uplink and frequency selective scheduling, the LTE uplink allows not only QPSK modulation as in HSUPA Release 6, but also for 16 QAM and optionally 64 QAM. Always-On Connectivity Finally, a key feature of next-generation mobile broadband technologies is to provide the end user with the feeling of so called “always-on” data con- nectivity; that is, the end user of a mobile broadband device should experience connectivity similar to the always-on wired broadband connections used in the home or office. The simple solution is to keep users fully connected to the wireless network as long as the device is powered on; however this will pose significant problems not only with device battery lifetime, but also may result in inefficiency in terms of air interface and base station channel card resources. In UMTS/HSPA, several levels of connectivity are defined through the radio resource control (RRC) states illustrated in Figure 10, which allow efficient Idle mode RRC connected mode Establish RRC connection Release RRC connection Triggered by data activity DCH—Dedicated channel FACH—Forward access channel PCH—Paging channel RRC—Radio resource control Triggered by data inactivity URA_PCH or CELL_PCH CELL_FACH CELL_DCH UMTS—Universal Mobile Telecommunications System URA—UTRAN registration area UTRAN—UMTS terrestrial radio network Figure 10. Radio resource control states for UMTS.
  • 19. DOI: 10.1002/bltj Bell Labs Technical Journal 25 management of terminal battery lifetime as well as base station channel card resources. The RRC con- nected states are characterized as follows: • URA_PCH/CELL_PCH allows the terminal to be kept in a dormant (or standby) mode while retaining an RRC connection with the RAN, as well as a signaling and bearer plane connection to the core network (i.e., the terminal retains its IP address). This is very efficient in terms of terminal battery life, as the device transceiver powers-on periodically only to listen for paging messages, or when it needs to send signaling messages. • CELL_FACH allows for connectionless packet data transfer that involves the use of the shared for- ward access channel (FACH) in the downlink, and the contention-based random access channel (RACH) in the uplink. This state is used when only small amounts of data need to be exchanged, i.e., via short message service (SMS), or as a transi- tional state in which signaling messages are exchanged between the terminal and the network, i.e., to move from URA/CELL_PCH to CELL_DCH. • CELL_DCH is a fully connected state in which large amounts of data can be exchanged efficiently between the terminal and the network; note that both HSDPA and HSUPA are only applicable to the CELL_DCH state in UMTS Release 6. In Release 6, this state is characterized in the uplink by the use of an always-on dedicated physical control channel (DPCCH) which continuously transmits a pilot, even in the absence of data, so that power control can track the fading channel. While the number of RRC connected sub-states offers flexibility and efficiency in terms of terminal battery life, air interface usage, and base station chan- nel card resource utilization, it also results in relatively long latencies when the terminal needs to transition between dormant states (i.e., URA_PCH/CELL_PCH) and fully active states (i.e., CELL_DCH), which makes it difficult to give the end user the always-on data con- nectivity experience. In LTE, the states and state transitions are simpli- fied significantly, as illustrated in Figure 11. Only an RRC_IDLE and RRC_CONNECTED state are defined, and in the RRC_CONNECTED state data can be quickly exchanged between the terminal and the network while terminal battery life can be managed via highly flexible discontinuous receive (DRX) periods which are under the control of the eNB. As the EPS archi- tecture contains only a single network element in the RAN (the eNB), the terminal can quickly move from being dormant to being active as signaling only needs to be exchanged between the terminal and the eNB. In addition, there is no continuously transmitted pilot channel in the LTE uplink as there is in HSUPA Release 6; a data demodulation reference signal (DM-RS) is transmitted by the terminal only when there is data transmitted on the uplink, which is more RRC_IDLE Establish RRC connection Release RRC connection RRC_CONNECTED LTE—Long term evolution RRC—Radio resource control Figure 11. Radio resource control states for LTE.
  • 20. 26 Bell Labs Technical Journal DOI: 10.1002/bltj efficient from both an air interface point of view and a terminal battery lifetime point of view. HSPA؉ Uplink Enhancements The main enhancements for the HSPAϩ uplink include the addition of 16 QAM in order to improve peak user data rates as well as the continuous packet connectivity feature which allows for an improved always-on connectivity experience. Neither of these features results in significant improvements in uplink spectral efficiency, particularly in the typical macro- cellular deployment conditions; hence HSPAϩ uplink data capacity still falls short of LTE. 16 QAM Modulation HSPAϩ extends the uplink peak rate of HSUPA from 5.76Mbps to 11.52Mbps through the addition of 16 QAM modulation. HSUPA Release 6 only allowed QPSK modulation, more specifically, multi-code binary phase shift keying (BPSK). 16 QAM is only available with the 2 ms transport time interval (TTI) length. As significantly higher SINRs are required to support the extended rates offered by 16 QAM, an advanced receiver at the node B, such as an LMMSE sub-chip equalizer, is needed in order to prevent SINR saturation due to self-noise in highly dispersive channels. In addition, 16 QAM requires a stronger phase reference than QPSK, so 3GPP has introduced the concept of boosting the enhanced dedicated physi- cal control channel (E-DPCCH) power level when 16 QAM modulation is used, with the intent that the E-DPCCH be used as additional pilot in the demodu- lation. Due to the range of SINRs experienced in typi- cal macro-cellular deployments, 16 QAM offers little gain in terms of average spectral efficiency; however environments which offer a higher degree of cell isolation (e.g., femtocells) may benefit more from 16 QAM. It should be pointed out that if we consider an LTE system in 5MHz and consider that the minimum amount of bandwidth is reserved for the physical uplink control channel (PUCCH) is two resource blocks, and also account for standards-based restrictions on the number of subcarriers that can be allocated to a single user, we find with 16 QAM (the highest mandatory modulation in LTE) the peak user data rate in 5MHz of exactly 11.52Mbps, which is precisely the uplink peak rate offered by HSPAϩ. Continuous Packet Connectivity The importance of the always-on connectivity experience has already been discussed. It was recog- nized in 2005 that HSPA would require enhance- ments to efficiently support this feature, and a work item called continuous packet connectivity was intro- duced in 3GPP. Recall from Figure 10 that a user needs to be in the CELL_DCH state in order to effi- ciently exchange large amounts of data between the user and the network; however, in the CELL_DCH state the terminal is continuously transmitting the DPCCH (consisting mainly of pilot and power control bits). The DPCCH always transmits in the CELL_DCH state even when the terminal has no data to transmit in the uplink. In HSPA,it is common to move the user into a CELL_PCH or URA_PCH state when there has been a sufficiently long period of data inactivity from the terminal; doing so not only reduces the uplink interference level, but also extends terminal battery life and saves base station channel card resources. When data arrives in the terminal’s buffer, signaling messages must be exchanged between the terminal and the RNC in the CELL_FACH state in order to move the terminal back into the CELL_DCH state. This process involves a random access procedure and establishment of radio bearers, which can take 700ms to 1 second depending on radio conditions. The latency involved in moving a user between CELL_DCH and CELL/URA_PCH repeatedly during a packet session immediately detracts from the feeling of being always connected. Hence, HSPAϩ introduced the CPC fea- ture in order to make it more efficient to keep a user in the CELL_DCH state. As illustrated in Figure 12, the DPCCH is gated off when there is no data to trans- mit on the uplink, which acts as a very quick way to get the benefits offered by the CELL_PCH and URA_PCH states; i.e., it reduces the level of interfer- ence generated in the uplink as well as preserves the terminal battery life. A DPCCH transmission cycle is defined so that the DPCCH is transmitted periodically even during data inactivity, in order to loosely main- tain the power control state and synchronization with the network. This allows the terminal to remain in
  • 21. DOI: 10.1002/bltj Bell Labs Technical Journal 27 an efficient semi-dormant sub-state, and the transi- tion back to a fully active sub-state can occur in less than 50 ms; essentially this is the time required to fully regain the power control state. Use of HSDPA and HSUPA in CELL_FACH The use of CPC in CELL_DCH state does not eliminate the need for the URA_PCH or CELL_PCH states, as it is still more efficient from a terminal bat- tery lifetime point of view to be in the URA_PCH or CELL_PCH states. In addition, when in CELL_DCH state, the user mobility between cells is controlled by the network, which incurs a higher signaling overhead compared to the user-controlled mobility through cell reselection in the URA_PCH and CELL_PCH states. CPC allows for the expiration timer to be set at much longer intervals before the network decides to move the terminal into the URA_PCH or CELL_PCH state, so that the user can experience the feeling of always-on connectivity during an extended data session. To further improve the always-on experience, HSPAϩ introduced enhancements known as enhanced CELL_FACH and enhanced uplink in CELL_FACH which allows the use of HSDPA and HSUPA in the CELL_FACH state, respectively. HSDPA is used in lieu of the secondary common control physical channel (S-CCPCH) to carry both FACH and paging information and HSUPA is used in lieu of sending RACH messages. In addi- tion, it has been agreed that HSUPA can be used in lieu of RACH messages even in the transition from idle mode, i.e., for transmission of the common con- trol channel. The intent here is to allow for faster exchange of signaling messages to move the user more quickly from idle, URA_PCH, or CELL_PCH states to the CELL_DCH state. CPC, combined with the use of HSDPA and HSUPA in the CELL_FACH state, is the HSPAϩ solution to provide the user experience of always-on data connectivity. Note that the CPC feature does not directly translate into sig- nificant improvement in spectral efficiency for best effort data applications. Uplink Performance Comparison Given that the HSPAϩ uplink features do not result in significant spectral efficiency improvement for best effort data applications, we focus here on comparing the performance of LTE with HSUPA Release 6 to check if the desired performance require- ments of LTE were met. As in the case of the downlink, the simulation assumptions from the next-generation mobile networks (NGMNs) forum in [5] have been Without CPC (Release 6) With CPC (Release 7) CPC—Continuous packet connectivity DPCCH—Dedicated physical control channel DPCCH Data burst Figure 12. Comparison of DPCCH transmission with and without CPC.
  • 22. 28 Bell Labs Technical Journal DOI: 10.1002/bltj used; detailed assumptions can be found in [6]. We have provided simulation results for the average cell spectral efficiency in Table III for HSUPA and LTE using the NGMN simulation assumptions. Results for LTE have been given for a 5 MHz carrier for direct comparison with HSUPA, as well as with a 10 MHz carrier which is a more likely deployment scenario for LTE. Unlike the NGMN simulation assumptions, which simulate ten UEs per cell for both HSUPA in 5MHz and LTE in 10MHz, we have taken a more fair approach and assumed five UEs per cell for HSUPA as well as LTE in 5MHz, and assumed ten UEs per cell for LTE in 10MHz. This ensures that the number of UEs scales proportionally with the bandwidth, allow- ing for a fair comparison of user throughputs. Table IV provides the cell edge user data rates for the NGMN simulation assumptions. Note from Table III and Table IV that LTE is able to provide the targeted two to three times improvement in spectral efficiency and cell edge user data rate compared to HSUPA in Release 6. While the NGMN set of simulation assumptions described in [5] do provide a framework in which results can be compared across different companies, we feel that it is not necessarily a very realistic set of assumptions even for the purposes of evaluating rela- tive performance gains. One point in particular we would like to highlight is that the NGMN assumption set uses a typical urban (TU) channel model with a 3 kilometer/hour (km/hr) velocity. This channel type has significant frequency selectivity (i.e., significant multi-path delay spread) which hinders the performance HSUPA Release 6 LTE LTE (5 UEs/cell in (5 UEs/cell in (10 UEs/cell in 5MHz) 5MHz) 10MHz) Improvement Case 1 (1 ϫ 2) 0.26 0.72 0.75 2.8–2.9x Case 1 (1 ϫ 4) 0.36 0.96 0.97 2.6–2.7x Case 3 (1 ϫ 2) 0.27 0.61 0.67 2.3–2.5x Case 3 (1 ϫ 4) 0.33 0.83 0.93 2.5–2.8x Table III. Average uplink spectral efficiency (bps/Hz/cell) with NGMN assumptions. HSUPA—High speed uplink packet access LTE—Long term evolution NGMN—Next-generation mobile network UE—User equipment HSUPA Release 6 LTE LTE (5 UEs/cell in (5 UEs/cell in (10 UEs/cell in 5MHz) 5MHz) 10MHz) Improvement Case 1 (1 ϫ 2) 125 245 306 2.0–2.4x Case 1 (1 ϫ 4) 168 368 430 2.2–2.6x Case 3 (1 ϫ 2) 35 50 60 1.4–1.7x Case 3 (1 ϫ 4) 42 85 125 2.0–3.0x Table IV. Five percent CDF uplink user throughput (kbps) with NGMN assumptions. CDF—Cumulative distribution function HSUPA—High speed uplink packet access LTE—Long term evolution NGMN—Next-generation mobile network UE—User equipment
  • 23. DOI: 10.1002/bltj Bell Labs Technical Journal 29 of the HSUPA Release 6 system using a rake receiver at the base station, while providing an advantage to the LTE system, which capitalizes on the frequency selectivity and low Doppler with its frequency selec- tive scheduling feature. For additional insight on the comparison between HSUPA Release 6 and LTE per- formance, Table V and Table VI provide the per- formance figures using all the NGMN assumptions with the exception that the channel model is changed to a mixture of ITU channel types given by the fol- lowing: 30 percent Pedestrian A 3km/hr, 30 percent Pedestrian B 10 km/hr, 20 percent Vehicular A 30km/hr, 10 percent Pedestrian A 120km/hr, and 10 percent Ricean with K factor of 10dB. This channel mix uses channel types with both low and high fre- quency selectivity as well as low and high Doppler. We see from Tables V and VI that with this channel mixture, LTE now improves performance only by a factor of one to two over HSUPA Release 6. An overview of LTE performance, with a more compre- hensive set of realistic assumptions compared to those used by NGMN, is given in [11]. Voice Over IP Transmission While there is an increasing demand for high speed data transmission over cellular networks, voice still remains the dominant application today. Compared to the traditional circuit-switched transmission, Voice over IP offers a great deal of flexibility in providing value-added services to the consumer, such as enhanced caller identification (ID), call waiting and voice mail features. In addition, VoIP is much easier to integrate into multimedia applications which make use of voice features (i.e., video calling, push-to-talk, and HSUPA Release 6 LTE LTE (5 UEs/cell in (5 UEs/cell in (10 UEs/cell in 5MHz) 5MHz) 10MHz) Improvement Case 1 (1 ϫ 2) 0.42 0.74 0.75 1.8x Case 1 (1 ϫ 4) 0.64 1.01 1.03 1.6x Case 3 (1 ϫ 2) 0.38 0.58 0.63 1.5–1.7x Case 3 (1 ϫ 4) 0.58 0.83 0.94 1.4–1.6x Table V. Average uplink spectral efficiency (bps/Hz/cell) using channel mixture. HSUPA—High speed uplink packet access LTE—Long term evolution UE—User equipment HSUPA Release 6 LTE LTE (5 UEs/cell in (5 UEs/cell in (10 UEs/cell in 5MHz) 5MHz) 10MHz) Improvement Case 1 (1 ϫ 2) 200 216 238 1.1–1.2x Case 1 (1 ϫ 4) 308 380 435 1.2–1.4x Case 3 (1 ϫ 2) 42 42 45 1.1x Case 3 (1 ϫ 4) 50 65 100 1.3–2.0x Table VI. Five percent CDF uplink user throughput (kbps) using channel mixture. CDF—Cumulative distribution function HSUPA—High speed uplink packet access LTE—Long term evolution NGMN—Next-generation mobile network UE—User equipment
  • 24. 30 Bell Labs Technical Journal DOI: 10.1002/bltj interactive gaming). Another inherent advantage of VoIP, especially in an end-to-end VoIP call, is that the speech packets no longer need to traverse the public switched telephone network (PSTN), which has a lim- ited bandwidth and hence only supports an 8 KHz (narrowband) sampling frequency. Significant improvements in voice quality are possible by moving to wideband vocoders which utilize a 16kHz sampling rate, and this sampling rate can be maintained as the packets traverse an IP network as opposed to the PSTN. In a VoIP system, the speech signal, after being dig- itized and compressed by a vocoder, is packetized into voice frames of a fixed duration in the application layer called the Real Time Transport Protocol (RTP). A voice frame duration of 20ms is used by adaptive multi rate (AMR) vocoders that are used in LTE and HSPA net- works. The output bit-rate of AMR vocoders can be adjusted between 12.2kbps (best quality, highest bit- rate) to 7.95kbps to 5.9kbps (lowest bit-rate, at the expense of some voice quality). The RTP packets are then transported in the network using a transport pro- tocol such as User Datagram Protocol (UDP), and routed using the Internet Protocol. Since a large over- head of 40 bytes (more than 100 percent overhead) is added by the RTP/UDP/IP version 4 (IPv4) layers, a technique to compress the header information, called robust header compression (RoHC), can be used to sig- nificantly reduce the overhead. For example, in the absence of RoHC, the 244 source bits that comprise a 20ms AMR 12.2kbps speech packet increases to 576 bits with the addition of headers and other overhead before it reaches the LTE packet core. RoHC compresses the 40 byte RTP/UDP/IPv4 header down to just 4 bytes. Then we have two bytes of radio link control (RLC) and MAC header that get added in the RAN. VoIP Transmission Over LTE Both LTE and HSPAϩ have incorporated air inter- face enhancements in the MAC and physical layers to enable efficient transmission of VoIP packets. In this section we describe these features and provide results of performance analysis for LTE and HSPAϩ. Scheduling In LTE, there is a choice of semi-persistent or dynamic scheduling for VoIP packets. Semi-persistent scheduling refers to the mode of scheduler operation where a set of dedicated resources in time and fre- quency are pre-allocated for the initial HARQ trans- mission of every MAC packet. This means that the network can allocate to a user a set of resource units at specific intervals of time (e.g., once every 20ms) on the downlink and/or uplink, which will be used for the transmission of initial HARQ transmissions of VoIP packets. Retransmissions for these packets will have to be scheduled dynamically, which on the uplink is dynamic only in the frequency domain due to syn- chronous HARQ operation. The benefit of semi- persistent scheduling is in the reduction of MAC control signaling that results from not having to trans- mit dynamic scheduling grants on the uplink and data associated signaling on the downlink for initial HARQ transmissions. A VoIP user may also be scheduled in a purely dynamic scheduling mode, similar to any other data user. Dynamic scheduling provides the flexibility of scheduling the user’s transmissions at any time and frequency, at the expense of higher control signaling. Frequency Hopping In many instances, it may not be possible or effi- cient to implement channel-selective scheduling for VoIP users in LTE. Channel sensitive scheduling cannot be used for persistently scheduled uplink transmissions as the resources are pre-allocated. For uplink transmissions that are dynamically scheduled, channel-sensitive scheduling requires the uplink channel sounding reference signal to be transmitted over multiple resource blocks, which consumes uplink bandwidth, and becomes infeasible when the number of users is large. On the downlink, channel- sensitive scheduling requires the mobile terminal to provide CQI feedback for different frequency resource blocks, which again becomes infeasible when the number of voice users in the cell is large. In situations where channel quality information can- not be used for scheduling in the frequency domain, frequency hopping can provide a significant diversity gain by ensuring that a slow-moving user is not “stuck” with a bad channel for a long time. Physical layer frequency hopping is allowed in LTE.
  • 25. DOI: 10.1002/bltj Bell Labs Technical Journal 31 Uplink Power Control The light load and low delay tolerance of VoIP traffic imply that we cannot rely on a large number of HARQ transmissions over a long period of time to average out channel variations. Some form of power control can hence be very useful to ensure that the required signal-to-interference-plus-noise ratio is consistently maintained and no more interference than necessary is generated. In LTE, uplink power control is applied on the transmit power spectral den- sity (PSD) at the mobile device, and defined as the transmitted power per physical resource block of 180 kHz. The PSD is computed using an open-loop portion that depends on the long-term path loss experienced at the mobile device, the average inter- ference level seen at the base station receiver, and a closed-loop portion that adjusts the open-loop set- point using power control commands that may be issued by the base station. Coverage Enhancement Techniques As discussed in the next subsection on capacity analysis, semi-persistently scheduled VoIP users on LTE uplink face a coverage limitation in macro-cell deployments with large building penetration losses. Two coverage enhancement techniques are currently being discussed in 3GPP to overcome this problem: one is to let a single HARQ transmission span several subframes before receiving an acknowledgement/ negative acknowledgement (ACK/NACK); the other is to segment voice frames within the MAC layer before transmission and use multiple HARQ processes to trans- mit the different segments. Both techniques aim to increase the amount of energy received from a cell- edge mobile device per unit of time, thereby enhanc- ing the cumulative received SINR for a voice frame. Capacity Analysis Similarly to most other air interfaces, VoIP capac- ity in LTE turns out to be uplink-limited. Using semi-persistent scheduling, frequency-hopping and closed-loop power control, we simulated uplink VoIP transmissions at the system level under the NGMN set of assumptions described in [5]. Capacity is defined as the largest value of the average number of users per cell for which no more than 5 percent of the users each experience larger than 2 percent voice frame outage. A voice frame is declared to be in outage if it is not received successfully at the receiver, or if it is received after the maximum tolerable one-way air interface delay of 50ms. The results are summarized in Table VII for case 1 using an AMR 12.2 kbps vocoder. We see that LTE provides approximately a 200 percent capacity improvement over Release 6 HSPA for case 1, which corresponds to a micro-cell deployment. For case 3, which corresponds to a large macro-cell deployment with 20dB in-building pene- tration loss, coverage was found to be inadequate for the LTE uplink. The coverage enhancement tech- niques outlined in the previous sub-section would HSUPA Release 6 HSPA؉ LTE (5MHz) (5MHz) (5MHz) Improvement Case 1 (1 ϫ 2) 73 100 220 3x HSUPA Release 6 2.2x HSPAϩ Table VII. VoIP capacity for uplink LTE and uplink HSPA for case 1 with NGMN assumptions, AMR 12.2kbps vocoder. AMR—Adaptive multi rate HSPA—High speed packet access HSPAϩ —HSPA evolution HSUPA—High speed uplink packet access LTE—Long term evolution NGMN—Next-generation mobile network UE—User equipment VoIP—Voice over Internet Protocol
  • 26. 32 Bell Labs Technical Journal DOI: 10.1002/bltj have to be used before evaluating capacity for this case. VoIP Transmission Over HSPA؉ HSPAϩ inherits all the key features of Release 6 HSPA that are useful for VoIP transmission, namely, the scheduled nature of transmission, HARQ retrans- missions, and link adaptation. The following are some new features in HSPAϩ that further enhance VoIP performance. Uplink CPC As described above in the section on HSPAϩ uplink enhancements, uplink CPC decreases the frac- tion of time that the mobile device is transmitting the pilot channel (DPCCH). This is useful not only during voice inactivity on the uplink, but even during a talk burst since not all HARQ processes are utilized when the 2ms TTI length is chosen for HSUPA. The reduc- tion in the DPCCH transmission, both during a talk spurt and also during voice inactivity, results in a reduction of the total interference level seen at the base station receiver, which allows a larger number of VoIP users to be supported for a given target loading. In Table VII, we have included the HSPAϩ VoIP capacity which utilizes the CPC feature, and we see a 37 percent improvement over HSUPA Release 6 VoIP capacity. However, LTE still offers a 120 percent improvement in VoIP capacity over HSPAϩ. HS-SCCH Less Operation If a system is loaded with a high number of low bit rate users, the HS-SCCH would use a significant amount of spreading codes and power on the HSDPA downlink. A means to avoid this is the introduction of the HS-SCCH less operation. A mobile device that takes part in this operation mode has to blindly decode all transport blocks sent over one spreading code of the high speed downlink shared channel (HS-DSCH). The corresponding transport blocks have only a limited set of code rates and sizes, which are configurable per mobile device, and sent with QPSK modulation only. Conclusion In this paper, we have described the key features of the two technology paths in 3GPP for global mobile broadband: the evolutionary HSPAϩ approach and the revolutionary LTE approach. HSPAϩ offers the advantage of backwards compatibility with earlier releases, allowing operators an easier upgrade while exploiting their current HSPA investment. On the other hand, LTE offers significant improvements in performance, especially in larger spectrum allocations, but does not offer backwards compatibility to HSPA. To deploy LTE, operators will need to consider new spectrum which is becoming available, and eventu- ally swap-out existing GSM/EDGE/UMTS/HSPA spec- trum as the LTE technology matures. The comparison provided in this paper illustrates that there are many feature similarities between HSPAϩ and LTE: the flatter network architecture with clean separation of the user plane and control plane, the availability of higher order modulations such as 64 QAM on the downlink and 16 QAM on the uplink, MIMO techniques, and the ability to provide the always-on data connectivity experience. While these similarities exist, there are fundamental differences between LTE and HSPAϩ; namely, the use of orthogo- nal multiple access in LTE (OFDMA in the downlink, SC-FDMA in the uplink) and the ability to exploit the frequency-selective nature of the channel in both the downlink and uplink. We have seen that LTE offers a considerable advantage in spectral efficiency for best effort data in the downlink and especially the uplink. We have also seen that while both LTE and HSPAϩ pro- vide features that enhance VoIP performance, LTE is able to offer twice the VoIP capacity compared to HSPAϩ for micocell deployments. Acknowledgements We gratefully acknowledge Lutz Schönerstedt for the LTE DL performance data and Michael Wilhelm for the HSDPA performance data. *Trademarks 3GPP is a trademark of the European Telecommuni- cations Standards Institute. CDMA2000 is a trademark of the Telecommunications Industry Association. GSM and Global System for Mobile Communications are registered trademarks of the GSM Association. References [1] 3rd Generation Partnership Project, “LS on UTRAN LTE Multiple Access Selection,” 3GPP TSG RAN Meeting #30, RP-050758, Nov. 2005, Ͻhttp://www.3gpp.orgϾ.
  • 27. DOI: 10.1002/bltj Bell Labs Technical Journal 33 [2] 3rd Generation Partnership Project, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN) (Release 7),” 3GPP TR 25.913, v7.3.0, Mar. 2006, Ͻhttp://www. 3gpp. org/ftp/Specs/html-info/25913.htmϾ. [3] 3rd Generation Partnership Project, “Scope of Future FDD HSPA Evolution,” 3GPP RAN Plenary #31, RP-060217, Mar. 2006, Ͻhttp:// www.3gpp.orgϾ. [4] 3rd Generation Partnership Project, “Physical Layer Aspects for Evolved Universal Terrestrial Radio Access (UTRA) (Release 7),” 3GPP TR 25.814, v7.1.0, Sept. 2006, Ͻhttp://www. 3gpp.org/ftp/Specs/html-info/25814.htmϾ. [5] 3rd Generation Partnership Project, “LTE Physical Layer Framework for Performance Verification,” 3GPP TSG-RAN1 #48, R1-070674, Orange, China Mobile, KPN, NTT DoCoMo, Sprint, T-Mobile, Vodafone, Telecom Italia, Feb. 2007, Ͻhttp://www.3gpp.orgϾ. [6] 3rd Generation Partnership Project, “Uplink E- UTRA Performance Checkpoint,” 3GPP TSG- RAN WG1, R1-071989, Alcatel-Lucent, Apr. 2007, Ͻhttp://www.3gpp.orgϾ. [7] 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA), Physical Channels and Modulation (Release 8),” 3GPP TS 36.211, v8.1.0, Nov. 2007, Ͻhttp:// www.3gpp.org/ftp/Specs/html-info/36211.htmϾ. [8] 3rd Generation Partnership Project, “High Speed Packet Access (HSPA) Evolution, Frequency Division Duplex (FDD) (Release 7),” 3GPP TR 25.999, v7.0.1, Dec. 2007, Ͻhttp://www. 3gpp.org/ftp/Specs/html-info/25999.htmϾ. [9] 3rd Generation Partnership Project “High Speed Downlink Packet Access (HSDPA), Overall Description, Stage 2 (Release 7),” 3GPP TS 25.308, v7.5.0, Dec. 2007, Ͻhttp://www. 3gpp.org/ftp/Specs/html-info/25308.htmϾ. [10] 3rd Generation Partnership Project, “Evolved Universal Terrestrial Radio Access (E-UTRA), Base Station (BS) Radio Transmission and Reception (Release 8),” 3GPP TS 36.104, v8.0.0, Dec. 2007, Ͻhttp://www.3gpp.org/ ftp/Specs/html-info/36104.htmϾ. [11] K. Balachandran, Q. Bi, A. Rudrapatna, J. Seymour, R. Soni, and A. Weber, “Performance Assessment of Next-Generation Wireless Mobile Systems,” Bell Labs Tech. J., 13:4 (2009), 35–58. [12] Informa Telecoms & Media, WCIS, and 3G Americas, Global UMTS and HSPA Operator Status, Feb. 27, 2008. (Manuscript approved October 2008) ANIL M. RAO is a member of technical staff in Alcatel-Lucent’s wireless research and development (R&D) organization in Naperville, Illinois. He received a B.S. in applied mathematics from the University of Alaska, Fairbanks, and M.S. and Ph.D. degrees in electrical engineering from the University of Illinois at Urbana Champaign where he held a National Science Foundation graduate research fellowship. Dr. Rao joined Alcatel-Lucent after assignments with NASA’s Jet Propulsion Laboratory and TRW. His work at Alcatel-Lucent has involved various aspects of system design, performance analysis, and algorithm development for UMTS, HSPA/HSPAϩ, and LTE. He has actively contributed to both the standardization and product realization of these technologies. His interests include intelligent antennas, scheduling and resource allocation algorithms, and optimizing the end-to-end performance of mobile broadband wireless systems. ANDREAS WEBER is team leader of the mobile system performance evaluation group in Bell Labs’ Radio Access domain in Stuttgart, Germany. He received Dipl.-Ing. and Dr.-Ing. degrees in electrical engineering from the University of Stuttgart, Germany. Prior to joining Alcatel-Lucent, Dr. Weber worked in the field of satellite communications as a member of scientific staff at the Institute of Communications Switching and Data Technics, University of Stuttgart. During his tenure at Alcatel Research & Innovation and later at Bell Labs, he worked on the performance evaluation and optimization of 2G, 3G, and beyond 3G mobile communication systems. Currently, he and his team work on LTE Advanced and WiMAX. SRIDHAR GOLLAMUDI is a member of technical staff with Alcatel-Lucent’s Wireless research and development (R&D) organization in Whippany, New Jersey. He received his Ph.D. in electrical engineering from the University of Notre Dame, Indiana. Dr. Gollamudi worked at Motorola Inc. before beginning his career at Alcatel-Lucent. His research interests include statistical signal processing, resource allocation in wireless systems, physical and MAC layer algorithm design, and performance analysis of communications systems.
  • 28. 34 Bell Labs Technical Journal DOI: 10.1002/bltj ROBERT SONI is a technical manager in Alcatel-Lucent’s Wireless business group in Whippany, New Jersey. He supervises a group which is investigating and developing new advanced antenna, physical layer and MAC layer technologies for 3G/4G cellular systems. He received a Ph.D. and MSEE in electrical engineering from the University of Illinois at Urbana-Champaign, and received his BSEE, summa cum laude, from the University of Cincinnati in Ohio. Dr. Soni began his career as a member of technical staff at Alcatel-Lucent ten years ago. He also teaches part-time at Columbia University in New York City, and the New Jersey Institute of Technology in Newark, New Jersey. ◆