Application NoteHigh Speed Downlink Packet AccessAn Introduction, Challenges and Protocol Test ExamplesGlobal deployment of 3G Networks and UMTS is finally underway and current estimatesshow that packet-switched traffic will overtake circuit–switched traffic in the not so farfuture. The growth of data used in communication will soon require networks which canhandle increasing data rates. HSDPA offers high speed data rates of up to 14 Mbps.This Application Note describes the technology and Challenges of High Speed DownlinkPacket Access (HSDPA) – an Evolution of the 3G UMTS networks to higher data rates.First describing the motivation and the different introduction phases of HSDPA, we will thenfocus on some challenges and typical protocol test issues. This paper is as well includingan overview of the basic features and impacts to an existing UMTS network infrastructure.
High Speed Downlink Packet Access Application NoteUMTS or W-CDMA Networks are developed within the The second phase of HSDPA is currently being specified inthird Generation Partnership Project Workgroup (3GPP) 3GPP release 6 and is aimed to achieve data rates of up toand consist of several releases that can be seen as 28.8 Mbps.the evolution of the standard. In Release 99 the 5 MHz It will introduce antenna array technologies such as beamUMTS carrier was defined to provide capacity and user forming and Multiple Input Multiple Output (MIMO).performance advantages over predecessor technologies Beam forming can be described as focussing the transmit-such as GSM, GPRS and EDGE. ted power of an antenna in a beam towards the user’s direction. Knowing that the limiting resources are the transmission power of the base station sector, one can understand that beam forming is a mean of increasing this power. MIMO uses multiple antennas at the sending and receiving side. The third phase of HSDPA which still is a long way down the road will concentrate on the air interface. It will intro- duce a new Air Interface with Orthogonal Frequency Division Multiplexing and higher modulation schemes. Phase three of HSDPA aims at data rates of up to 50 Mbps Additions to the UMTS 3GPP Figure 1. Mobile Networks Evolution Specification In order to support HSDPA, new physical channels, logicalR4 of UMTS provided nominal enhancements to the channels as well as changes to protocols have been addedtransport, radio interface and features defined in R99 and to the UMTS Specification.Release 5 extends the R99 and R4 specifications, offering Channel changes and additionsan enhancement called High Speed Downlink PacketAccess (HSDPA). This evolution of evolving UMTS delivers The two new physical channels introduced with HSDPAmore throughput and performance. HSDPA will provide are the High Speed Physical Downlink Shared Channeltheoretical peak data rates of up to 14.4Mbps. (HS-PDSCH) as well as the High Speed Physical Control Channel (HS-DPCCH).Different evolution steps of The HS-PDSCH is the transport mechanism for theHSDPA newly introduced logical channels. It carries the actual data, uses adaptive modulation and is power controlledThe idea of HSDPA is to increase the possible downlink by the Node-B.data rate by increasing the spectral efficiency. The focuson the downlink data rate is originated in High data rate In addition to the code multiplexing of traditionaldemanding Services such as Internet Access and file W-CDMA channels, where user data is transmitteddownloads. via dedicated channels, HSDPA also introduces time multiplexing. This means that several user share theThe First phase of HSDPA has been specified in 3GPP same channel and at times where one user is notrelease 5. Phase one introduces new basic functions and using an available resource it is becoming availableis aimed to achieve peak data rates of 14.4 Mbps. to others. The reasoning behind this approach isNewly introduced are the High Speed Downlink Shared that user traffic is becoming more of a bursty nature,Channels (HS-DSCH), the adaptive modulation QPSK and so that a large number of users can use the same16QAM and the High Speed Medium Access protocol time-multiplexed channel and efficiently use the available(MAC-hs) in the Node-B radio network resources.2 www.tektronix.com/signaling
High Speed Downlink Packet Access Application Note The other newly introduced physical channel, the HS- The HS-SCCH is a downlink signalling channel providing DPCCH is an uplink control channel. It carries signalling- information to the UE. The information provided is around and channel quality information from the User Equipment timing and coding and amongst others contains the (UE) to the Node-B. This information is used by the channel code set, the modulation scheme, the transport Node-B to perform the adaptive modulation, and coding block size and the UE identity. This data enables the of the above described HS-PDSCH. We will talk about user equipment to “listen” to the HS-DPCH in an opti- the adaptive modulation at a later point in this paper. mized way, at the right time and with the correct codec’s The transmitted signalling information also contains in order to decode the received data. It enables a acknowledgements or non-acknowledgements for each connection without wasting precious radio resources. received user data block. Protocol changes and additionsLogical Channel additions are the High Speed Downlink The main changes where introduced for the MediumShared Channel (HS-DSCH) as well as the High Speed Access Channel protocol (MAC). The MAC decides onShared Control Channel (HS-SCCH). which channel the Protocol Data Units (PDU’s) will be The HS-DSCH provides the logical transfer mechanism transmitted. The traditional MAC protocol resides in the for the data that is transported on the physical channel Radio Network Controller, whereas for HSDPA or more HS-PDSCH precisely for the High Speed Physical Downlink Channel the High Speed MAC (MAC-hs) has been introduced. The MAC-hs resides in the Base Station. It takes care of the transport block scheduling, channel allocation and the transport format selection. Further tasks of the MAC-hs amongst others are are: Adaptive Modulation and Coding (AMC) Fast packet scheduling mechanism Hybrid Automatic Repeat Request (HARQ) Adaptive Modulation and Coding (AMC) is one of the major changes in HSDPA. In UMTS release99 modulation techniques where applied to provide a reliable connection under changing environmental conditions. With decreasing signal to noise ratio more errors are transmitted with the signal. The higher the coding rate applied, the better the chances of an UE to decode the original data. But on the other hand, the higher the coding rate, the more bits are sent to transmit the information which means that more bandwidth is used. In HSDPA the UE is actively feeding back information about the channel conditions which is used by the Node-B to determine Modulation and coding scheme. For each Transmit Time Interval (TTI) the UE feedback is taken into account the best possible modulation and coding is chosen and the highest possible transmission rate is obtained. Figure 2. HS-DSCH data Frame Structure www.tektronix.com/signaling 3
High Speed Downlink Packet Access Application NoteThe Fast scheduling mechanism handles the logical channel Network Element Changesresources and determines which particular user should The mainly affected network Element in HSDPA are thebe served within a 2ms time interval. This mechanism also Node-B and the RNC.takes into account the information sent by the individual The Node-B in HSDPA, is taking over several functionsUE’s. The knowledge of the instantaneous quality of a that where previously performed by the RNC. The MAC-hschannel makes it possible to avoid sending data packets protocol with all its new functions such as flow controlduring channel fades and rather schedule a UE in better towards the Iub, data buffering, the earlier describedconditions. This might cause that certain UE’s will obtain a scheduling the HARQ termination, link adaptation andlower throughput than others. The challenge for this packet the power control are all functions that have not beenscheduling function is to optimize the cell capacity and at performed by Node-B’s so far.the same time fulfil QoS requirements defined in NetworkOperators policies. Depending on the Node-B currently deployed, Network Operators might face software or software and hardwareHybrid Automatic Repeat Request (HARQ) is an enhanced upgrades in order to deploy HSDPA. The maximumform of the Automatic Repeat Request (ARQ) and is used achievable bit rate a Node-B can support depends onfor the packet retransmission. the hardware.In Rel.99, whenever a packet was not properly decoded The main new functionality for the RNC is the new Radioby the UE, it was discarded and resent (ARQ). Resource Management. The addition of shared channels to the dedicated ones will have a major impact on this function. The capacity of a cell needs to be split between dedicated channels and HSDPA channels. New to the Streaming services is the requirment of an intelligent algo- rithm with a dynamic behaviour when sharing dedicated and shared channels. Also new to the RNC is the HS-DSCH Framing Protocol for the user plane which has been added to the protocol stack. Challenges for network test engineers As stated before, the Node-B as well as the RNC are Figure 3. Packet Retransmission with ARQ vs. HARQ becoming key elements in HSDPA. And Test engineers will be faced to test this new functionality. Decoding ofNot so in HSDPA with HARQ. In HSDPA the erroneous the new protocols, analysis of the obtained data as wellpacket is not discarded but stored by the UE. The packet as the correlation of data over several Interfaces will playis resent and both, the previous one as well as the newly an essential role for the deployment of HSDPA Networks.sent packets are used to extract the original information. The following chapter will describe specific test challengesThis approach has a much better error rate performance, that Equipment Manufacturer (EM) and Network Operatorscauses packets to be resent less often and therefore (NO) are facing today.enables to use the bandwidth more efficiently.The retransmission is performed by the Node-B, so thatthere is no retransmission via the Iub interface (figure 3).4 www.tektronix.com/signaling
High Speed Downlink Packet Access Application Note data could also affect the traffic speed. If the traffic control mechanism is not working properly, some user could receive very high throughput rate (e.g. 4Mbps constantly) whereas the rest would only receive low speed traffic. Monitoring the actual IP throughput on the Iub interface will help analyzing the actual traffic condition. Analyzing multiple user’s traffic in one and the same cell will provide information about the balance of throughput amongst different user. The IP packet retransmission ratio on the Iub interface is one of the critical parameters for the reliability of a service. This also means that monitoring and decoding of user- Figure 4. HSDPA Setup Signaling plane IP data helps to analyze the network performance on the Iub user-plane.Signalling Test: QoS:In HSDPA Networks, new messages like the HS-DSCHCapacity Request and Allocation messages are used on Delaythe Node-B for data traffic control. The amount of traffic on The Node-B with its new functionality will perform datathe Iub Interface, and therefore the achieved throughput is buffering and scheduling at the Iub side. It will analyze thedepending on message-parameters like the Packed Data air interface conditions and choose the optimal modulationUnit (PDU) size, the Interval, and the Repetition Period. The scheme at the air interface (Uu) side (i.e. 16QAM or QPSK).Node-B buffering and the scheduling will also be affected One of the critical functions for an HSDPA enabled Node-Bby several parameters like the conditions of the air interface would be the packet scheduling functionality and HARQ.the retransmission control mechanism and others. The packet process mechanism on both, the Iub and UuMonitoring the behaviour and development of each param- interface of any Node-B is a critical factor for minimizingeter helps to analyze the functionality of a signalling proce- the latency. Monitoring of the delay between the Iub anddure and the traffic flow control functionality. (See Fig-4). Uu will help analyzing the HSDPA scheduling and theThe Tektronix K15 Monitor and K1297-G20 functional tester packet retransmission functionality. The delay measurementare equipped with decoding functionalities to monitor and is critical for HSDPA service performance.test above parameters ThroughputUser Data Test: Data throughput on both Iub and Uu could give us some key performance information of the Node-B. AnalyzingHSDPA will provide a theoretical throughput of 14.4Mbps the data generation timing from Node-B and the relatedper cell and is aimed to achieve over 1Mbps of actual timing information in NBAP, RRC message could be usefulpacket service speed. However, the actual traffic speed for the performance analysis. Figure 5 illustrates a possibleis depending on several factors like the number of users, test setup to analyze the air interface (Uu) using a spectrumthe Node-B buffer condition, the air interface condition, analyzer triggered by an event on the lub interfaces.and the Node-B data transmission timing. Different typesof data user accesses, i.e. short packet data or long packet www.tektronix.com/signaling 5
High Speed Downlink Packet Access Application Note Figure 5. Uu, lub Test ExampleFuture test challenge: The ability of mobile test equipment to measure above parameters is becoming more and more important. ItOnce HSDPA will be deployed and become more mature, enables Equipment Manufacturer and Network Operatorsthe Node-B resource allocation will become a critical to deploy their networks more rapidly and maintainfactor for network optimization. HSDPA Networks will their quality of service requirements. Mobile test equipmentrequire offering larger resources and quality values than like the Tektronix K15 or K1297-G20 are equipped toother mobile networks. meet these challenges and offer a large variety of protocolKey Performance Indicators (KPI) will help analyzing network decoding capabilities as well as applications that willconditions and help optimizing the network deployment. ensure to manage above mentioned challenges.Following parameters are just some useful KPI’s that arerelevant in HSDPA and other mobile networks. Number of Voice calls Number of Video calls Number of Packet calls Spreading factor values Number of non-success calls - Released calls - Rejected calls - Failures Power control information6 www.tektronix.com/signaling
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