HSDPA – An Introduction


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HSDPA – An Introduction

  1. 1. HSDPA – An Introduction By Juha Korhonen A TTPCom White Paper TTPCom Headquarters, Melbourn, Cambridge World leading independent supplier of software and silicon IP for digital wireless terminals For more information – www.ttpcom.com
  2. 2. HSDPA – An Introduction 2 Contents 1 INTRODUCTION 3 2 HSDPA – THE PRIMER 4 2.1 Why is there a need for HSDPA? 4 2.2 What does HSDPA do? 5 2.3 When and where does HSDPA deploy? 6 3 HSDPA TECHNICAL DETAILS 7 3.1 What is HSDPA? 7 3.2 How does the Air Interface work? 10 3.3 What is the future for HSDPA? 13 4 GLOSSARY 16 ©TTPCom 2004 www.ttpcom.com
  3. 3. HSDPA – An Introduction 3 1 INTRODUCTION The field of telecommunications is full of peculiar abbreviations, and HSDPA is yet another new entrant. HSDPA stands for High Speed Downlink Packet Access. It is a new improved downlink packet data transfer scheme for 3GPP1 systems. Modern telecommunication networks are constantly under development, and new features are introduced regularly. However, HSDPA is not just a minor change to 3GPP system specifications, but a major upgrade that brings clear capacity improvements andmuch higher data speeds than the existing 3GPP systems. This white paper explains why this enhancement is needed, how it is achieved, and what kind of improvements it brings, as well as when it will happen. The first part of this paper discusses the general aspects of HSDPA. It includes an overview, discussion on the impact HSDPA has on services and applications and the probable timetable of HSDPA deployment. The second part is a technical discussion of HSDPA, including a presentation of HSDPA channel structures and procedures, followed by a detailed example of HSDPA data transmission procedure. In addition, the future development of HSDPA is discussed. 1 3G Partnership Project Organisation that deals with most of the 3G specifications. The 3G technical specifications can be found at www.3gpp.org ©TTPCom 2004 www.ttpcom.com
  4. 4. HSDPA – An Introduction 4 2 HSDPA – THE PRIMER 2.1 Why is there a need for HSDPA? Unlike two-way voice communications that are essentially "symmetric" in their use of radio, many 3G mobile services - such as web browsing or streaming live video - create more traffic coming to the user (downlink) than from the user (uplink). UMTS Forum report “3G Offered Traffic Characteristics”2 concludes that data traffic on downlink will exceed uplink traffic by a factor of 2:3. HSDPA offers a way to increase downlink capacity within the existing spectrum. There are no reliable studies available from open sources, but the estimates generally state that HSDPA increases the downlink air interface capacity 2-3 -fold. The first 3GPP networks conform to a 3GPP standard version called Release 99. Release 99 is a full 3G system, with clearly improved capabilities compared to 2G (the basic GSM) and 2.5 G (GPRS and EDGE) systems. The maximum data rate per user in Release 99 systems is typically 384kbit/s, whereas in 2.5 G systems it is a few tens of kbit/s, or just over 100 kbit/s at best. Current 3G technology can accommodate only a few maximum data rate users at a time before the cell capacity runs out in the downlink direction. A typical user consumes more downlink than uplink resources. Some applications, such as web browsing and many games, use uplink only for control signalling, whereas the downlink carries lots of payload data for those applications. Therefore, it is clear that a Release 99 system will first run out of capacity in the downlink. HSDPA aims to improve downlink capacity, and thus remove this potential bottleneck from the system. It increases both the system capacity as a whole, and the data rate that can be allocated for one user. The maximum theoretical data rate for one user is 14.4 Mbit/s, but in real systems, this is likely to be limited to around 2 Mbit/s at first. SYSTEM GSM GPRS EDGE 3G (R99) HSDPA Typical max. data 9.6 50 130 384 2048 rate (kbit/s) (or more) Theoretical max. 14.4 170 384 2048 14400 data rate (kbit/s) Table 1 Data rates of telecommunication systems (downlink) HSDPA is included in Release 5 in 3GPP standards, and it is the most important part of it. The HSDPA scheme adds an additional wideband downlink shared channel that is optimised for very high-speed data transfer. ThusS, HSDPA improves only the downlink throughput although a corresponding uplink enhancement is being specified for later releases. After this upgrade, 3GPP systems are expected to be capable of handling the mobile data transmission needs for several years to come. 2 UMTS Forum Report #33, “3G Offered Traffic Characteristics”, http://www.umtsforum.org, November 2003. ©TTPCom 2004 www.ttpcom.com
  5. 5. HSDPA – An Introduction 5 2.2 What does HSDPA do? HSDPA is most suitable for background and streaming class services. HSDPA is a downlink enhancement scheme, and thus it improves downlink throughput only. Both background and streaming class services typically generate much more downlink than uplink traffic. Background class services do not have very strict end- to-end delay requirements, and thus this enables the network to employ more efficient (throughput-wise) data scheduling algorithms. For example, the transmission resources can be allocated for UEs with the best radio channel conditions, which increases the throughput. Streaming class services are also typically asymmetric; there is more data in the downlink than in the uplink. Streaming class applications can also withstand transmission delays and delay variations quite well if large enough reception buffers are used. However, HSDPA can also be used for other data applications, even for conversational ones, as the network can employ data schedulers that give higher priority to real-time applications. In fact, in some cases HSDPA could be more efficient than a dedicated channel for real-time data, as HSDPA employs a shorter frame length, and thus it can react faster to problems in the radio channel. The selection of a suitable packet data scheduler is an important network performance issue for operators. Moreover, it is not only a performance issue, as it is also possible to give higher priority to data packets going to prime users. Thus, HSDPA is not only a pure technical method of enhancing radio network performance, but it can also be used as a marketing tool. HSDPA increases the typical user data rates up to 2Mbps. The basic HSDPA is unlikely to provide higher throughput, as higher data rates would consume too large a share of the code resources of a cell, causing the cell to become code limited. However, HSDPA technology can provide up to 10 Mbit/s user data rates. It is up to the operator to decide whether it wants to provide such rates to users, and of course, users must have high-end HSDPA terminals before they can enjoy these benefits. HSDPA capacity can be further increased by either by HSDPA-MIMO or by employing new frequency channels that would be allocated exclusively for HSDPA. However, in addition to increased capacity, HSDPA also provides another important enhancement; that is shorter delays. User perception of a fast connection is not only dependent on the bandwidth available, but also on the feedback delay of the channel. If the application reacts quickly on the commands given by the user, it gives the impression of a high-bandwidth application. HSDPA provides a short feedback delay. Short feedback delay also enables new applications, such as interactive networked games. Note that HSDPA is not the only method that can be used to increase downlink capacity. Nevertheless, it is probably one of the easiest ways to do it, and it saves spectrum, which itself is a scarce resource. ©TTPCom 2004 www.ttpcom.com
  6. 6. HSDPA – An Introduction 6 It is also clear that HSDPA is not suitable for applications with very low bandwidth requirements, such as voice. HSDPA channels employ spreading factor3 16, and using that kind of high-capacity channel for voice is clearly a waste of resources. As a summary, HSDPA is best for applications with highly variable bandwidth requirements, which can occasionally be very large. 2.3 When and where does HSDPA deploy? Are operators interested in HSDPA? Because HSDPA can be seen as a capacity enhancement for 3G networks, network operator interest is proportional to the number of subscribers planned on the network and the deployment of asymmetric high bandwidth services. There is already strong interest in HSDPA by some operators in Japan and in Korea. They have operated 3G networks for some time now, and their data services are highly advanced when compared to European offerings. Increased data traffic in their networks demands increased capacity, and the easiest way to deliver this is by implementing HSDPA. These operators are also most aggressive in delivering interactive and high-bandwidth applications. In both countries, the competition among 3G operators is fierce, and HSDPA enables new kind of applications that can attract new customers. The launch schedule estimates vary, but it seems quite certain that the first HSDPA launches will take place during 2005 in East Asia. The European schedule will be country specific and depend on the success of new 3G data services. However, some 3G operators may use HSDPA as a competitive advantage in Europe, and adopt it quite early. In countries where GSM-EDGE networks are deployed, 2.5G operators can provide quite similar services to early 3G networks, but they cannot compete with HSDPA upgraded networks. One factor that may speed deployment is that HSDPA is often only a software upgrade for the network, and thus easy and quick to undertake. The real bottleneck for the HSDPA service growth will be the availability of HSDPA capable handsets, as new models have to be redesigned to include new HSDPA features. 3 Spreading factor (SF) in a CDMA system indicates the number of chips that are used for spreading one data symbol. The higher the spreading factor, the lower the data rate as in each timeslot a fixed number of chips are transmitted (2560 chips per timeslot in the FDD mode). In 3GPP (FDD mode), downlink spreading factors vary between 4 and 512. ©TTPCom 2004 www.ttpcom.com
  7. 7. HSDPA – An Introduction 7 3 HSDPA TECHNICAL DETAILS 3.1 What is HSDPA? HSDPA is a scheme that supports a very high capacity shared data channel in the downlink direction. It encompasses three new channel types: two control channels and one data channel. In addition, it employs a new frame structure, a new fast retransmission scheme, and a new adaptive modulation scheme. The shared HSDPA data channel (HS-PDSCH) is a more capable upgrade of the Release 99 - specified Downlink Shared Channel (DSCH). A UE only has to support one of them, but the network has to support both, as not all users will have HSDPA capable phones. HS-PDSCH is a shared channel; it is shared between all active HSDPA users in the cell. This channel is shared in two dimensions: it is both time and code multiplexed (see Figure 1). Each standard 10 ms frame is divided into 2 ms sub- frames in HSDPA. Timeslots are still the same length as in Release 99; that is 0.67 ms. Thus there are 3 timeslots within one HSDPA sub-frame. The transmission resources can be re-allocated in each sub-frame, so the HS-PDSCH is time multiplexed. Furthermore each sub-frame can further be shared up to 16 users simultaneously because each active user is allocated at least one spreading code of SF=16. 1 frame = 10 ms 1 sub-frame = 2 ms HS-PDSCH user 1 user 1 user 3 user 2 user 1 user 2 user 3 user 2 user 2 user 2 user 3 user 3 user 5 user 3 user 3 spreading codes user 4 user 4 user 5 user 4 user 3 user 5 user 5 user 5 user 4 user 5 user 6 user 5 user 6 user 5 user 5 user 7 user 6 user 8 user 6 user 5 user 8 user 8 user 9 user 7 user 7 user 9 user 10 user 10 user 9 user 9 user 10 user 11 user 11 user 10 user 10 user 11 user 12 user 11 user 11 user 12 user 12 user 14 user 12 user 13 user 13 user 13 user 14 user 12 user 14 user 13 user 14 user 15 user 13 user 14 user 14 user 15 user 16 user 15 user 15 user 15 user 16 user 17 user 16 user 15 user 16 Figure 1 HS-PDSCH channel time and code multiplexing In addition to HS-PDSCH, an HSDPA UE will also need new control channels to support this function. The High Speed Shared Control Channel (HS-SCCH) is a downlink control channel that gives the UE the fast changing parameters that are needed for HS-PDSCH reception, and indicates when there is data on the HS-PDSCH that is addressed to this UE. In the uplink direction, there is the High Speed Dedicated Physical Control Channel (HS-DPCCH) that is a low bandwidth channel for sending back data packet acknowledgements and channel quality information. HS-DPCCH is always code multiplexed with the dedicated uplink control channel, and it cannot exist alone. See Figure 2 for the illustration of the new channels. ©TTPCom 2004 www.ttpcom.com
  8. 8. HSDPA – An Introduction 8 Standard 3G System HSDPA upgrade dedicated channels dedicated channels Uplink DCH Uplink DCH HS-DPCCH Downlink DCH Downlink DCH HS-SCCH UE UE PDSCH Node-B Node-B shared channels HS-PDSCH shared channels Figure 2 HSDPA upgrade HSDPA is a combination of several techniques which all contribute to the enhanced capabilities of the downlink channel. A Release 5 HSDPA capable handset will include both Hybrid ARQ (HARQ) and Adaptive Modulation and Coding (AMC) functionality. Release 6 will further enhance HSDPA capabilities by introducing Multiple Input Multiple Output (MIMO) antenna techniques. HARQ is a link adaptation scheme in which link layer acknowledgements are used for retransmission decisions in the UTRAN. In Release 99 retransmission functionality is part of the RLC layer. However, this kind of high-level retransmission scheme is too slow for the high-speed data transmissions envisaged for HSDPA. With HSDPA the HARQ retransmission buffers are located closer to the physical layer, specifically within the new MAC-hs logical entity that is just above the physical layer. The ARQ combining is based on incremental redundancy; that is, if a transmission fails, the received (corrupted) data is stored to a buffer regardless. Successive retransmissions will include more redundancy, and they are combined with the old data in the buffer. This is repeated until the data in the buffer is considered to be correctly received, or the maximum number of retransmissions is reached. Moreover, to make HARQ more efficient, a shorter frame, or Transmission Time Interval (TTI), length is needed. The new shorter TTI will be only 3 timeslots long (2 ms), compared to 15 timeslots (10 ms) employed by the other physical channels. When a shorter TTI is used, the UE can inform the network every 2 ms if the transmission failed. In the old scheme, 10 ms would have to pass before a failure could be reported. Shorter frames also mean that the system can respond more quickly to changing channel conditions, and re-assign capacity amongst users. Adaptive Modulation and Coding (or Link Adaptation) means that the shared channel transport format (i.e., the modulation scheme and the code rate) depends on the channel quality. This is monitored constantly, and the transport format used can be dynamically changed in every frame. The quality information is transmitted to the Node-Bs via the uplink control channels. That is, if the radio channel condition is good, the network can use higher-order modulation and less redundancy, whereas in poor conditions, a more robust modulation scheme can be employed and the data packets may have more redundancy in them. Release 5 employs two modulation schemes, namely QPSK and 16 Quadrature Amplitude Modulation (16QAM). Later releases may introduce other schemes, such as 64QAM. Note that older non-HSDPA releases only support QPSK. ©TTPCom 2004 www.ttpcom.com
  9. 9. HSDPA – An Introduction 9 All HS-PDSCH channels use a spreading factor (SF) of 16. However, to increase the throughput of a user, the network can allocate several such spreading codes to one user. The maximum number of multicodes a UE can support is a UE capability parameter, and can be 5, 10, or 15. Note that in principle only 16 spreading codes (of SF=16) are available in a cell, thus the use of 15 multicodes means that the cell could be very close to becoming a code-limited cell as only 1/16 of the code space is available for other purposes, such as for (mandatory) control channels. HSDPA is not suitable for all kinds of services. The HSDPA data channel is shared amongst all active HSDPA UEs in a cell. The shared character of the channel means that maximum transfer delays cannot be (easily) guaranteed, and applications that have strict real-time requirements should use dedicated channels and not the HSDPA. On the other hand, the resource allocation in HSDPA channels is very fast. The capacity allocation can be dynamically changed in every sub-frame (2 ms in HSDPA channels). Additionally, the scheduler in the network may favour higher priority real-time data streams. The HSDPA upgrade requires new handsets with the HSDPA capability. There will be different UE HSDPA capability classes (see Table 2). For example, low-end UEs may conform to the lowest category classes, and high-end UEs can implement the highest classes potentially having better throughput. Reference combination 1.2 Mbps 3.6 Mbps 7 Mbps class 10 Mbps class class class Corresponding FDD HS-DSCH Category 1 Category 5 Category 7 Category 9 category Table 2 FDD UE Radio Access Capability classes Each HSDPA capability class has been defined a corresponding physical layer category class that as a minimum is required so that the UE can achieve the capability class requirements. There are altogether 12 different physical layer HSDPA categories as seen in Table 3. Note that categories 11 and 12 include QPSK only support, so despite the high category number, these two categories are actually low-end HSDPA categories capacity-wise. ©TTPCom 2004 www.ttpcom.com
  10. 10. HSDPA – An Introduction 10 HS-DSCH category Minimum Maximum number Maximum Total inter-TTI of bits of an HS- number of number of interval HS-DSCH DSCH transport soft channel codes block received bits received within an HS-DSCH TTI Category 1 5 3 7298 19200 Category 2 5 3 7298 28800 Category 3 5 2 7298 28800 Category 4 5 2 7298 38400 Category 5 5 1 7298 57600 Category 6 5 1 7298 67200 Category 7 10 1 14411 115200 Category 8 10 1 14411 134400 Category 9 15 1 20251 172800 Category 10 15 1 27952 172800 Category 11 5 2 3630 14400 Category 12 5 1 3630 28800 Table 3 FDD HSDPA Physical Layer Categories The difference between these categories lies mainly in the number of multicodes supported, and the length of the inter-TTI gap during HS-PDSCH reception (i.e., the ability of the UE to receive HSDPA data in successive TTIs, or sub-frames). HSDPA related categories and other capability information is defined in the 3GPP standard4. 3.2 How does the Air Interface work? In this section, we will discuss how HSDPA scheme works in the air interface. It involves the interworking of three physical channels: HS-PDSCH, HS-SCCH, and HS-DPCCH. HS-PDSCH and HS-SCCH are shared downlink channels, whereas HS- DPCCH is a dedicated uplink channel. There can be up to 4 HS-SCCH channels configured for a UE, and these have to be monitored simultaneously as the Node-B can use any of these (but only one at a time). Note that there can be many more HS-SCCH channels in a cell, but only maximum of four can be allocated to one UE. Of course, each of these channels has a different spreading code, so they can be received simultaneously. However, if the UE has already received a HS-SCCH addressed to it in one frame, it is sufficient for it to monitor only this HS-SCCH during the next HS-SCCH sub frame. This arrangement is because now the UE is already receiving at least one HS-PDSCH channel, and it would be quite difficult to monitor in addition up to four HS-SCCHs simultaneously. HS-SCCH is a shared channel (time-wise); each HS-SCCH sub frame can be allocated to a different UE. 4 3GPP TS 25.306, v. 5.6.0, UE Radio Access capabilities (Release 5), September 2003. ©TTPCom 2004 www.ttpcom.com
  11. 11. HSDPA – An Introduction 11 The timing of these channels is shown in Figure 3. Note that the HS-SCCH frame and the corresponding HS-PDSCH frame are overlapping by one timeslot. This may seem strange at first, because the UE has to first receive and decode the HS-SCCH frame before it knows whether the corresponding HS-PDSCH is addressed to it. It all becomes clearer when we discover that the UE identity can be resolved after reception of the first HS-SCCH timeslot. The data in the said timeslot is masked using a bit string that is derived from the UE identity. Only the correct UE identity can decode this timeslot. The first timeslot also indicates the spreading code(s) used, and the modulation scheme employed, so that the UE can start receiving the HS-PDSCH if necessary. There is a gap of one timeslot (0.667 ms) between the end of the first timeslot of the HS-SCCH frame, and the start of the HS-PDSCH frame. 1 frame = 10 ms 1 sub-frame = 2 ms 1. ts 2.&3. timeslots HS-SCCH (downlink, control) HS-PDSCH (downlink, data) 2 timeslots = 19200 chips = 2.5 frames = 7.5 timeslots = 5 ms 1.33 ms HS-DPCCH (uplink, control) Figure 3 the timing of HSDPA channels The HS-SCCH frame structure is depicted in Figure 4. A HS-SCCH sub-frame contains the following information: • Channelization-code-set information (7 bits) • Modulation scheme information (1 bit) • Transport Block size information (6 bits) • Hybrid-ARQ process information (3 bits) • Redundancy and constellation version (3 bits) • New data indicator (1 bit) • UE identity (16 bits) Note that the number of bits indicated in the previous list refers to the raw data bits. In the rather complex coding process, these bits (38 altogether) are transformed into 120 channel-coded bits. ©TTPCom 2004 www.ttpcom.com
  12. 12. HSDPA – An Introduction 12 HS-SCCH 1 frame = 10 ms 1 sub-frame = 2 ms #0 #1 #2 #3 #4 SF = 128 Bits/slot = 40 code set, mod. other information scheme, UE id 1 timeslot = 0.67 ms 2 timeslots = 1.33 ms Figure 4 HS-SCCH frame structure After the UE has received the HS-PDSCH frame and successfully decoded it, it has to send an ACK (or NACK in case of errors) back to Node-B using a HS-DPCCH channel. Figure 5 depicts the structure of HS-DPCCH channel. The UE has 5 ms to spend for this procedure. Depending on the channel configuration, the UE may be required to repeat the ACK/NACK transmission over a number of consecutive HS- DPCCH frames. However, if repeated acknowledgements are used, then the UE will not be scheduled more downlink data in as many consecutive downlink frames after the received data frame. Note also that ACK/NACK channel coding is a very robust one, because the input consists of only one bit (ACK=1, NACK=0), and the channel coder simply multiplies this ten times, so the output is ten bits long. An active HSDPA may also be required to report the channel conditions back to the Node-B (this is in a way an HSDPA specific channel measurement procedure). The network signals whether the channel condition indicator (CQI) should be reported and how often it is repeated. The UE measures the received common pilot channel (CPICH; note that this is not an HSDPA specific channel but a common pilot). The reported value is not a straightforward reception level value, but a CQI value that indicates the maximum amount of data the UE estimates it could receive given the current channel conditions and UE capabilities (for example how many multicodes and what kind of modulation schemes it supports). The network can then use this value as a guideline when it schedules the next block of data. There are 30 different CQI values for each UE category, so a CQI can be addressed using 5 bits. However, CQI values are coded using a robust (20,5) code, so the channel coder output is 20 bits long, and fills completely the two slots allocated for CQI. ©TTPCom 2004 www.ttpcom.com
  13. 13. HSDPA – An Introduction 13 HS-DPCCH 1 frame = 10 ms 1 sub-frame = 2 ms #0 #1 #2 #3 #4 SF = 256 Bits/slot = 10 ACK/NACK CQI 1 timeslot = 0.67 ms 2 timeslots = 1.33 ms Figure 5 HS-DPCCH frame structure The HS-DPCCH channel has a SF=256, and it never exists alone, but it is code multiplexed with an uplink DPCCH. HSDPA channels need dedicated channels to accompany them, both in the uplink and in the downlink. This is because, as a minimum, the downlink-dedicated channel is needed to transfer configuration data for HSDPA channels, and in the uplink, acknowledgement and channel quality information is transmitted. The HSDPA protocol stack (i.e., changes to layer 2 and layer 3) is described in the Overall description of Release 55. For the physical layer a similar HSDPA-specific document does not exist, but the HSDPA upgrades are embedded in other physical layer specifications – Physical channels6, Multiplexing and channel coding7, and Physical layer procedures.8 3.3 What is the future for HSDPA? As most telecommunication systems, HSDPA will be continuously under development. New improvements will be introduced to increase data throughput and to save system resources. The pace of this development work will depend a lot on the success of 3G and especially on the success of HSDPA. If there is no demand for HSDPA, then it is unlikely to be developed much further. The need for HSDPA and its enhancements will depend on the success of the new bandwidth-hungry 3G services. 5 3GPP TS 25.308, v. 5.4.0, High Speed Downlink Packet Access (HSDPA); Overall description (Release 5), March 2003. 6 3GPP TS 25.211, v.5.3.0, Physical channels and mapping of transport channels onto physical channels (FDD) (Release 5), December 2002. 7 3GPP TS 25.212, v. 5.4.0, Multiplexing and channel coding (FDD) (Release 5), March 2003. 8 3GPP TS 25.214, v. 5.4.0, Physical layer procedures (FDD) (Release 5), March 2003. ©TTPCom 2004 www.ttpcom.com
  14. 14. HSDPA – An Introduction 14 However, some improvements are already under work, even before the first HSDPA systems are launched. Multiple-Input Multiple-Output (MIMO) antenna systems have long been seen as a potential enhancement for HSDPA systems, although MIMO is by no means an HSDPA-specific scheme, and will be used in many other systems. MIMO introduces a new way of handling the radio interface channel resources. Previously transmission channels were thought to be shared and allocated amongst users by means of frequency and time (TDMA systems such as GSM), or by means of frequency, time, and code (CDMA systems such as UMTS). However, MIMO introduces a new spatial dimension. It has been shown that it is possible to separate two transmissions in the receiver even if they have been sent using the same frequency, time, and code, if their spatial signatures are sufficiently different. MIMO systems can achieve this by using several transmit and receive antennas. In optimal conditions, these can form several parallel transmission channels, which can still employ the same frequency, time and code space, thus increasing the system capacity considerably. A MIMO system with m transmit and n receive antennas can have up to c = min (m, n) independent sub-channels. The main problem in MIMO systems is the antenna cross-correlation, which can cancel the capacity gain because of the increased interference. In addition, after a feasibility study, 3GPP has decided to continue the standardisation work of three other HSDPA improvements: • CQI enhancement for FDD mode • ACK/NACK transmit power reduction for HS-DPCCH with preamble and postamble • Fractional dedicated physical channel CQI enhancement means a more efficient channel quality reporting towards the network. In Release 5, the reporting rate is fixed. This is easy to implement, but the problem is that a fixed rate is not suitable for all occasions. The network needs to know as up-to-date channel quality as possible because this information is used for data scheduling and coding decisions. The more frequent the indications, the better the channel estimate, but on the other hand, these indications themselves increase the uplink interference. In addition, during periods of inactivity in the downlink no indications are required. ACK/NACK transmit power reduction for HS-DPCCH with preamble and postamble is a logically complex scheme. HS-DPCCH channel is used for transmitting ACK/NACK bursts back to Node-B. The coding of these bursts is quite robust and probability of an erroneous decoding decision is small. However, one particular scenario seems to cause problems. If the UE misses a data burst, and thus does not send anything to Node-B, AND Node-B erroneously decodes this missed burst (=DTX) as ACK, then the network assumes that the transmission was successful even though it was not. The situation will only be corrected once the higher-layer retransmission scheme (in RLC) notices the error. The proposed enhancement is to send special Preamble and Postamble bursts in the uplink HS-DPCCH before and after an ACK/NACK burst. This method reduces the probability of a DTX->ACK error in the Node-B, because now the Node-B has to decode at least two successive timeslots erroneously before the scenario described earlier could take place. ©TTPCom 2004 www.ttpcom.com
  15. 15. HSDPA – An Introduction 15 The rationale for fractional dedicated physical channel enhancement is that the associated dedicated channels for HSDPA channels may be quite unused in typical usage scenarios. If HSDPA is used for bulk data transfer, and there is no conversational component in the session, then the dedicated channel will most probably only transfer power control bits and pilots, and occasional RRC control messages related to HSDPA channels. Allocating a full DPCH to relay these few bits is a bit of overkill, but so far, there has not been an alternative for this. Fractional dedicated physical channel is proposed to fix this problem. This scheme proposes to time-multiplex several DPCH channels into one code channel. The new DPCH sub channels would only carry power control and pilot bits. Possible RRC control signalling would be relayed over the air interface using HS-DSCH. If the existing numbers of power control and pilot bits are used in the proposed enhancement, then it is possible to multiplex three F-DPCH channels into one DPCH. However, this does not come free, as the timing of the power control and pilot bits in the combined DPCH channel would be different from the normal Release 5 DPCH. This would cause many changes to several specifications, and the gain would only be a few spreading codes of SF=256. One has to remember that all HS-DSCH channels would have SF=16, thus the gain in comparison to the main channel is minimal. It is also possible that new frequency bands will be assigned for 3GPP, and some of those frequency carriers could be used exclusively for HSDPA channels. ©TTPCom 2004 www.ttpcom.com
  16. 16. HSDPA – An Introduction 16 4 GLOSSARY 16QAM 16 Quadrature Amplitude Modulation 3GPP 3rd Generation Partnership Project 64QAM 64 Quadrature Amplitude Modulation AMC Adaptive Modulation and Coding CDMA Code Division Multiple Access CPICH Common Pilot Channel CQI Channel Condition Indicator DCH Dedicated Channel DPCCH Dedicated Physical Control Channel DPCH Dedicated Physical Channel DTX Discontinuous Transmission EDGE Enhanced Data rates for GSM Evolution FDD Frequency Division Duplex F-DPCH Fractional Dedicated Physical Channel GSM Global System for Mobile communications GPRS General Packet Radio System HARQ Hybrid Automatic Repeat Request HSDPA High Speed Downlink Packet Access HS-DPCCH High Speed Dedicated Physical Control Channel HS-PDSCH High Speed Physical Downlink Shared Channel HS-SCCH High Speed Shared Control Channel ITU International Telecommunication Union MAC-hs Medium Access Control – high speed MIMO Multiple Input Multiple Output PDSCH Physical Downlink Shared Channel QPSK Quadrature (Quaternary) Phase Shift Keying RLC Radio Link Control RRC Radio Resource Control SF Spreading Factor TDMA Time Division Multiple Access TTI Transmission Time Interval UE User Equipment UMTS Universal Mobile Telecommunications System UTRAN Universal Terrestrial Radio Access Network ©TTPCom 2004 www.ttpcom.com