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  1. 1. HSDPA – An IntroductionByJuha KorhonenA TTPCom White Paper TTPCom Headquarters, Melbourn, CambridgeWorld leading independent supplier of software and silicon IP for digital wireless terminals For more information – www.ttpcom.com
  2. 2. HSDPA – An Introduction 2Contents1 INTRODUCTION 32 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? 63 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? 134 GLOSSARY 16©TTPCom 2004 www.ttpcom.com
  3. 3. HSDPA – An Introduction 31 INTRODUCTIONThe field of telecommunications is full of peculiar abbreviations, and HSDPA is yetanother new entrant. HSDPA stands for High Speed Downlink Packet Access. It is anew improved downlink packet data transfer scheme for 3GPP1 systems.Modern telecommunication networks are constantly under development, and newfeatures are introduced regularly. However, HSDPA is not just a minor change to3GPP system specifications, but a major upgrade that brings clear capacityimprovements andmuch higher data speeds than the existing 3GPP systems. Thiswhite paper explains why this enhancement is needed, how it is achieved, and whatkind 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 anoverview, discussion on the impact HSDPA has on services and applications and theprobable timetable of HSDPA deployment.The second part is a technical discussion of HSDPA, including a presentation ofHSDPA channel structures and procedures, followed by a detailed example ofHSDPA data transmission procedure. In addition, the future development of HSDPAis discussed.1 3G Partnership Project Organisation that deals with most of the 3G specifications. The 3Gtechnical specifications can be found at www.3gpp.org©TTPCom 2004 www.ttpcom.com
  4. 4. HSDPA – An Introduction 42 HSDPA – THE PRIMER2.1 Why is there a need for HSDPA?Unlike two-way voice communications that are essentially "symmetric" in their use ofradio, 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). UMTSForum report “3G Offered Traffic Characteristics”2 concludes that data traffic ondownlink will exceed uplink traffic by a factor of 2:3. HSDPA offers a way to increasedownlink capacity within the existing spectrum. There are no reliable studiesavailable from open sources, but the estimates generally state that HSDPA increasesthe 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 (thebasic GSM) and 2.5 G (GPRS and EDGE) systems. The maximum data rate per user inRelease 99 systems is typically 384kbit/s, whereas in 2.5 G systems it is a few tens ofkbit/s, or just over 100 kbit/s at best.Current 3G technology can accommodate only a few maximum data rate users at atime before the cell capacity runs out in the downlink direction. A typical userconsumes more downlink than uplink resources. Some applications, such as webbrowsing and many games, use uplink only for control signalling, whereas thedownlink carries lots of payload data for those applications. Therefore, it is clear thata Release 99 system will first run out of capacity in the downlink.HSDPA aims to improve downlink capacity, and thus remove this potentialbottleneck from the system. It increases both the system capacity as a whole, and thedata rate that can be allocated for one user. The maximum theoretical data rate forone user is 14.4 Mbit/s, but in real systems, this is likely to be limited to around 2Mbit/s at first.SYSTEM GSM GPRS EDGE 3G (R99) HSDPATypical max. data 9.6 50 130 384 2048rate (kbit/s) (or more)Theoretical max. 14.4 170 384 2048 14400data 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 partof it. The HSDPA scheme adds an additional wideband downlink shared channelthat is optimised for very high-speed data transfer. ThusS, HSDPA improves only thedownlink throughput although a corresponding uplink enhancement is beingspecified for later releases. After this upgrade, 3GPP systems are expected to becapable 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 52.2 What does HSDPA do?HSDPA is most suitable for background and streaming class services. HSDPA is adownlink enhancement scheme, and thus it improves downlink throughput only.Both background and streaming class services typically generate much moredownlink than uplink traffic. Background class services do not have very strict end-to-end delay requirements, and thus this enables the network to employ moreefficient (throughput-wise) data scheduling algorithms. For example, thetransmission resources can be allocated for UEs with the best radio channelconditions, which increases the throughput. Streaming class services are also typicallyasymmetric; there is more data in the downlink than in the uplink. Streaming classapplications can also withstand transmission delays and delay variations quite well iflarge enough reception buffers are used.However, HSDPA can also be used for other data applications, even forconversational ones, as the network can employ data schedulers that give higherpriority to real-time applications. In fact, in some cases HSDPA could be moreefficient than a dedicated channel for real-time data, as HSDPA employs a shorterframe length, and thus it can react faster to problems in the radio channel.The selection of a suitable packet data scheduler is an important networkperformance issue for operators. Moreover, it is not only a performance issue, as it isalso possible to give higher priority to data packets going to prime users. Thus,HSDPA is not only a pure technical method of enhancing radio networkperformance, but it can also be used as a marketing tool.HSDPA increases the typical user data rates up to 2Mbps. The basic HSDPA isunlikely to provide higher throughput, as higher data rates would consume too largea 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 tothe 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 byemploying new frequency channels that would be allocated exclusively for HSDPA.However, in addition to increased capacity, HSDPA also provides another importantenhancement; that is shorter delays. User perception of a fast connection is not onlydependent on the bandwidth available, but also on the feedback delay of thechannel. If the application reacts quickly on the commands given by the user, it givesthe impression of a high-bandwidth application. HSDPA provides a short feedbackdelay. Short feedback delay also enables new applications, such as interactivenetworked games.Note that HSDPA is not the only method that can be used to increase downlinkcapacity. Nevertheless, it is probably one of the easiest ways to do it, and it savesspectrum, which itself is a scarce resource.©TTPCom 2004 www.ttpcom.com
  6. 6. HSDPA – An Introduction 6It is also clear that HSDPA is not suitable for applications with very low bandwidthrequirements, such as voice. HSDPA channels employ spreading factor3 16, and usingthat kind of high-capacity channel for voice is clearly a waste of resources.As a summary, HSDPA is best for applications with highly variable bandwidthrequirements, 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 capacityenhancement for 3G networks, network operator interest is proportional to thenumber of subscribers planned on the network and the deployment of asymmetrichigh 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 arehighly advanced when compared to European offerings. Increased data traffic intheir networks demands increased capacity, and the easiest way to deliver this is byimplementing HSDPA. These operators are also most aggressive in deliveringinteractive and high-bandwidth applications. In both countries, the competitionamong 3G operators is fierce, and HSDPA enables new kind of applications that canattract new customers.The launch schedule estimates vary, but it seems quite certain that the first HSDPAlaunches will take place during 2005 in East Asia. The European schedule will becountry specific and depend on the success of new 3G data services. However, some3G operators may use HSDPA as a competitive advantage in Europe, and adopt itquite early. In countries where GSM-EDGE networks are deployed, 2.5G operatorscan provide quite similar services to early 3G networks, but they cannot competewith HSDPA upgraded networks.One factor that may speed deployment is that HSDPA is often only a softwareupgrade for the network, and thus easy and quick to undertake. The real bottleneckfor the HSDPA service growth will be the availability of HSDPA capable handsets, asnew 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 forspreading one data symbol. The higher the spreading factor, the lower the data rate as in eachtimeslot a fixed number of chips are transmitted (2560 chips per timeslot in the FDD mode). In3GPP (FDD mode), downlink spreading factors vary between 4 and 512.©TTPCom 2004 www.ttpcom.com
  7. 7. HSDPA – An Introduction 73 HSDPA TECHNICAL DETAILS3.1 What is HSDPA?HSDPA is a scheme that supports a very high capacity shared data channel in thedownlink direction. It encompasses three new channel types: two control channelsand one data channel. In addition, it employs a new frame structure, a new fastretransmission scheme, and a new adaptive modulation scheme.The shared HSDPA data channel (HS-PDSCH) is a more capable upgrade of theRelease 99 - specified Downlink Shared Channel (DSCH). A UE only has to supportone of them, but the network has to support both, as not all users will have HSDPAcapable phones. HS-PDSCH is a shared channel; it is shared between all activeHSDPA users in the cell. This channel is shared in two dimensions: it is both time andcode 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 resourcescan 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 simultaneouslybecause 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 16Figure 1 HS-PDSCH channel time and code multiplexingIn addition to HS-PDSCH, an HSDPA UE will also need new control channels tosupport this function. The High Speed Shared Control Channel (HS-SCCH) is adownlink control channel that gives the UE the fast changing parameters that areneeded for HS-PDSCH reception, and indicates when there is data on the HS-PDSCHthat is addressed to this UE. In the uplink direction, there is the High SpeedDedicated Physical Control Channel (HS-DPCCH) that is a low bandwidth channelfor 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 channelsFigure 2 HSDPA upgradeHSDPA is a combination of several techniques which all contribute to the enhancedcapabilities of the downlink channel. A Release 5 HSDPA capable handset willinclude both Hybrid ARQ (HARQ) and Adaptive Modulation and Coding (AMC)functionality. Release 6 will further enhance HSDPA capabilities by introducingMultiple Input Multiple Output (MIMO) antenna techniques.HARQ is a link adaptation scheme in which link layer acknowledgements are usedfor retransmission decisions in the UTRAN. In Release 99 retransmission functionalityis part of the RLC layer. However, this kind of high-level retransmission scheme istoo slow for the high-speed data transmissions envisaged for HSDPA. With HSDPAthe HARQ retransmission buffers are located closer to the physical layer, specificallywithin the new MAC-hs logical entity that is just above the physical layer. The ARQcombining is based on incremental redundancy; that is, if a transmission fails, thereceived (corrupted) data is stored to a buffer regardless. Successive retransmissionswill 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, orthe maximum number of retransmissions is reached. Moreover, to make HARQ moreefficient, a shorter frame, or Transmission Time Interval (TTI), length is needed. Thenew 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 caninform the network every 2 ms if the transmission failed. In the old scheme, 10 mswould have to pass before a failure could be reported. Shorter frames also mean thatthe system can respond more quickly to changing channel conditions, and re-assigncapacity amongst users.Adaptive Modulation and Coding (or Link Adaptation) means that the sharedchannel transport format (i.e., the modulation scheme and the code rate) depends onthe channel quality. This is monitored constantly, and the transport format used canbe dynamically changed in every frame. The quality information is transmitted to theNode-Bs via the uplink control channels. That is, if the radio channel condition isgood, the network can use higher-order modulation and less redundancy, whereas inpoor conditions, a more robust modulation scheme can be employed and the datapackets may have more redundancy in them. Release 5 employs two modulationschemes, namely QPSK and 16 Quadrature Amplitude Modulation (16QAM). Laterreleases may introduce other schemes, such as 64QAM. Note that older non-HSDPAreleases 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 classCorresponding FDD HS-DSCH Category 1 Category 5 Category 7 Category 9category 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 28800Table 3 FDD HSDPA Physical Layer CategoriesThe difference between these categories lies mainly in the number of multicodessupported, and the length of the inter-TTI gap during HS-PDSCH reception (i.e., theability of the UE to receive HSDPA data in successive TTIs, or sub-frames).HSDPA related categories and other capability information is defined in the 3GPPstandard4.3.2 How does the Air Interface work?In this section, we will discuss how HSDPA scheme works in the air interface. Itinvolves the interworking of three physical channels: HS-PDSCH, HS-SCCH, andHS-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 bemonitored simultaneously as the Node-B can use any of these (but only one at atime). Note that there can be many more HS-SCCH channels in a cell, but onlymaximum of four can be allocated to one UE. Of course, each of these channels has adifferent spreading code, so they can be received simultaneously. However, if the UEhas already received a HS-SCCH addressed to it in one frame, it is sufficient for it tomonitor only this HS-SCCH during the next HS-SCCH sub frame. This arrangementis because now the UE is already receiving at least one HS-PDSCH channel, and itwould 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 allocatedto 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 11The timing of these channels is shown in Figure 3. Note that the HS-SCCH frame andthe corresponding HS-PDSCH frame are overlapping by one timeslot. This may seemstrange at first, because the UE has to first receive and decode the HS-SCCH framebefore it knows whether the corresponding HS-PDSCH is addressed to it. It allbecomes clearer when we discover that the UE identity can be resolved afterreception of the first HS-SCCH timeslot. The data in the said timeslot is masked usinga bit string that is derived from the UE identity. Only the correct UE identity candecode this timeslot. The first timeslot also indicates the spreading code(s) used, andthe modulation scheme employed, so that the UE can start receiving the HS-PDSCHif necessary. There is a gap of one timeslot (0.667 ms) between the end of the firsttimeslot 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 channelsThe HS-SCCH frame structure is depicted in Figure 4. A HS-SCCH sub-framecontains 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 into120 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 msFigure 4 HS-SCCH frame structureAfter the UE has received the HS-PDSCH frame and successfully decoded it, it has tosend an ACK (or NACK in case of errors) back to Node-B using a HS-DPCCHchannel. Figure 5 depicts the structure of HS-DPCCH channel. The UE has 5 ms tospend for this procedure. Depending on the channel configuration, the UE may berequired to repeat the ACK/NACK transmission over a number of consecutive HS-DPCCH frames. However, if repeated acknowledgements are used, then the UE willnot be scheduled more downlink data in as many consecutive downlink frames afterthe received data frame. Note also that ACK/NACK channel coding is a very robustone, because the input consists of only one bit (ACK=1, NACK=0), and the channelcoder 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 theNode-B (this is in a way an HSDPA specific channel measurement procedure). Thenetwork signals whether the channel condition indicator (CQI) should be reportedand 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). Thereported value is not a straightforward reception level value, but a CQI value thatindicates the maximum amount of data the UE estimates it could receive given thecurrent channel conditions and UE capabilities (for example how many multicodesand what kind of modulation schemes it supports). The network can then use thisvalue as a guideline when it schedules the next block of data. There are 30 differentCQI 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 bitslong, 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 msFigure 5 HS-DPCCH frame structureThe HS-DPCCH channel has a SF=256, and it never exists alone, but it is codemultiplexed with an uplink DPCCH. HSDPA channels need dedicated channels toaccompany them, both in the uplink and in the downlink. This is because, as aminimum, the downlink-dedicated channel is needed to transfer configuration datafor HSDPA channels, and in the uplink, acknowledgement and channel qualityinformation is transmitted.The HSDPA protocol stack (i.e., changes to layer 2 and layer 3) is described in theOverall description of Release 55. For the physical layer a similar HSDPA-specificdocument does not exist, but the HSDPA upgrades are embedded in other physicallayer specifications – Physical channels6, Multiplexing and channel coding7, andPhysical layer procedures.83.3 What is the future for HSDPA?As most telecommunication systems, HSDPA will be continuously underdevelopment. New improvements will be introduced to increase data throughputand to save system resources. The pace of this development work will depend a loton the success of 3G and especially on the success of HSDPA. If there is no demandfor HSDPA, then it is unlikely to be developed much further. The need for HSDPAand its enhancements will depend on the success of the new bandwidth-hungry 3Gservices.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 physicalchannels (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 14However, some improvements are already under work, even before the first HSDPAsystems are launched. Multiple-Input Multiple-Output (MIMO) antenna systemshave long been seen as a potential enhancement for HSDPA systems, althoughMIMO is by no means an HSDPA-specific scheme, and will be used in many othersystems. MIMO introduces a new way of handling the radio interface channelresources. Previously transmission channels were thought to be shared and allocatedamongst users by means of frequency and time (TDMA systems such as GSM), or bymeans 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 toseparate two transmissions in the receiver even if they have been sent using the samefrequency, time, and code, if their spatial signatures are sufficiently different. MIMOsystems can achieve this by using several transmit and receive antennas. In optimalconditions, these can form several parallel transmission channels, which can stillemploy the same frequency, time and code space, thus increasing the system capacityconsiderably. A MIMO system with m transmit and n receive antennas can have upto c = min (m, n) independent sub-channels. The main problem in MIMO systems isthe antenna cross-correlation, which can cancel the capacity gain because of theincreased interference.In addition, after a feasibility study, 3GPP has decided to continue thestandardisation 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 channelCQI enhancement means a more efficient channel quality reporting towards thenetwork. In Release 5, the reporting rate is fixed. This is easy to implement, but theproblem is that a fixed rate is not suitable for all occasions. The network needs toknow as up-to-date channel quality as possible because this information is used fordata scheduling and coding decisions. The more frequent the indications, the betterthe channel estimate, but on the other hand, these indications themselves increasethe uplink interference. In addition, during periods of inactivity in the downlink noindications are required.ACK/NACK transmit power reduction for HS-DPCCH with preamble and postambleis a logically complex scheme. HS-DPCCH channel is used for transmittingACK/NACK bursts back to Node-B. The coding of these bursts is quite robust andprobability of an erroneous decoding decision is small. However, one particularscenario seems to cause problems. If the UE misses a data burst, and thus does notsend anything to Node-B, AND Node-B erroneously decodes this missed burst(=DTX) as ACK, then the network assumes that the transmission was successful eventhough it was not. The situation will only be corrected once the higher-layerretransmission scheme (in RLC) notices the error. The proposed enhancement is tosend special Preamble and Postamble bursts in the uplink HS-DPCCH before andafter an ACK/NACK burst. This method reduces the probability of a DTX->ACK errorin the Node-B, because now the Node-B has to decode at least two successivetimeslots erroneously before the scenario described earlier could take place.©TTPCom 2004 www.ttpcom.com
  15. 15. HSDPA – An Introduction 15The rationale for fractional dedicated physical channel enhancement is that theassociated dedicated channels for HSDPA channels may be quite unused in typicalusage scenarios. If HSDPA is used for bulk data transfer, and there is noconversational component in the session, then the dedicated channel will mostprobably only transfer power control bits and pilots, and occasional RRC controlmessages related to HSDPA channels. Allocating a full DPCH to relay these few bits isa 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 schemeproposes to time-multiplex several DPCH channels into one code channel. The newDPCH sub channels would only carry power control and pilot bits. Possible RRCcontrol signalling would be relayed over the air interface using HS-DSCH. If theexisting numbers of power control and pilot bits are used in the proposedenhancement, 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 inthe 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 bea few spreading codes of SF=256. One has to remember that all HS-DSCH channelswould 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 ofthose frequency carriers could be used exclusively for HSDPA channels.©TTPCom 2004 www.ttpcom.com
  16. 16. HSDPA – An Introduction 164 GLOSSARY16QAM 16 Quadrature Amplitude Modulation3GPP 3rd Generation Partnership Project64QAM 64 Quadrature Amplitude ModulationAMC Adaptive Modulation and CodingCDMA Code Division Multiple AccessCPICH Common Pilot ChannelCQI Channel Condition IndicatorDCH Dedicated ChannelDPCCH Dedicated Physical Control ChannelDPCH Dedicated Physical ChannelDTX Discontinuous TransmissionEDGE Enhanced Data rates for GSM EvolutionFDD Frequency Division DuplexF-DPCH Fractional Dedicated Physical ChannelGSM Global System for Mobile communicationsGPRS General Packet Radio SystemHARQ Hybrid Automatic Repeat RequestHSDPA High Speed Downlink Packet AccessHS-DPCCH High Speed Dedicated Physical Control ChannelHS-PDSCH High Speed Physical Downlink Shared ChannelHS-SCCH High Speed Shared Control ChannelITU International Telecommunication UnionMAC-hs Medium Access Control – high speedMIMO Multiple Input Multiple OutputPDSCH Physical Downlink Shared ChannelQPSK Quadrature (Quaternary) Phase Shift KeyingRLC Radio Link ControlRRC Radio Resource ControlSF Spreading FactorTDMA Time Division Multiple AccessTTI Transmission Time IntervalUE User EquipmentUMTS Universal Mobile Telecommunications SystemUTRAN Universal Terrestrial Radio Access Network©TTPCom 2004 www.ttpcom.com