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High Speed Downlink Packet Access (HSDPA) is a new mobile telecommunications protocol, which hasalso been called 3.5G. The standard is a packet-based data service in W-CDMA downlink with datatransmission up to 8Mbps to 10Mbps (and 20Mbps for MIMO systems) over a 5MHz bandwidth inWCDMA downlink.

HSDPA implementations include Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output(MIMO), Hybrid Automatic Request (HARQ), fast cell search, and advanced receiver design.In 3rd generation partnership project (3GPP) standards, Release 4 specifications provide efficient IPsupport enabling provision of services through an all-IP core network; Release 5 specifications focus onHSDPA to provide data rates up to approximately 10Mbps to support packet-based multimedia services.MIMO systems are the work item in Release 6 specifications, which will support even higher datatransmission rates up to 20Mbps.HSDPA is evolved from, and backward compatible with, Release 99 WCDMA systems. HSDPA marks asimilar boost for WCDMA that EDGE does for GSM. It provides a two-fold increase in air interfacecapacity and a five-fold increase in data speeds in the downlink direction. HSDPA also shortens theround-trip time between the network and terminals and reduces variance in downlink transmission delay.

HSDPA IMPROVEMENTS

The standard combines some key functions, such as scheduling of data packet transmission andprocessing of retransmissions (in case of transmission errors) into the base station (closer to the airinterface). The system utilises a short frame length to further accelerate packet scheduling fortransmission and employs incremental redundancy for minimizing the air-interface load caused byretransmissions.The standard also incorporates a new transport channel type, known as High Speed Downlink SharedChannel (HS-DSCH) to facilitate air interface channel sharing between several users. The system canalso adapt the modulation scheme and coding according to the quality of the radio link.

ADAPTIVE MODULATION AND CODING (AMC) AND MULTICODETRANSMISSION

Traditionally, WCDMA has used fast power control for link adaptation, but HSDPA holds the transmissionpower constant over the TTI and uses Adaptive Modulation and Coding (AMC) as an alternative link

adaptation method to power control in order to improve the spectral efficiency. The Node-B determinesthe transmission data rate based on CQI reports as well as power measurements on the associatedchannels. The data rate is adjusted by modifying the modulation scheme, the effective code rate as wellas the number of HS-PDSCH codes.In a system with AMC, users close to the Node-B are typically assigned higher order modulation withhigher code rates (e.g. 16 QAM with a 3/4 code rate), and the modulation-order and/or code rategenerally decreases as the distance to the Node-B increases.

H-ARQ (HYBRID AUTOMATIC REQUEST)

When operating HSDPA near the highest possible spectral efficiency the Block Error Rate (BLER) afterfirst transmission is in the order of 10% to 20%. With the HSDPA concept, an H-ARQ mechanism hasbeen introduced to reduce the delay and increase the efficiency of retransmitting data.The H-ARQ protocol used for HSDPA is Stop And Wait (SAW). With SAW, the transmitting side persists

on the transmission of the current block until it has successfully been received by the user equipment. Inorder to utilise the time when the Node-B awaits acknowledgements, N parallel SAW-ARQ processesmay be set for the user equipment, so different processes transmit in separate TTIs. The value of N maymaximally be 8, but in practice N will be around 4 to 6. The minimum delay between the original and thefirst retransmission is 12ms for HSDPA.The control of the L1 H-ARQ is located in the Node-B, so that storage of unacknowledged data packetsand the following scheduling of retransmissions do not involve the RNC. Hence, the Iub delay is avoidedand the resulting retransmission delay is significantly lower than the delay caused by conventional RLCretransmissions. The HSDPA concept supports both the Incremental Redundancy (IR) and ChaseCombing (CC) retransmission strategies.

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PACKET SCHEDULING

The packet scheduling for HSDPA is located in the medium access layer, MAC-hs. The MAC-hs islocated in the Node-B, which means that the packet scheduling decisions are almost instantaneouslyexecuted. In addition to this, the TTI length has been shortened to 2ms.A typically considered packet scheduling strategy is the Round-Robin in time scheduler where users are

served in sequential order so they all get the same average allocation time. However, the high schedulingrate combined with the large AMC dynamic range available with the HSDPA concept also facilitatesadvanced scheduling methods where channel allocation is conducted according to the current radioconditions.A popular packet scheduling method is the proportional fair packet scheduler. With this type of scheduler,the serve order is determined by the highest instantaneous relative channel quality, i.e. it attempts totrack the fast fading behaviour of the radio channel. Since the selection is based on relative conditions,every user still gets approximately the same amount of allocation time but the increase in system capacityeasily exceeds 50%.

HSDPA PERFORMANCE

The HSDPA concept facilitates peak data rates exceeding 2Mbps (theoretically up to and exceeding10Mbps) and the cell throughput gain over previous UTRA-FDD releases has been evaluated to be in theorder of 50% to 100% or even more, highly dependent on factors such as the radio environment and theservice provision strategy of the network operator.Practical HSDPA user bit rates, even in large macro cells, can be similar to broadband home DSL lines.As HSDPA enables more bits to be transferred with the same radio frequency, it also enables lower costper bit than Release 99 based WCDMA.

TAKE-UP OF HSDPA

HSDPA is beginning to reach deployment status in North America. Cingular has announced that they willbegin to deploy UMTS with expansion to HSDPA in 2005. Cingular faces competitive pressure fromoperators such as Verizon who use a competing 3G technology, CDMA-2000 1x-EvDO, and who havealready deployed a similar high speed data service.In Japan, KDDI has been commercially deploying 3.5G services based on a nationwide CDMA-2000 1x-EvDO network since December 2003. Docomo announced in 2005 that it will introduce HSDPA from2006. In Germany, T-Mobile will officially introduce its HSDPA service at the CeBIT 2006 exhibition.

There is much talk about the competition between HSDPA and WiMax (a new wireless broadbandstandard). However although superficially the two standards will be in competition they are fundametallydifferent. In the early stages, HSDPA will still be about mobility and data and voice from a mobiletelecoms platform and WiMax will be about wireless broadband data to underserved areas. For WiMax,HSDPA serves as a form of competition that's not as fast. WiMax promises speeds of up to 70Mbps, butmuch more mobile. One key difference between the two technologies is the fact that HSDPA won'trequire any new infrastructure, just new software, but brand new infrastructure is needed for WiMax.