News, views and contacts from the global telecommunications industry

HSDPA (High Speed Downlink Packet Access)




Key Data


High Speed Downlink Packet Access (HSDPA) is a new mobile telecommunications protocol, which has also been called 3.5G. The standard is a packet-based data service in W-CDMA downlink with data transmission up to 8Mbps to 10Mbps (and 20Mbps for MIMO systems) over a 5MHz bandwidth in WCDMA 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 IP support enabling provision of services through an all-IP core network; Release 5 specifications focus on HSDPA 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 data transmission rates up to 20Mbps.

HSDPA is evolved from, and backward compatible with, Release 99 WCDMA systems. HSDPA marks a similar boost for WCDMA that EDGE does for GSM. It provides a two-fold increase in air interface capacity and a five-fold increase in data speeds in the downlink direction. HSDPA also shortens the round-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 and processing of retransmissions (in case of transmission errors) into the base station (closer to the air interface). The system utilises a short frame length to further accelerate packet scheduling for transmission and employs incremental redundancy for minimizing the air-interface load caused by retransmissions.

The standard also incorporates a new transport channel type, known as High Speed Downlink Shared Channel (HS-DSCH) to facilitate air interface channel sharing between several users. The system can also adapt the modulation scheme and coding according to the quality of the radio link.

ADAPTIVE MODULATION AND CODING (AMC) AND MULTICODE TRANSMISSION

Traditionally, WCDMA has used fast power control for link adaptation, but HSDPA holds the transmission power 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 determines the transmission data rate based on CQI reports as well as power measurements on the associated channels. The data rate is adjusted by modifying the modulation scheme, the effective code rate as well as the number of HS-PDSCH codes.

In a system with AMC, users close to the Node-B are typically assigned higher order modulation with higher code rates (e.g. 16 QAM with a 3/4 code rate), and the modulation-order and/or code rate generally 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) after first transmission is in the order of 10% to 20%. With the HSDPA concept, an H-ARQ mechanism has been 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. In order to utilise the time when the Node-B awaits acknowledgements, N parallel SAW-ARQ processes may be set for the user equipment, so different processes transmit in separate TTIs. The value of N may maximally be 8, but in practice N will be around 4 to 6. The minimum delay between the original and the first retransmission is 12ms for HSDPA.

The control of the L1 H-ARQ is located in the Node-B, so that storage of unacknowledged data packets and the following scheduling of retransmissions do not involve the RNC. Hence, the Iub delay is avoided and the resulting retransmission delay is significantly lower than the delay caused by conventional RLC retransmissions. The HSDPA concept supports both the Incremental Redundancy (IR) and Chase Combing (CC) retransmission strategies.

PACKET SCHEDULING

The packet scheduling for HSDPA is located in the medium access layer, MAC-hs. The MAC-hs is located in the Node-B, which means that the packet scheduling decisions are almost instantaneously executed. 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 scheduling rate combined with the large AMC dynamic range available with the HSDPA concept also facilitates advanced scheduling methods where channel allocation is conducted according to the current radio conditions.

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 to track 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 capacity easily exceeds 50%.

HSDPA PERFORMANCE

The HSDPA concept facilitates peak data rates exceeding 2Mbps (theoretically up to and exceeding 10Mbps) and the cell throughput gain over previous UTRA-FDD releases has been evaluated to be in the order of 50% to 100% or even more, highly dependent on factors such as the radio environment and the service 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 cost per 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 will begin to deploy UMTS with expansion to HSDPA in 2005. Cingular faces competitive pressure from operators such as Verizon who use a competing 3G technology, CDMA-2000 1x-EvDO, and who have already 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 from 2006. 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 broadband standard). However although superficially the two standards will be in competition they are fundametally different. In the early stages, HSDPA will still be about mobility and data and voice from a mobile telecoms 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, but much more mobile. One key difference between the two technologies is the fact that HSDPA won't require any new infrastructure, just new software, but brand new infrastructure is needed for WiMax.

CONCLUSION

The success of this technology as a GSM-replacement, vis-a-vis other contenders like CDMA2000 EvDO and not yet finished cellular datacommunication standards like WiMax (802.16), is still unclear. This is especially true considering that KDDI's CDMA2000 is generally considered as being much more successful and smooth than DoCoMo's and Vodafone's UMTS/WCDMA introduction in Japan, which are slower than initially hoped.