Universal Mobile Telecommunication System (UMTS)
HSDPA

The HSDPA is newly introduced in UMTS Release 5. It adopts new techniques in order to significantly enhance data bit rates in downlink direction (data rate in uplink direction remains the same). As a result, the highest theoretical data bit rates per cell is increased from 2 Mbps up to 14.4 Mbps. Changes in networks are done especially in UTRAN and the key idea is to move several radio resource management procedures to NodeB instead of RNC as in prior UMTS releases (i.e., Release 99 and Release 4). The advantage of this step is that NodeB is much “closer” to the UE. Thus it can much more effectively react to varying radio channel quality. The procedures that have been moved to the NodeB are depicted in Figure 17.

The HSDPA enhancements to NodeB – Comparison with Release 99 and Release 4

## Fast data scheduling

The important aspect regarding HSDPA is that most of the resources are shared between individual active users. Note that the rest of radio resources are dedicated for users with active voice or media stream (more will be described in chapter 19). The purpose of data scheduling is to assign current NodeB’s radio resources to its users. The allocation period, also called as Time Transmission Interval (TTI), is set only to 2 ms when compared to UMTS Release 99 or Release 4 where the minimal TTI of 10 ms is used. This is the main reason why we are talking about “Fast” data scheduling. The scheduling interval could be shortened since the NodeB directly communicates with attached UE and receives up to date information regarding their requirements and channel quality. Several strategies can be adopted by NodeB to allocate effectively radio resource in downlink as illustrated in Figure 18.

Scheduling methods in HSDPA

The most popular and the simplest scheduling algorithm is Round Robin (RR) where HSDPA users are scheduled with equal probability independently on radio channel conditions. The main disadvantage of this approach is that radio resources are not efficiently utilized (users with poor channel conditions are not able to transmit at high bit rates). The second scheduling method, Maximum Carrier-to-Interference ratio (Max-C/I), assigns all radio resources to users with highest channel quality and thus to maximize HSDPA cell throughput. Nevertheless this approach is not convenient as well, since users close to the NodeB are preferred to users with poor channel quality. The reasonable trade-off between both scheduling mechanisms mentioned above is to implement Proportional Fair (PF) method. The probability that the UE can receive data depends both on channel quality and amount of received data in the past. Consequently, if the UE is inactive for a long time, its priority is increasing.

## Fast data retransmissions

If the UE is not able to decode received data packet correctly, it requests immediately its retransmission from NodeB. In prior UMTS releases, the request for retransmission is sent to RNC, which is responsible for this operation. In HSDPA, the retransmission procedure is handled solely by NodeB (that is why we are referring to a procedure as “fast” data retransmissions). The basic principle is depicted in the following figure.

Fast data retransmission procedure in HSDPA

The additional novelty in HSDPA is the principle of a retransmission mechanism. While in original UMTS standard, simple Automatic Repeat request (ARQ) is assumed, HSDPA introduces its modified version, known as Hybrid ARQ (HARQ). The HARQ is able to temporarily store corrupted data in a buffer that could be additionally combined with newly received data packet to increase the probability of successful decoding. This way, the amount of retransmissions is effectively minimized and thus expensive radio resources are saved for other transmissions.

## Fast link adaptation

The link adaptation means continuous parameters adjustment of transmission link depending on current quality of radio channel. The fast link adaptation is again enabled by moving this functionality to NodeB. In UMTS Release 99 and UMTS Release 4, link adaptation is mainly achieved by fast power control. On the other hand, HSDPA introduces new procedure known as Adaptive Modulation and Coding (AMC) that dynamically selects appropriate Modulation and Coding Scheme (MCS) used in downlink transmission. Consequently, if channel quality is poor, more robust MCS scheme is utilized resulting in lower data bit rates but guaranteeing low Packet Error Rate (PER). As soon as the radio channel characteristics are improved sufficiently, that is, higher Carrier to Interference ration (C/I), NodeB selects more efficient MCS in order to make data transmission more efficient (see Figure 20). Note that other important difference when compared to previous UMTS releases is that HSDPA supports also 16 QAM modulation scheme in addition to QPSK.

Fast link adaptation in HSDPA

## Evolution of HSDPA

The further enhancements to support higher data transmission in downlink are introduced in Release 7 where MIMO with 2x2 configuration is supported for the first time (more detailed explanation of MIMO principle will be described later in Section 23). This way, the former theoretical downlink transmission rate per cell 14 Mbps can be doubled to 28 Mbps. In addition, downlink data bit rates are also increased up to 42 Mbps by introduction of 64 QAM modulation in Release 7. More than that, the downlink throughput can be enhanced by DC method. The principle of DC is to aggregate more carrier (in most cases two carrier with resulting bandwidth equal to 10 MHz). As a result, theoretical data bit rate in downlink is up to 84 Mbps for Release 8.

The data bit rates mentioned above are only theoretical and the user will experience only a small fraction. This is due to several reasons. The first important aspect necessary to be taken into account is that the maximal theoretical data bit rates cannot be achieved in real system. The available capacity per one cell is strongly dependent on aspects, such as quality of radio channel (utilized MCS), maximum allowed transmitting power or signalling overhead required for management of control procedures. In addition, real capacity per one cell is further divided between individual active UEs. Other important restriction is related to UE capabilities. The UEs do not need to support all available bit rates. As a result, current networks are able to support only bit rates up to several Mbps per one user.