The Channel Quality Indicator (CQI) is the key indicator for HSDPA down- link channel quality. The CQI value that the terminal reports is a function of: multipath environment, terminal receiver type, ratio of the interference of the own base station compared with others and expected BTS HSDPA power availability. Thus, the value reported does not just correspond to the signal to interference ratio (SIR) that the terminal is experiencing.
When a UE reports a particular CQI, it is reporting that under the cur- rent radio conditions (including the current power settings), the UE is able to receive the reported CQI and lower CQI’s, at single-transmission BLER no greater than 0.1. User equipments report CQI and HARQ-ACK feed- back information to the Node-B through the uplink HS-DPCCH. The clear benefit of the approach is that the solution defined will automatically ac- commodate the various possible receiver implementations and environment variations and, thus, gives an indication of the best data rates needed by the terminal to cope with the environment in question. This removes the need from the network end to have to consider the delay profile characteristics of the cell/sector, [2]. Possible implementatios of the CQI evolution can be found in [20].
The HS-DPCCH carries uplink feedback signalling related to the downlink HS-DSCH transmission. This signalling consists of HARQ-ACK and CQI as shown in Figure 5.1. Each 2-ms subframe, like those of the downlink physi- cal channels, consists of three slots, each with 2560 chips. The HARQ-ACK is carried in the first slot of the HS-DPCCH subframe and the CQI in the second and third slots. A system parameter k controls the CQI transmission frequency, and also control repetition for both slots is controlled by a sep- arate parameter. Repetition over multiple 2-ms periods is needed in some cases, like cell edge operation when the available power would not ensure sufficient quality for feedback reception, [2].
Figure 5.1: High-Speed Dedicated Physical Control Channel that carries the uplink.
Furthermore, we can explain some cases in which the CQI is used for the performance of HSDPA, say:
• Link adaptation: as already said, the CQI is used for the dynamic HS-DSCH link adaptation, in addition to the scheduling decision. The user equipment sends every 2-ms TTI a CQI to the serving HS-DSCH cell on the uplink HS-DPCCH. By means of the CQI, the maximum
TBS that can be received correctly with at least 90% probability is indicated. The range of CQI values is 0-30, and each step corresponds appoximately to a 1-dB step in HS-DSCH SINR, [9]. The HS-DSCH link adaptation algorithm at the Node B adjusts the transmit bit rate on the HS-DSCH every TTI when a user is scheduled for transmission. Ideally, the HS-DSCH transmit bit rate should be adjusted as a function of the per-TTI HS-DSCH signal-to-interference plus noise ratio (SINR) experienced at the user end. Figure 5.2 illustrates the general principle HS-DSCH link adaptation, [2].
Figure 5.2: HS-DSCH link adaptation principle: (1) the UE reports low- quality channel information and the Node B allocates a low bit rate; (2) the UE reports high-quality channel information and the Node B allocates a high bit rate, [2].
As shown in Figure 5.3, several factors contribute to variance of HS- DSCH SINR although the HS-DSCH transmit power is supossed con- stant. Due to the transmission of power controlled DCHs, the total transmit power from the serving HS-DSCH cell is time variant, like the downlink radio channel in case than the user is moving, and the other cell interference experienced at the user end is also time variant. The signaling delay on the uplink is used for the case that the SINR at the user has changed from the time when the CQI was estimated until the actual transmission on the HS-DSCH. In such a case, the use of the recommended transmission format by the CQI report for the
HS-DSCH transmission does not necessarily guarantee that the BLEP is lower than 0.1, [4].
Figure 5.3: Block diagram showing the received signal at the HSDPA user and report of the CQI to the serving HS-DSCH cell, [4].
A simple link adaptation algorithm would directly follow the CQI values reported by the UE, but there may be a need to adjust the UE-reported CQI by adding an offset because the HS-DSCH transmit power from the Node B to the user might be different from the assumed HS-DSCH transmit power by the UE at the time it derived the CQI report; be- sides, the UE assumes a received HS-PDSCH power level PHS−P DSCH
derived from the observed P/S-CPICH power level PCP ICH according
to PHS−P DSCH = PCP ICH + G, where G is a power offset parameter
signalled to the UE via RRC signalling from the RNC. The effect of feedback delays on link adaptation performance has previously been ad- dressed in [21], [22]. In order to further adjust the CQI index received from a user before applying it for adjustment of the HS-DSCH transmis- sion format, these studies indicate a need for an outer loop HS-DSCH link adaptation algorithm, which can be based on ACKs/NACKs from past transmissions. The algorithm adjusts the offset values to ascer- tain the average targetted retransmission probability. If too many re- transmissions occur, an unnecessary delay is added, whereas too few
retransmissions indicate that the transport block sizes used are not large enough, unnecessarily lowering throughput, [2].
• HS-SCCH power control: sufficient power should be allocated to trans- mission of the HS-SCCH to ensure reliable reception quality of the HS- SCCH, which is important since the transport block on the HS-DSCH can only be decoded if the HS-SCCH has been correctly received. Fur- thermore, reduce HS-SCCH transmission power is also desirable, in order to decrease the interference levels in the network. Hence, it is generally recommended to control the HS-SCCH power every TTI. The 3GPP specifications do not explicitly specify any power control mecha- nism for the HS-SCCH. The HS-SCCH transmit power can be adjusted relative to the transmit power of the associated downlink DPCCH, or as a function of the CQI report received from the user. This is possible if there is an internal table at the Node B expressing a power offset be- tween each CQI index and the required HS-SCCH power. In both cases it is possible to implement a pseudo closed loop power control scheme for the HS-SCCH, relying on either feedback information from the user about the reception quality of the associated DPCH or the HS-DSCH (CQI). Common to both approaches is the Node B need for a priori knowledge of a power offset parameter before it can adjust HS-SCCH transmit power as a function of either DPCCH power or CQI.