Last week I wrote about UMTS power control with reference to R99 channels. Today’s post continues the discussion with reference to HSDPA and HSUPA.
Let us begin with HSDPA for which power control is significantly different from R99 channels. HSDPA does not have fast power control as in R99 DL DPCH. However, power control is just as important for HSDPA. Inner loop power control happens implicitly in HSDPA based on Adaptive Modulation and Coding (AMC). This can be understood by acknowledging that WCDMA is an interference limited system. Although reducing power, results in reduced interference this does not have significant impact for users close to the Node-B. Such users do not benefit much by decreasing power nor does system capacity increase. It is known that power control dynamics is only 20 dB in DL and 70 dB in UL . Beyond the 20 dB limit, new techniques such as AMC are more beneficial.
Thus, when users close to the Node-B experience excellent channel conditions, modulation could be changed from QPSK to 16 QAM with an additional increase in coding rate (less redundancy). This increases bandwidth for a given user. Within the cell/sector, fast scheduling (another new feature in HSDPA) enables quick decisions so that one user may be given/denied more resources for relatively short time periods to make the best use of channel conditions at that moment in time. This increases overall system capacity towards what is optimal. Such benefits were not possible with only R99 fast power control.
We can infer that power control is done implicitly because when modulation and coding are changed based on CQI feedback received from the UE on UL HS-DPCCH, Node-B will also adjust its transmission power. Every CQI is associated with a reference power offset that is used by the UE when it calculates the CQI to send to the Node-B [TS 25.214, 6A]. This calculation is also dependent on the UE category.
Of course, open loop power control exists. The Node-B is commanded by the CRNC with specific power offsets. There is a power offset for HS-SCCH that’s relative to DL DPCCH power. For HS-PDSCH, where multiple codes are used, all codes will be transmitted with the same power [TS 25.214, 5.2.11]. The transmission that the Node-B can use for HS-PDSCH is indicated as a percentage of the total power available to the Node-B. This is indicated for every priority class with weights for every UE. Thus, differentiation in terms of priority and UE is partly implemented with these power weights [TS 25.433, 188.8.131.52lc]. It is then left to the Node-B to manage these power profiles efficiently. This is an implementation issue and has been the subject of many patents.
HSDPA has an associated channel in the UL (HS-DPCCH) that carries channel quality feedback and acknowledgments. CQI, ACK and NACK – all have their power offsets which are relative to UL DPCCH [TS 25.213, 184.108.40.206]. These offsets are signalled using NBAP to the Node-B.
Coming to HSUPA, the scenario is a little closer to the power control R99 UL channels. In DL DPCCH, there are always non-zero bits of TPC (Transmit Power Control) which are used for inner loop power control of the UL DPCCH and UL DPDCH. Power control of UL DPDCH is closely tied to UL DPCCH. There is defined offset between these two channels in terms of gain factors. These are either signalled at RRC or computed at Layer 1. Every TFC (Transport Format Combination) has a defined pair of gain factors. When computed, rate matching attribute, number of transport channel bits after radio frame segmentation and number of DPDCH are taken in account [TS 25.214, 220.127.116.11.3].
In HSUPA, the channels in question are E-DPCCH and E-DPDCH. Power control of HSUPA UL channels are tied to DPCCH. In fact, there is a requirement that E-DPCCH can be transmitted only if DPCCH is also transmitted in the same slot [TS 25.211, 18.104.22.168].
Complexity of HSUPA power control comes from the fact that inner loop and open loop controls are intimately tied to each other. For example, E-DPCCH power is signalled by RRC and it is relative to DPCCH power offset [TS 25.213, 22.214.171.124]. This offset does not change often because it is fixed by RRC at SRNC. However, inner loop controls the power of DPCCH which in turn affects the power of E-DPCCH.
RRC also signals the power offsets for each E-DPDCH. These offsets or gain factors are associated with E-TFC. This design gives flexibility to use higher power when data rate is higher. Actual gain factors for each E-TFC is calculated by the UE. Calculations take into account the HARQ power profile which is signalled at RRC [TS 25.213, 126.96.36.199]. This is similar to the weights given for priority classes for HSDPA. The computed gain factors are quantized to reference gain factors signalled at RRC.
The idea of using these reference gain factors is that every MAC-d flow is associated with a certain Q0S. The HARQ power profile allows this QoS to be met. If a MAC-e PDU carries only one MAC-d flow the power profile can be easily met. More commonly, data from different flows may need to be multiplexed in the same PDU to make the best of available bandwidth. For this reason, RRC signals a sets of compatible MAC-d flows that could be multiplexed in the same MAC-e PDU.
Of course, the UE ought to choose an E-TFC according to the amount of data it has to send. This is the case with R99 when UE chooses a TFC. The additional control for E-TFC is that amount of power the UE is allowed to use. This affects the UE MAC in its choice of E-TFC for the next transmission. These grants which are basically expressed in relation to power are sent on E-AGCH and/or E-RGCH channels. Thus, these two DL channels introduced for HSUPA play an important part in HSUPA power control by not just changing the power but also affecting the amount of data the UE is allowed to send. This can be seen as an enhancement or constraint to R99 inner loop power control.
An additional comment is that whatever happens, total transmission power from either the Node-B or the UE is fixed to a certain level. This level should never be exceeded. So if UL DPCCH is transmitted at a certain power, offsets and grants can be applied bearing in mind the upper limit. At the Node-B, there is an optional parameter that specifies the maximum power of HS-PDSCH, HS-SCCH, E-AGCH, E-RGCH and E-HICH taken together.
We now know that HSUPA inner loop power control is partly done using TPC bits of DL DPCCH. What happens if only HSDPA is required in the DL and DL DPDCH is redundant? This problem has been solved with the introduction of a new DL channel called Fractional DPCH (F-DPCH). This is a light-weight channel that functions just like DL DPCCH. The reason for introducing a new channel is to have minimum impact on previous releases of the standard. It was difficult to modify the slot formats of DL DPCH in a backward compatible manner since DL DPCCH and DL DPDCH are time multiplexed.
- Harri Holma and Antti Toskala, WCDMA for UMTS, Second Edition, John Wiley & Sons, 2002.