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Power Control in HSPA

December 17, 2007 by itsme

Last week I wrote about UMTS power control with reference to R99 channels. Todays 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 [1].

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 thats 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,

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9.2.1.31lc]. 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, 4.2.1.2]. 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, 5.1.2.5.3].

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, 5.2.1.3].

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, 4.2.1.3]. 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, 4.2.1.3]. This is similar to the weights given for priority classes forHSDPA. 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.

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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.