WIRELESS MULTIMEDIA

CONTINUOUS PACKET CONNECTIVITY (CPC), POWER

AMPLIFIER (PA) BACK-OFF AND THE CUBIC METRIC (CM)

Terence E. Dodgson and Mark Horgan

Siemens Roke Manor Research Ltd., Romsey, Hampshire, SO51 0ZN, United Kingdom

Keywords: UMTS, WCDMA, Radio Coverage, Resource Efficiency, Cubic Metric, Continuous Packet Connectivity.

Abstract: With the every-increasing complexity of (e.g. multi media) signal waveforms generated by transmitters of

modern digital communications systems, there comes a corresponding demand for transmitters to continue

to operate efficiently (e.g. from a cost per bit and power output point of view) and to produce minimum

distortion, within specified limits. This paper looks at how a particular CPC scheme (for efficient resource

usage) might be devised and what the resulting impact is on the CM measure. The work uses, as a basis for

simulations, mainly the Uplink of the third generation (3G) Wideband Code Division Multiple Access

(WCDMA) Release six (R6) system.

1 INTRODUCTION

When looking at services which require high user bit

rates, wireless communication systems have tended

to lag behind wired systems due to problems caused

by the less benign radio transmission environment.

To the end user however, service is all important.

The current perception is that users would like to

have the same service offered over a wireless link as

they would be able to obtain over a wired one. This

has led to the design of cellular wireless systems

which are capable of working with relatively high

data rates, in particular over the wireless link.

2 CONTINUOUS PACKET

CONNECTIVITY (CPC)

The increase in data rate that allows for the delivery

of enhanced user services needs to be complimented

by the “efficient” delivery of that service, that is to

say, resources should not be needlessly wasted.

Optimisation of system resource is a necessary task

in order to drive down the cost per bit to the end

user. Multi-media (e.g. internet) service users

require relatively high data rates combined with low

data access delays. Internet sessions tend to be

bursty in nature, with defined periods of activity and

inactivity. Users require that they stay in a CPC

mode during periods of inactivity, i.e. they should

stay continuously connected. To ensure efficient

resource usage, users should be assigned only the

minimum resources which would allow them to

remain connected during these inactive times.

The CPC scheme given in Section 5 is thus

aimed at reducing the power level of the Uplink

(UL) Dedicated Physical Control Channel (DPCCH)

when there is no data to be transmitted on the UL

Enhanced Dedicated Physical Data Channel (E-

DPDCH).

3 CUBIC METRIC

HSDPA and HSUPA/E-DCH involve the

deployment of an increased number of channels and

associated channel configurations. The way these

channels are configured, and their subsequent

processing by the PA, has an impact on PA

distortion. In the past (i.e. with second generation,

2G, systems) the calculation of a signal’s PAPR has

been sufficient to determine what the PA back-off

should be. However, recent literature e.g. (3GPP

TSG RAN WG4#31, 2004), (3GPP TSG RAN

WG4#33, 2004), (3GPP TSG RAN WG4#38, 2006),

(Rohde & Schwarz, see ref) has suggested that the

PAPR measure is not sufficiently accurate when

working with the, increasingly, complex signals that

occur with enhanced 3G systems. Reference (3GPP

101

E. Dodgson T. and Horgan M. (2007).

WIRELESS MULTIMEDIA CONTINUOUS PACKET CONNECTIVITY (CPC), POWER AMPLIFIER (PA) BACK-OFF AND THE CUBIC METRIC (CM).

In Proceedings of the Second International Conference on Wireless Information Networks and Systems, pages 101-104

DOI: 10.5220/0002145801010104

 SciTePress

TSG RAN WG4#31, 2004) indicates that the CM

has the required accuracy for determining what the

PA back-off should be for any given channel format

and combination for current, modern, cellular

systems, such as WCDMA.

4 CM DERIVATION

Modulator output can be represented as a complex

stream of N samples, {v

}, as indicated in Figure 1;

Figure 1: Modulation Output.

From the literature, e.g. (3GPP TS 25.101 v6.12.0,

see ref), the CM is given by;

CM = CEIL([ 20*log10( (v_norm

)

rms

) -

20*log10( (v_norm_ref

)

rms

) ] / k)

(1)

Where,

v_norm is the normalised voltage waveform of the

input signal

- v_norm_ref is the normalised voltage

waveform of the reference signal (12.2kbps

AMR Speech) and 20*log10(

(v_norm_ref3)rms = 1.52dB

- k is 1.85 for signals where all

channelisation codes meet the following

criteria CSF,N where N < SF/2

- k is 1.56 for signals whe

- re any channelisation codes meet the

following criteria CSF,N where N > SF/2

- CEIL{x,0.5} means rounding upwards to

the nearest 0.5dB, i.e. CM ∈

[0,0.5,1.0,1.5,2.0,2.5,3.0,3.5

A more explicit form of the CM can be derived;

CM = CEIL([20*log10(c

rms

) -

20*log10(

ref

rms

)]/ k)

(2)

where,

rms

= N. ( (P

cubed

)/(P

total

) )

(1/2)

(3)

5 CPC CONCEPT

HSUPA/E-DCH defines a new radio interface for

the uplink communication, with the main overall

goal of improving coverage and throughput as well

as reducing the delay of the uplink dedicated

transport channels.

The key technical capabilities introduced with

HSUPA/E-DCH are;

1) A new, dedicated Transport Channel, the uplink

E-DCH which maps onto a group of new

Physical Channels for both signalling and traffic.

2) Introduction of Hybrid Auto Repeat ReQuest (H-

ARQ) - including Chase and Incremental

Redundancy (IR) combining.

3) Fast Node B Scheduling to control the Set of

Transport Formats, within set Radio Network

Controller (RNC) limits, from which each User

Equipment (UE) may choose – enabling

improvement in coverage and capacity in the

uplink.

4) Fast Retransmissions based on a H-ARQ

Protocol for error recovery at the Physical layer.

From item (1), the introduction of a new dedicated

Transport Channel (and the resulting increase in the

number of uplink Physical channels) has the knock-

on effect of increasing the PAPR and puts higher

requirements on accurately specifying the required

back off of the UE Power Amplifier.

During Multimedia (e.g. internet) sessions UE’s

should be assigned minimum resources during

inactive periods. During an inactive period a user

may be, for example, reading recent information

obtained as a result of a request sent to an

appropriate website, before responding – and

thereby moving into an active period of the session.

In particular it may not be necessary for inactive

UEs to continue communications using a relatively

high power level for control signalling when lower

power levels may be sufficient. If control signalling

power is reduced, the level of cell interference is

correspondingly reduced and more users are able to

be catered for in a cell (and in this way the system

becomes more efficient).

One way in which this could be carried out is to

use a concept referred to as SIR_target reduction, i.e.

Layer 1 signalling (3GPP TR 25.903 v0.4.0, see ref),

which is applied by the Node B when inactivity of

some UE on the E-DCH is detected. This detection

might in fact be done looking at the corresponding

Medium Access Control (MAC) packet activity. The

Layer 1 signalling amounts to the Node B reducing

the Uplink (UL) SIR target to a predetermined value

(the Radio Network Controller, RNC, is not

involved in the immediate process and thus

relatively long delays associated with Radio

Resource Control, RRC, are avoided and the user

stays in the cell dedicated channel, CELL_DCH,

state). The predetermined value is set to a level

which ensures reliable transmit power control (TPC)

detection. WCDMA inner loop power control then

converges to this, reduced target, value, thus

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

102

reducing the transmit power of the UL Dedicated

Physical Control Channel (DPCCH) and thereby

also reducing overall cell noise/interference.

As an example, the above, SIR_target reduction

scheme might use the following UL channels during

its application;

1) Dedicated Physical Control Channel (DPCCH)

which consists of;

a. Two Transmit Power Control, TPC, bits, and,

b. Eight Pilot Bits.

2) High Speed Dedicated Physical Control Channel

(HS-DPCCH) which consists of;

a. Channel Quality Indication (CQI) bits

b. Acknowledgement/Negative

Acknowledgement (ACK/NACK) bits

Having reduced the power usage (and hence

interference) during periods of inactivity, a method

is then required to ensure that signals (on the HS-

DPCCH) are transmitted at their correct levels when

UEs are then active (e.g. when new data arrives in

the UL MAC-enhanced dedicated channel, MAC-e,

buffer). This is achieved through β boosting (3GPP

TR 25.903 v0.4.0, see ref). SIR_target reduction

together with β boosting allows UEs to operate in

CPC fashion. Figure 2 and Figure 3 show a

simplistic view of waveforms during active and

inactive periods.

In order to obtain any gain during the active

periods the total transmit power of the DPCCH and

the HS-DPCCH after SIR_target reduction and

boosting should be less than the total transmit power

that occurs when using normal SIR target operation.

It is noted that the two methods of operation may not

result in the same performance results since there

may be a penalty associated with knock-on effects of

channel estimation under (possibly) worse

conditions (3GPP TR 25.903 v0.4.0, see ref).

Figure 2: Signal to Interference Ratio (SIR)/Power of

Waveforms during periods of activity and inactivity, for a

100% HS-DPCCH duty cycle.

It is noted that the 3GPP standard of ref (3GPP TS

25.213 v6.5.0, see ref) indicates how to set the

(gain) values of appropriate signals. Of particular

interest is the β

value, as derived from a quantized

amplitude ratio,

, which is translated from Δ

ACK

ΝACK

and Δ

CQI

signalling coming from higher layers.

ACK

, Δ

ΝACK

and Δ

CQI

are carried by the HS-DPCCH

channel and are associated with acknowledgment,

negative acknowledgement and channel quality

indication respectively

Figure 3: Signal to Interference Ratio (SIR)/Power of

Waveforms during periods of activity and inactivity, for a

50% HS-DPCCH duty cycle – indicated by equally spaced

dashed lines.

6 SIGNAL GENERATION

& RESULTS

In order to estimate whether the proposed CPC

scheme, is likely to lead to distortion effects due to

incorrect PA back-off setting (through determination

of the CM), representative signals are simulated and

the CM calculation is performed. Comparison of

calculated CMs of other 3G WCDMA R6 signals is

then undertaken (with and without the inclusion of

the proposed CPC scheme). Using the empirical

results from ref (3GPP TSG RAN WG4#31, 2004),

in the form of a graph, the calculated CM values can

be mapped onto the required PA power back-off

values. From these simulations, comparisons, and

mapping a conclusion might then be reached as to

whether such a scheme would significantly add

distortion when such CPC signals are processed and

transmitted by the PA. decibels (dBs) above the

DPCCH.

Table shows some of the various signals

considered – Signal A is the reference signal (3GPP

TSG RAN WG4#31, 2004), (3GPP TSG RAN

WG4#33, 2004), (3GPP TS 25.101 v6.12.0, see ref

This signal can be used to verify the “raw CM”

value of 0.0. Values obtained are in agreement with

the previous literature. Signals B to I are those

which might occur when the CPC scheme is

implemented, i.e. they reflect various power offsets

during an inactive period and which might be the

result of boosting the β

value so that the HS-

DPCCH is a certain number of decibels (dBs) above

the DPCCH.

WIRELESS MULTIMEDIA Continuous Packet Connectivity (CPC), Power Amplifier (PA) Back-off and the Cubic Metric

(CM)

103

Table 1: Considered Signals.

Note: the raw_CM value does not include

normalization by the factor K of ref (3GPP TS

25.101 v6.12.0, see ref), and does not include the

quantization indicated by the “CEIL” (rounding up

to the nearest 0.5dB) function found in the same

reference. The column headed CM(dB) does take

this into account and values are seen to stay within

the 0 to 3dB range (consistent with the requirement

of ref (3GPP TS 25.101 v6.12.0, see ref)).

Values for β

of 42.25, 59.76 and 84.43

correspond to power offsets, 20log

(β

/β

), of

20log

(42.25/15),20log

(59.76/15),20log

(84.43/

15), i.e. 9dB, 12dB and 15dB respectively.

It is noted that the relationship of PAPR or the

CM to amplifier back-off are derived empirically,

e.g. (3GPP TSG RAN WG4#31, 2004), (3GPP TSG

RAN WG4#33, 2004) using a limited number of

amplifiers, with curves fitted to experimentally

obtained data. Using these references and the

resulting empirical result, the raw_CM values of

decibels (dBs) above the DPCCH.

Table are mapped onto the corresponding power

back-off value. The resulting graph can be seen in

Figure 4.

Figure 4: PA De-rating/Back-off versus raw_CM (dB).

The points on the graph are the simulation results mapped

onto the PA back-off value.

7 CONCLUSION

The PAPR or CM relationship with amplifier back-

off is dependent on the complexity of modulated

signals. For current systems, the PAPR/CM versus

PA back-off relationship can be determined

empirically through the generation of appropriate

Uplink Modulated Waveforms. The CM is a value

which can be determined through simulation (as in

this paper).

Through comparison of appropriately modulated

waveforms that comprise the CPC scheme given in

this report, it has been seen that such a scheme

produces CM values close to those of similar non

CPC waveforms, laying in the same 0 to 3dB range

(3GPP TSG RAN WG4#31, 2004), (3GPP TSG

RAN WG4#33, 2004), (3GPP TSG RAN WG4#38,

2006).

It can therefore be concluded that the impact of

the reported CPC scheme on the CM and the

subsequent PA amplifier back-off is no worse than

that which currently exists, i.e. when no such CPC

scheme is implemented. Indeed, if an issue had been

found, or is ever found, it would undermined the

CMs usage suitability, in terms of its ability to be

used with any channel combination and format and

any system concept.

Implementation of any CPC scheme that alters

the gain value of the HS-DPCCH requires that the

appropriate standard, e.g. ref (3GPP TS 25.213

v6.5.0, see ref), or future standards are modified to

reflect the change in relationship between the HS-

DPCCH and the DPCCH.

Finally, a different (more explicit) procedure

from that of (3GPP TS 25.101 v6.12.0, see ref) has

been derived in this paper.

REFERENCES

3GPP TSG RAN WG4#31, Tdoc#R4-040367,

“Comparison of PAR and Cubic Metric for Power De-

rating”, Beijing, China, May 10-14, 2004.

3GPP TSG RAN WG4#33, Tdoc#R4-040721, “Mapping

of cubic metric to additional PA headroom”, Shin-

Yokohama, Japan, 15-19 November 2004.

3GPP TSG RAN WG4#38, Tdoc#R4-060343, Change

Request “UE maximum output power with HS-

DPCCH and E-DCH”, Denver, Colorado, USA, 13th

– 17th February 2006.

Rohde & Schwarz, Application Note, “High Speed

Downlink Packet Access (HSDPA): “Challenges for

UE Power and Amplifier Design”, http://www.rohde-

schwarz.com.

3GPP TS 25.213 v6.5.0, “Spreading and Modulation

(FDD) Release 6”, Section 4.2.1.1.

3GPP TS 25.214 v6.9.0, “Physical Layer Procedures

(FDD) Release 6”, Section 5.

3GPP TR 25.903 v0.4.0, “Continuous Connectivity for

Packet Data Users, Release 7”, Section 4.3.

3GPP TS 25.101 v6.12.0 “User Equipment (UE) radio

transmission and reception (FDD) Release 6”, Section

6.2.2.

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