USING RATE MATCHING ATTRIBUTES FOR DIFFERENTIATED QoS IN UMTS NETWORKS

USING RATE MATCHING ATTRIBUTES FOR DIFFERENTIATED QoS IN UMTSNETWORKS

R.M.Karthik and Joy Kuri

CEDT, IISc, Bangalore-560012.email: [email protected], [email protected].

ABSTRACTWe consider

�transport channels on the downlink of an UMTS

network, with Rate Matching Attribute (RMA) values �� ,�� ,. . . �� , that are multiplexed on a single Coded Com-posite Transport Channel (CCTrCh). We derive analytical expres-sions for the Bit Error Ratios (BERs) seen by each channel in themultiplex. Simulations indicate that the expressions are very ac-curate. We also provide a procedure that gives the RMA valuesrequired to achieve a target set of BERs for a specified signal-to-noise ratio; when no feasible RMA values exist, the procedure in-dicates that as well. The results of this paper will be of use toUMTS service providers who wish to provide differentiated QoSwhile maintaining high spectral efficiency.

1. INTRODUCTION AND SYSTEM MODEL

In UMTS technology, “transport channels” are used by WCDMAphysical layer to offer services to the higher layers. Transportchannels are always undirectional and are either common, sharedamong several users or dedicated to a single user. Data arrives ontransport channels in the form of transport blocks. A variable num-ber of blocks arrives on the transport channel at each transmissiontime instant. The size of the transport blocks is, in general, dif-ferent for different transport channels. The time interval betweenconsecutive transmission time instants is called the transmissiontime interval (TTI) and is limited to the set � 10, 20, 40, 80 ms ��

.

TrCh 1

TrCh 2

TrCh I

PhCh 1

PhCh 2

PhCh I

Code 2

Spreading

Spreading

Code 1

Spreading

Code I

Fig. 1. Transport-channels using separate spreading codes inWCDMA

Consider the situation depicted in Fig.�

on the downlinkwhere

�transport channels are destined to the same mobile.

�

spreading codes and therefore�

separate physical channels areused. A spreading code is a scarce resource. It would be desir-able to use fewer codes (ideally one). It is also possible that the biterror probability achieved using separate spreading codes is bet-ter than what is actually required by a particular application; thisindicates a wastage of resources.

To conserve spreading codes, when several transport chan-nels are destined to the same mobile, UMTS technology providesthe possibility of transporting multiple parallel services (transportchannels) with different quality requirements on one physical layerconnection. Parallel transport channels (TrCh-

�to TrCh-

�) are

separately channel coded and interleaved. The coded transportchannels are then time-multiplexed into a single Coded CompositeTransport Channel (CC-TrCh). The number of bits available in aCCTrCh per radio frame is fixed; these bits must be shared amongthe

�transport channels that are multixplexed as shown in Fig. � .

Rate Matching Attributes (RMAs) are used to determine the shareof each channel in the CCTrCh. The procedure to determine eachchannel’s share is given in Section 1.2.

If the RMA values determine a share for TrCh � that is morethan the number of bits provided by the channel, then one or morebits of the channel must be repeated one or more times. On theother hand, if the share for TrCh � is less than the number of bitsprovided by the channel, then some bits of TrCh � will be punc-tured. It can be expected that punctured channels will see a de-graded QoS, while the quality of repeated channels will improve.Thus RMA values can be used as an effective mechanism to con-serve spreading codes, and yet achieve a differentiated QoS formultiple connections on the downlink and uplink.

TrCh 1 codingChannel

Channel

Channel

coding

coding

TrCh 2

TrCh I

Rate

Multiplex CCTrCh

TrCh 1From

TrCh 2From

FromTrCh I

PhCh

SingleSpreadingCode

Attribute

Attribute

Matching

Matching

AttributeMatchingRate

Rate

Puncturing/repeating

Fig. 2. Transport-channel coding and multiplexing to a single CC-TrCh

1.1. Parameters

In this section, we list the parameters that are used in the study.��- Transport Block Size on TrCh � .� �

- Number of bits in a radio frame before rate matching onTrCh � .� ��

- If positive, the number of bits to be repeated in a radio frameon TrCh � . If negative, the number of bits to be punctured.��

- Transmission Time interval on TrCh � in msecs.�� - Semi Static Rate Matching Attribute on TrCh � .��

- Total number of bits available for the CCTrCh in a radioframe.�

- Number of Transport Channels in the CCTrCh.�� - Bit error ratio for TrCh � .��- Energy per bit over noise power spectral density.

BPSK signalling is used for transmission over the AWGNchannel. The objective is to determine the BER in each transportchannel in the CCTrCh when no channel coding is used.

1.2. Determination of the number of bits to be repeated orpunctured in each transport channel

The number of bits to be repeated or punctured is determined as in� � � . Given the RMA values, �� for the�channels, variables � � �� are defined, and

� � �, �� , are obtained as follows:� � � ��

= !#"#$!%! $� � �'& (�) �*,+ � ��

* � *�- �� ) *,+ � ��* � * . / �0� � � �� (1)

� � � �1� �32 � �54 � 2 � � / �6� � � �7�� (2)

2. APPROACH

As a first step, we consider the simple case in which the channelcoding blocks in Fig. � are absent. Consider the �

98transmitted

bit : � . The � 98

received bit is given by;<� � : �#=?>@�where

> �is the noise sample from a white Gaussian noise pro-

cess with two sided noise power spectral density��

/2 corrupting: � . Since the channel adds white Gaussian noise, the successivenoise samples added are uncorrelated and hence independent. Theprobability of bit error for BPSK signalling

� A �in an AWGN envi-

ronment is given by B6� � CEDGF � � ��IH (3)

where CKJ9L#MN� �O �QPSRUTVXW 4 ZY�[ ��\^]

2.1. Determination of BER for a punctured transport channel

Let _ � denote the event that a bit on TrCh � is punctured. Recallingthat� � �

is negative for a punctured channel, the probability ofthis event,

B J9_ � M is given byB J9_ � M`� 2 � � �� Let� �

denote the event that a bit of TrCh � is incorrectly detected.The probability of this event is given byB J �� M`� B J9_ � M B J ��ba _ � M =?B Jdc_ � M B J ��a c_ � M (4)

where c_ � denotes the complementary event that the bit on TrCh� is not punctured. When a bit is punctured, it is detected at thereceiver as � or

�with equal probability. Then, the probability of

error,B J � � a _ � M is given byB J ��ba _ � MG� �

�For a non-punctured bit, the probability of error is given byB J � � a c_ � MG� CeDGF � �� HThe overall probability of error is thereforeB J � � MG� �

�gf 2 � ��ih = f � = � ��jh CeD F � � �� H (5)

This procedure is repeated for all channels in which the bits arepunctured and the corresponding BERs are determined.

2.2. Determination of BER for a repeated transport channel

For kml � , let �In 4 � be the event that a bit is repeated J5k 2 � Mtimes. When bit : � is repeated J5k 2 � M times, the correspondingk -bit transmitted sequence is given by]bo $ � � :^p �rq� ��:sp �bq� ��:sp n q� �where : p �rq� �t: p �bq� �u��i: p n q� �t: � since the bit has beenrepeated.

The noise sequence added to the k -bit sequence is given by> o $ � � > p �rq� � > p �bq� �� > p n q� �

where> p �rq� � > p �bq� �� > p n q� are independent samples from the

same Gaussian noise process with variance�� . The received

sequence is given by; o $ � � ; p �rq� � ; p �bq� �� ; p n q� �

where ; p v q� � : p v q� =U> p v q� � w�� 7��xk#�In the rate-dematching block, this sequence is averaged over k togive ;<� � �k nyv + � Jz:sp v q� =?> p v q� M� : � = �k nyv + � > p v q�

For this particular bit, the noise variance � � is given by

� � �� D �k nyv + � > p v q� H� �k � nyv + � �� J > p v q� MAll noise samples are independent samples from the same distri-bution. Therefore the noise variance is given by

� � � �k J � � � �sMTherefore the SNR for this bit is given by� � � � � �� J � k M � k � � �� The probability of error associated with this bit is therefore givenby � ��

= C ("! � n$#&%')( -There are two cases which are to be considered.

CASE A: �+*-, ' $' $/. �

Let � be the event that a bit is repeated. If �0* , ' $' $1. �, the

probability of event � is given byB J �MG� � �� In this case, a bit is repeated just once (if at all). The probabilityof error is given byB J �� MG� B J �M B J ��ba �M = B J%c�M B J ��ba c�M (6)

where c� denotes the complementary event that a bit is not re-peated. For the case of a bit repeated once, the probability of errorB J ��ba �M is given byB J � � a �MG�1CED F A �� IH (7)

For a non-repeated bit, the probability of error is given byB J � � a c�MG�1C D F � � �� HTherfore the overall probability of error

B J �� M is given byB J � � M`� f � �� jh C'DGF A �� H = f � 2 � �� h CeDNF � �� H(8)

This procedure is repeated for all channels in which the bits arerepeated once and the corresponding BERs are determined.

CASE B: , ' $' $/2 �

The number of times each bit is to be repeated is determined fromthe number of repeated bits and the block size. If , ' $' $32 �

, then

every bit in the original radio-frame of size� �

is repeated at leastonce. To find the number of times a bit is repeated, we proceed asfollows. For intergers k and J5k = � M , letk . ( , ' $' $ - * J5k = � M

In this case, some bits are repeated k times and others J5k = � Mtimes. Let

� p n q� bits in the original radio frame of size� �

berepeated k times and

� p n�4 �rq� bits be repeated J5k = � M times.Then� p n q� =?� p n�4 �rq� � � �(9)J5k = � M � p n q� = J5k = �sM � p n�4 �rq� � J � � = � � � M (10)

Solving the above two equations, we get� p n$4 �rq� � J ��#= � �� M 2 J5k = � M �� p n q� � J5k = � M �� 2 � ��Recalling that � n is the event that a bit is repeated k times, wehave B J � n M � J5k = � M �� 2 � ��B J � n�4 �bM � f � 2 J5k = � M � � 2 � � �� hThe probability of error is given byB J �� MG� B J � n M B J ��xa � n M =?B J � n$4 ��M B J ��ba � n$4 � M� f J5k = � M � �32 � � �� h C D F � J5k = � M � �� H =5 � 2 JZk = � M �� 2 � �� 6 C D F ��J5k = � M � �� H (11)

This procedure is repeated for all channels in which bits are re-peated and the corresponding BERs are determined.

Thus using equations (5), (8) and (11), the BERs of all thechannels in the CCTrCh can be determined, given the RMA valuesand� � ��

.

3. DETERMINATION OF RMAs, GIVEN BERs FOR ALLTHE

�CHANNELS

Given� � ��

, the probability of bit error is given by (3). Thegiven target BERs,

� � � � � � � ��7�� , are compared withB �.

CASE 1:If� � � � 2 B � , then the channel � can be punctured. As seen

before, the BER for punctured channel is given by�� f 2 � � �� th = f � = � � �� h C D F � � �� H

From this equation2 � � �

can be calculated and leads to the fol-lowing 2 � � � � � �� ( �� 2 C ( ! � # %'7( -<-

� 2 � C ( ! � # %'7( - (12)

�� B p ! q� B p�� q� B p ! q� B p�� q�6 15 2160 2160 0.2274 0.2273 0.0141 0.0142

10 10 2000 2000 0.0213 0.0212 0.0213 0.021410 12 2000 2000 0.0317 0.0317 0.0192 0.019310 14 2000 2000 0.0706 0.0705 0.0176 0.017510 14 2160 2160 0.1024 0.1024 0.0195 0.019610 15 2160 2160 0.1183 0.1185 0.0188 0.018710 30 2160 2160 0.2615 0.2616 0.0127 0.012720 24 1000 3320 0.0868 0.0866 0.0221 0.022120 24 2160 2160 0.0665 0.0665 0.0210 0.0210

Table 1. Comparison of BERs with�� : on both channels.

CASE 2:If�� * B � , then channel � must be repeated. Two integers k

and J5k = � M are found such thatC (�! � n$# %')( - * �� * C ( ! � p n$4 �rq # %')( -This implies that some of the bits must be repeated J5k 2 � M timesand others k times. The

�� in this case is given byB J � � MG� B J �In M B J � � a ��n^M = B J ��n$4 � M B J � � a �In$4 � M� f k �� 2 � �� h C DGF � k � �� H =� � 2 k � � 2 � � �� C D F ��J5k = � M � �� H (13)

From this equation,� � �

can be obtained as�� k C ( ! � n$# %'7( - 2 J5k 2 � M C ( ! � p n�4 �rq # %')( - 2 �� C ( ! � n$# %')( - 2 C ( ! � p n$4 �rq # %'7( - � ��(14)

Evaluating equations (12) and (14), the value of� � �

for all thechannels are determined. Then we verify whether the followingequation is satisfied:

y � + � J ��#= � �� M . � �� (15)

If Equation (15) is not satisfied, then the given�� : cannot be

satisfied for the specified��

with any value of the RMAs onthe transport channels.

If Equation (15) is satisfied, the RMA values can be obtainedusing the procedure given in Section 1.2. Combining equations(�) and ( � ) with equations (12) and (14), and solving them we

can obtain the values of RMAs ( �� ) given the��

( �0� � � �� ).

4. COMPARISON OF ANALYTICAL AND SIMULATEDRESULTS

The RadioLab 3G tool� � �

is used for comparison. The simulatedBERs are compared with the analytical expressions for the BERsfor the case of

�= 2, 3 and 4. We use

� �� = 4320 bits and� � ��

= 3 dB. The simulations are performed for various RMAvalues, different transport block sizes and different TTIs in thechannels. Tables

� 2 �compare the analytical and simulated BERs

for different values of RMAs, block sizes (in bits) and TTIs in the2 transport channels. In the Tables, the BERs are denoted by theletter

B. Tables � and compare the results for

� �� and� � A

respectively. In all the cases, the match is observed to be excellent.

5. CONCLUSION AND FUTURE WORK

We presented a method for the evaluation of BERs of the differentservices on the transport channels, given the corresponding RMAsand a specified

� � ��on the UMTS downlink. The procedure can

be extended to the uplink also. It is found that the simulated resultsclosely match the derived formula. In addition, we obtained a pro-cedure to determine whether it is possible to satisfy the given BERsat the specified

� � � � �on the transport channels using the RMAs.

The results considered in this paper are for the case where no chan-nel coding was done. We are currently extending the results to thecase where transport channels use convolutional coding.

6. REFERENCES

[1] Erik Dahlman, Bjorn Gudmundson, Mats Nilsson and JohanSkold, “UMTS/IMT-2000 Based on WCDMA,” IEEE Commu-nications Magazine, vol. 36, Issue 8, pp. 1094–1105, Septem-ber 1998.

[2] 3GPP TS 25.212 V5.0.0, Technical Specification, Multiplex-ing and Channel Coding (FDD).

[3] Insoo Sohn and Seung Chang Bang, “Performance Studiesof Rate Matching for WCDMA Mobile Receiver,” IEEE VTC2000, vol. 6, pp. 2661-2665, September 2000.

[4] Bernard Sklar, “Digital Communications, Fundamentals andApplications,” �� edition, Pearson Education, Asia.

[5] “User guide for RadioLab 3G,” www.radioscape.com.

�� B p ! q� B p�� q� B p ! q� B p�� q�10 12 2160 2160 0.0178 0.0179 0.0127 0.012710 15 2160 2160 0.0200 0.0201 0.0082 0.008310 20 2160 2160 0.0229 0.023 0.0024 0.002410 30 2160 2160 0.1185 0.1183 0.0015 0.001620 24 2160 4320 0.0665 0.0665 0.0210 0.0210

Table 2. Comparison of BERs with�� `� � � � : and

�� : .�� d� � �� : and

�� ,� A � � : �� d� � � � : and�� ,� � �� :�� B p ! q� B p�� q� B p ! q� B p�� q� �� B p ! q� B p � q� B p ! q� B p�� q�

10 12 2160 8640 0.0665 0.0665 0.0210 0.0211 17280 0.0665 0.0665 0.0210 0.021110 12 2160 6480 0.0218 0.0220 0.0175 0.0175 12960 0.0218 0.0220 0.0175 0.017610 15 2160 8640 0.1183 0.1185 0.0188 0.0187 17280 0.1183 0.1185 0.0188 0.0187

Table 3. Comparison of BERs with the mentioned�� : .

�� Q�� : and�� A � � : �� : and

�� :�� B p ! q� B p�� q� B p ! q� B p�� q� �� B p ! q� B p � q� B p ! q� B p�� q�10 12 4320 8640 0.0665 0.0665 0.0210 0.0211 17280 0.0665 0.0665 0.0210 0.021210 15 4320 8640 0.1183 0.1185 0.0188 0.0187 17280 0.1183 0.1185 0.0188 0.0187


�� d� A �� : and�� ,� A � � : �� d� A � � : and

�� ,� � �� :�� B p ! q� B p�� q� B p ! q� B p�� q� �� B p ! q� B p�� q� B p ! q� B p�� q�10 12 8640 12960 0.1594 0.1594 0.0909 0.0911 17280 0.0665 0.0665 0.0210 0.021110 15 8640 12960 0.2065 0.2067 0.0596 0.0597 17280 0.1183 0.1185 0.0188 0.0187


�� B p ! q� B p � q�10 1440 0.1024 0.102612 1440 0.1422 0.142312 1440 0.1349 0.135112 1080 0.1099 0.1099

�� B p ! q� B p � q�12 1440 0.0229 0.022914 1440 0.0229 0.022915 1440 0.0430 0.043015 1440 0.0224 0.0226

�� B p ! q� B p � q�

14 1440 0.0195 0.019516 1440 0.0178 0.017820 1440 0.0172 0.017420 1440 0.0154 0.0154

Table 6. Comparison of BERs for � transport channelswith

�� S� � �� :s� �� : and��

� �� : .

�� B p ! q� B p�� q�10 1080 0.0229 0.023010 1080 0.1333 0.133710 1080 0.1872 0.1873

�� B p ! q� B p�� q�10 1080 0.0229 0.022912 1080 0.0596 0.059614 1080 0.0622 0.0623

�� B p ! q� B p�� q�

10 1080 0.0229 0.022914 1080 0.0213 0.021516 1080 0.0205 0.0206

�� B p ! q� B p�� q�

10 1080 0.0229 0.022916 1080 0.0182 0.018220 1080 0.0166 0.0166

Table 7. Comparison of BERs forA

transport channelswith�� :s� �� I� � � � :s� �� :

and��

�,� � �� : .

USING RATE MATCHING ATTRIBUTES FOR DIFFERENTIATED QoS IN UMTS NETWORKS

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