For an integrated services UMTS network serving speech and data calls, we propose, evaluate and compare different scheduling schemes, which dynamically adapt the shared data transport channel rates to the varying speech traffic load. within each cell, the assigned data transfer resources are distributed over the present data flows according to certain fairness objectives. The performance of the adaptive schemes is numerically evaluated by means of analytical performance optimisation methods in combination with Monte Carlo simulations.\ud
\u

Berg, Hans van den

Litjens, Remco

English

For an integrated services UMTS network serving speech and data calls, we propose, evaluate and compare different scheduling schemes, which dynamically adapt the shared data transport channel rates to the varying speech traffic load. within each cell, the assigned data transfer resources are distributed over the present data flows according to certain fairness objectives. The performance of the adaptive schemes is numerically evaluated by means of analytical performance optimisation methods in combination with Monte Carlo simulations

van den Berg, Hans Leo

University of Twente Research Information

Performance Analysis of Adaptive Scheduling in Integrated Services UMTS Networks Remco Litjens* and Hans van den Berg*%t *Knowledge Innovation Center, KF" Research, The Netherlands. {R.Litjens, l.L.vandenBerg}@kpn.com 'Faculty of Mathematical Sciences, University of Twente, The Netherlands, l.L.vandenBerg@math.utwente.nl Abslrac-For an integrated semces UMTS network semng speech and data calls, we propose, evaluate and compare different schedul- ing schemes, which dynamically adapt the shared data transport channel rates to the varying speech traffic load. Within each cell, the assigned data transfer resources are distributed over the present data flows according to certain fairness objectives. The perfor- mance nfthe adaptive schemes is numerically evaluated by means nf analytical performance nptimisation methods in combination with Monte Carlo simulations. Keywords-UMTS, HSDPA, integrated semces networks, adaptive scheduling, Quality of Semce, performance evaluation. 1. INTRODUCTION The third-generation universal Mobile Telecommunications system (UMTS) [5 ]  is designed to support a wide variety of ser- vices, e.g. speech and video telephony, file transfer and WWW browsing, with distinct Quality of service (QOS) requirements. A number of technological improvements of the initial UMTS system release is standardised under the denominator High- speed Downlink packet Access (HSDPA) [XI, whose objective it is to enhance the data transfer capabilities and enable the support of downlink peak rates in the range of 8 - 10 Mbitds for best ef- fort packet data services. To this end, HSDPA upgrades UMTS'S Downlink shared CHannel (DSCH) to a H i g h  speed DSCH (HS- DSCH), which supports a variety of enhanced technologies, in- cluding higher order modulation combined with fast link adapta- tion, and fast scheduling centered at the Base Transceiver station (BTS) with a proposed smaller transmission time interval, in or- der to reduce delays and facilitate better lracking of the channel variations. The viability of fast link adaptation allows parameter- isation of the HS-DSCH by an assigned transmission power [8] ,  where the experienced data rate depends on the radio link quality of the served data flow. An important advantage is the reduced variability of the exerted power levels, since the path gain differ- ences of the served data calls can be dealt with in the time rather than the power domain. Operational in different forms at different timescales, aahp- tive scheduling on HS-DSCHS is foreseen to offer significant Qos enhancements for data services. At a timescale of millisec- onds, C/I-based scheduling exploits the variability of the radio link due to fast fading by multiplexing the data flows in accor- dance with their instantaneous CII. This optimises the system throughput but may result in a significant degree of unfairness among data flows [Z, 6,7]. At a timescale of seconds, the appropriate up- and downgrad- ing of the data transport channels in an integrated services UMTS network is another form of adaptive scheduling which exploits both the traffic load variability of a prioritised e.g. speech ser- vice and the delay tolerance of data services. The objective of the present paper is to assess the performance gain that can be achieved by the latter type of adaptive scheduling, combining an- alytical optimisation techniques with Monte Carlo simulations. Scarce earlier related work includes [3,9], where a load control scheme is assessed which (de)activates the dedicated transport channels conveying speech traffic in support of the varying data traffic load, and [9], which investigates the effectiveness of de- creasing data rates to reduce interference levels as a form of con- gestion control. While [3,9] concentrate on heuristic and rather myopic schemes, the adaptive schemes considered here aim to oprimise performance in a network-wide sense. The outline of the paper is as follows. Section U sets the frame- work for the performance evaluation of the different scheduling schemes which are specified in Section 111. The performance evaluation method is outlined in Section N. Section V presents a numerical study and discusses the results. Section VI ends the main body of this paper with some concluding remarks. 11. NETWORK AND TRAFFIC MODEL We consider the downlink in a network consisting of a set B of omnidirectional UMTS BTSS each serving a cell with a radius of R km. Denote by p,, each BTS'S transmission power budget and let U be the spatially uniform thermal noise level. A single- path radio propagation model is used with path loss exponent < and correlated lognormal shadowing [lo] with a standard devi- ation of U dB. The downlink orthogonality factor is denoted w. The system chip rate is fixed at &hip Mchipsls. The considered UMTS network integrates speech and data ser- vices. Speech calls are handled on Dedicated transport CHannels (DCHS) with a fixed information bit rate of Rspeech kbitds, and require a carrier-to-interference ratio no less than (̂rpeech E 0-7803-7605-6/0U$17.00 02002 IEEE 3 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 31,2010 at 09:34:02 UTC from IEEE Xplore.  Restrictions apply. (&peech/&hip) 10'--h''o with CspeEch (in dB) the speech ser- vice's target. Data transfers are multiplexed on HS-USCHs (one per BTS). Utilising HSDPA'S link adaptation feature, the data transfer rate experienced by a given data flow is proportional to its actual carrier-to-interference ratio ydafa: given an E,/N,  target of either service type is uniformly sampled over the network. (in dB). The location of calls of 111. SCHEDULING SCHEMES The scheduling schemes consist of a macro- and a micro- component. The macro-scheduling scheme dimensions the HS- DSCHs in accordance with the actual traffic conditions and is parametrised byp$&. If pb denotes the fraction of data flows that is served by BTS b, then the HS-DSCH transmission power assigned to a BTS b serving at least a single data flow is equal to (2) mi" P L C "  = P"S.DSC" + vbffHs.mscn> b E B, where the aggregate adaptive HS-DSCH transmission power component BHS.DSCH is to be maximised under the condi- tion that no speech calls are caused to violate their CII  require- ments. While ss%,,, is a minimum per-BTS resource assign- ment, the incorporation of the pb's establishes a fair sharing of the scheme's adaptivity gain over the data calls. The special case of pg?Db,,, = 0 specifies a purely adaptive scheme, while on the other hand, prefixing B,,.,,,, at 0 specifies a purely fixed HS- USCH power assignment, which is used for reference purposes. At each BTS, either a Round Robin (RR) or weighted Round Robin (WRR) rnicm-scheduling scheme is selected to multiplex the present data flows over the HS-DSCH as dimensioned at the macrwlevel. Whereas RR scheduling distributes the BTSS' HS- DSCH resources fairly over the different data flows, the WRR scheme establishes equal throughputs within each cell, by appro- priately specifying scheduling weights based on the data flows' (average) path gains. Denote by & the scheduling weight of data flow m served by BTS b such that 4: = 1, b E B. Figure 1 illustrates the different scheduling timescales. At each adaptive scheduling instant, advocated to correspond with significant changes in the traffic loads, the macro- and micro- scheduling parameters are re-determined. At the fixed heart- beat determined by the HSDPA transmission time intervals, the data flows are then multiplexed according to their current micro- scheduling weights. adaptive mimDSChedUling Scheduling period instant / ~ ' -  1 1 1 . . I I I I  ... time f adaptive Scheduling instant - Fig. 1. Scheduling fimescales. IV. PERFORMANCE EVALUATION The relative performance of the presented scheduling schemes is evaluated by means of analytical optimisation in combina- tion with Monte Carlo simulations. The speech performance is expressed in terms of the outage probability P,, defined as the probability that an arbitrary speech call fails to meet its CII target, while the data performance is measured by the ex- pected throughput Rd per data flow. Additionally, the condi- tional speech (P. (a)) and data (Rd (0)) performance are de- termined as a function of a call's geometry 0, given by Pm&M5bMS a,, P,,,A,* + U' b f b w  where GMS b denotes the path gain of call MS w.r.t. BTS b. At a given scheduling instant, the consfellation of speech and data calls is specified by the set M, (1, . . .  , M,} p d d  = [ 1,. . , Md))  of speech (data) calls, the path gains G,, m E M, U M d ,  b E B, and the serving BTS b, assigned to each call m E M (based on a 'maximum path gain' criterion). Let M: = {m E M, : b, = b}  a n d M i  In order to evaluate the adaptive scheduling schemes, the first step is to assess whether the prioritised speech calls experience an outage for the minimum HS-DSCH assignment ofpz&,,, i.e. whether speech transmission powers p. 1 (p,, m E Ms) exist such that all speech calls meet their C J I  target, {m E Md : b, = b}. with I, (m) = U (bm,bm,)pm,Gmb,, (12" (m) = CbeB:M;,o w (b,, b) Gmb)  the amount of interference call m experiences from speech (data) transmissions (per exerted Watt ofp~:",,,.), and w (b, b') w (1) if b = b' (b # b') is the appli- cable orthogonality factor. An additional restriction is given by the BTSS' power budget, P, + {Mi # 0) 5 pmar, b E B. -€U: 4 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 31,2010 at 09:34:02 UTC from IEEE Xplore.  Restrictions apply. A distributed implementable power control scheme similar to that proposed in e.g. [I] can be applied to determine the speech performance under the minimum HS-DSCH assignment of p&,,. This implicitly determines the speech performance in terms of the number of outages. Only if no speech outages occur in the considered constella- tion for the minimum HS-DSCH assignment of pg?Dscs,  or in case of a fixed scheduling scheme the next step is to optimise the adaptivity parameter zP,,.,scH under the condition that no speech outages are induced. Otherwise, ZPHS.DSCH is fixed at 0. Theorem If in a given constellation the minimum HS-DSCH power assignment of pz.;",,,, induces no speech outages, the op- timal value of BHS.DSCH is given by P m u  - P%"1 {Mi # 0) + c; (1 - H)-'T: + p b  -c; (1 - 7t-1 {p&c,,Ty" + G }  (3) 8~,.,,,,, s min bEB where the entries of matrix H are given by 0 m = m '  form,  m' E M,, where I,"(m) = ~ a E s ~ ( b m , b ) ~ & m b ,  and G, 3 (1 { b ,  = b }  , m E M,) indicates which speech calls are served by BTS b E B. Proof See appendix. rn At this point, all HS-DSCH powers are set according to Eq. (2). In order to determine the data Q O S ,  the distinction in micro- scheduling schemes (RR or WRR) needs to be effectuated. Re- call that since data flows inherently meet their CII targets via HSDPA'S link adaptation feature, data flow m E Md achieves a throughput of (cf Eq. (1)) khitsis whenever it is served, so that its expected throughput dur- ing the scheduling period is given by R, = d>Rk, where the micro-scheduling weights $2 depend on the deployed micro- scheduling alternative: V. NUMERICAL RESULTS A Monte Carlo simulation study is carried out to evaluate the scheduling schemes with respect to the speech and data perfor- mance. M, and Md are assumed to be Poisson random variables with mean pspgrh and pdam, respectively. A single cell scenario is considered. The system and traffic parameters are given by R = l/& !a, p,,  = 42 dBm (= 15,849 Watt), Y = -99.157 dBm (= 1.21410-'3 Watt), c = 3.523, U = 8 dB, w = 0.40, &hip = 3.84 Mchipsis, & p e d  = 12.2 kbitsis, Cspecch = 5 dB and Cdata = 4 dB. For (pspeesh, pdata) = (75,25) Fig. 2 depicts the speech and data performance as a function of pF:& for both the fixed and adaptive macro-scheduling schemes, and for the RR (left) and WRR (right) micro-scheduling alternatives. The vertical bars along the Rd curves indicate the standard deviation of the data throughput. Observe first the trivial trends that both P, and Rd are increasing in ~g?~",,,,, and that P, is inherently insensitive to both the scheme's adaptivity at the macro-level and the choice of micro-scheduling scheme. With regard to Rd, the adaptive scheme appears to exploit its minimum guarantee ofpz&, not until it exceeds about 5 Wan, beyond which the adaptivity gain vanishes as both throughputs cumes converge. Assessing the im- pact of the micro-scheduling scheme, observe that the 'rate-fair' WRR scheme yields lower expected throughputs. This can be in- tuitivitly explained by the insight that the WRR scheme assigns a disproportionate amount of resources to the data flow at the worst location, thereby reducing the overall data performance. In contrast, under the 'power fair' RR scheme such a badly lo- cated data flow receives a fair share of the resources, and thus does not inflict its own suffering upon the other data flows. 5 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 31,2010 at 09:34:02 UTC from IEEE Xplore.  Restrictions apply. POWER FAIRNESS RATE FAIRNESS . R  Fig, 2 .  speech and data performance versus P & ~ ~ ~  (P,,.~ = 75. Pdats = 25) POWER FAIRNESS YI 45 I RATE FAIRNESS Fig. 3 .  Conditional speech and data performance for different pEDSCH (ps- = 75. pdB,. = 25). SPEECH PERFORMANCE DATA PERFORMANCE Fig. 4. Speech and data performance versus (psoncn, pdata). 6 Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on May 31,2010 at 09:34:02 UTC from IEEE Xplore.  Restrictions apply. Fig. 2 further demonstrates that a purely adaptive scheme can improve both speech and data performance jointly, compared to a scheme withfired HS-DSCH powers. Fig. 3 shows the conditional speech and data performance un- der the adaptive scheduling scheme with pg&H E {0,3,6} Watt, and for the RR (left) and WRR (right) micro-scheduling al- ternative. As expected, the experienced speech and data perfor- mance improve as a terminal is located nearer to the BTS (higher geometry). The principal result demonstrated by the figure is the impact ofthe micro-scheduling scheme: due to the 'rate fairness' ofthe WRR scheme, distant terminals typically experience higher throughputs than under the R R  scheme, which favours near ter- minals. In this single cell case, the RR scheme outperforms the WRR scheme w.r.t. to both the expected throughput and the con- ditional expected throughput experienced at the modal geometry. Fig. 4 shows P. and Rd under the purely adaptive schedul- ing scheme @$DIDSCH = 0 Watt) with RR micro-scheduling as a function of the traffic mix (pspecch, pdara) . Aside from the inher- ent insensitivity of P, to pdata under the purely adaptive schedul- ing scheme, performance generally worsens under heavier traf- fic. Observe that whereas P. increases exponentially in prpesch. Rd becomes less and less sensitive as the traffic load increases. For light traffic loads Rd is most sensitive to pdata, while for rel- atively heavy traffic loads the relative impact of pspeerh becomes slightly dominant. In case of WRR micro-scheduling, the Rd curve lies below that for the R R  scheme (not shown), with the discrepancy increasing in pdaa due to the growing significance of the badly located terminals. VI.  CONCLUDING REMARKS A performance evaluation of adaptive scheduling schemes has been presented for the downlink in a UMTS network integrating prioritised speech and delay-tolerant data calls. We have derived a closed-form expression for the optimal transmission power as- signed to the HS-DSCHS that are applied to multiplex the data flows, while protecting the speech calls' CII performance. Us- ing a fixed HS-DSCH power assignment as a reference scheme, the numerical experiments have demonstrated that it it possible to improve both the speech and data performance by means of adaptive scheduling. A minimum HS-DSCH power assignment was demonstrated to be an effective policy parameter to attain a desired tradeoff between speech and data performance. For further research we recommend to apply the derived ana- lytical results in dynamic simulations (see also [4]) in order to futher investigate the implications of the demonstrated spatial unfairness as the system evolves in time, and to devise heuristic derivations of the optimal scheme for practical implementation. ACKNOWLEDGMENTS The authors appreciate the valuable comments of Richard J .  Boucherie of the University of Twente, The Netherlands. This research has been carried out within the BEYOND 3c project, and is partially supported by the Ministry of Economic Affairs, The Netherlands. APPENDIX: PROOF OF THEOREM Proof Along similar lines as followed in e.g. [l], the speech calls' CII  conditions for a given value of BHS.DSCH can be written in matrix form: Momentarily ignoring the restrictions posed by the BTS power budget p,,, the Pareto optimal vector of speech transmission powers is given by which is nonnegative and increasing in the aggregate adaptive HS-DSCH ower component B,,,.,,,, 2 0, since the matrix (T - N)- IS positive componentwise [I]. A necessruy and suf- ficient restriction for BHS.DSCH from BTS b's perspective is then P. . . for each b E B. Since these restrictions on BHS.,,,, must hold w for all BTSs,  Eq. (3) immediately follows. REFERENCES [ I ]  N.D.Bambos.S.C. 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Performance analysis of adaptive scheduling in integrated services UMTS networks

CDMA: principles of spread specrmm communicotian.&quot;

ChenandG.I.Pottic,&quot;Radiolinkadmissionalgo~Uuns for wireless networks with power control and active link quality protection:'

Congestion conuol in WCDMA with respect to different service EI~SSOS,&quot;

Downlink Radio Resource Estimation and Control

Dynamic rate conml algorithms for HDR throughput aptirnisation,&quot;

WCDMA/OT UMTS: radionccess/or rhirdgeneronon mobile communicolions,&quot;John Wiley & Sons,

https://ris.utwente.nl/ws/files/5477542/2002-14LitjensBerg.pdf

Performance analysis of adaptive scheduling in integrated services UMTS networks

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