Transmit Power Control Algorithms in IEEE h Based Networks

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1 Transmt Power Control Algorthms n IEEE 82.h Based Networks Andreas J. Könsgen, Zakr Hossan, Carmelta Görg Department of Communcaton Networks Center for Informaton and Communcaton Technology (IKOM) Unversty of Bremen Otto-Hahn-Allee, Bremen, Germany Emal: {ajk zakr cg}@comnets.un-bremen.de Abstract In ths paper, Transmt Power Control algorthms are dscussed to support the Spectrum Management requred by the Wreless LAN standard IEEE 82.h. A centralsed algorthm for the mode and an algorthm workng on a lnk-by-lnk bass for networks are dscussed. The performance of the algorthms, n partcular the reducton of the transmt power and the enhancements of the QoS parameters delay and throughput are consdered. It s shown that both algorthms sgnfcantly reduce the transmt power and mprove the qualty-of-servce characterstcs of wreless networks. I. INTRODUCTION The rapdly ncreasng number of moble devces lke laptops and PDAs results n a hgh demand for wreless network access. To ensure the compatblty between the wreless LAN hardware of dfferent manufacturers, the IEEE has standartsed the protocols to be used on the ar nterface. Currently IEEE 82.a [][2] s beng ntroduced nto the market, whch works on the 5 GHz band and provdes enhanced data rates up to 54 Mbt/s. The ncreasng amount of devces, however, results n a hgher spatal densty and thus n an hgher mutual nterference on the ndvdual frequency channels. Ths problem occurs n partcular n areas where the assgnment of the avalable frequency channels to wreless devces s uncoordnated, lke n exhbton halls, offce buldngs wth a large number of small companes or resdental areas. The mutual nterference between devces of neghbourng wreless networks can result n a performance degradaton of connectons. Therefore, there s a demand to control the usage of the lmted avalable frequency spectrum by effcent spectrum management methods whch are specfed n the IEEE 82.h standard [3]. Besdes Dynamc Frequency Selecton dscussed n [4], another spectrum management method specfed n the standard s Transmt Power Control (TPC). The prncple of TPC s the control of the power by the sendng staton n such a way that the recevng staton gets the sgnal at a C/I whch s suffcent to receve the data below a specfed bt or packet error rate. Ths stategy reduces the nterference and also the power consumpton of the sendng devces, whch s n partcular mportant for small portable unts lke PDAs or WLAN/VoIP based telephone handsets. The basc dea of the sgnallng specfed by the standard s the ntroducton of two new management packet types termed as TPC Request and TPC Report. These management packets allow a staton to provde nformaton about the own transmt power and the lnk margn at whch the sgnal of the communcaton lnk partner was receved. The standard, however, only specfes the sgnallng; the algorthms to control the transmt power are not covered. In ths paper, algorthms for TPC whch support the power control specfed n IEEE 82.h are descrbed. A centralsed algorthm whch orgnally has been desgned for cellular telephone networks s adapted here for the use wth Wreless LAN. Ths algorthm s based on a group of access ponts whch are connected by a wrded backbone. An algorthm for networks s also nvestgated whch determnes the transmt power on a lnk-by-lnk bass. Ths algorthm s based on a fuzzy controller. Moreover, to determne the effect of TPC on the connecton qualty and to show the behavour of the dfferent algorthms, scenaros ncludng a number of wreless networks, TPC-enabled and not TPC-enabled, are smulated to gather nformaton about the performance enhancements suppled by TPC. For a wreless staton, three types of transmsson ranges can be dstngushed: The goodput range r g specfes the maxmum dstance between the staton and a communcaton partner to whch a successful communcaton can be establshed (more precsely, where a mnmum C/I or a maxmum acceptable bt error rate s met). The nterference range r specfes the range nsde whch a staton nterferes the transmsson of neghbourng statons, thus causng a collson. The carrer sense range r c specfes the dstance nsde whch the sgnal of a neghbourng staton can be sensed, wthout the requrement of decodng t. Ths range s mportant to judge the channel occupaton when controllng the backoff, snce a staton stops the backoff tmer once the wreless meda has been sensed busy. The approach of TPC s that the goodput range r g does not have to be hgher than the dstance between the sender and the recever. Reducng t to the necessary mnmum results n a mnmsaton of the TX power and as well n a reducton of r and r c. Due to ths, the number of statons whose recepton

2 s nterfered or whch stop ther backoff due to a busy channel s reduced. As a result, neghbourng statons can be closer to the sendng staton wthout beng nterfered, whch ncreases the spatal reuse of a frequency channel. The remanng part of ths paper s organsed as follows: In secton II, other approaches of Transmt Power Control are shortly dscussed. After that, the detals of the TPC algorthms proposed n ths paper are gven n secton III. Secton IV gves an overvew on the smulaton setup. The results are shown and evaluated n secton V. Secton VI concludes the paper. II. RELATED WORK Transmt power control algorthms for networks were orgnally developed for cellular telephone networks are descrbed wdely n the lterature, as for example shown n the overvew gven n [5]. For ths paper, the dea of the balancng algorthm proposed n [6][7], whch s descrbed n detal n secton III, has been adapted for the usage wth 82.h Wreless LANs. For networks, algorthms whch work on a lnk-bylnk have been nvestgated: In [9], a strategy s dscussed whch modfes the RTS/CTS sgnallng of the 82. protocol. [] ntroduces a power control scheme whch determnes the transmt power by measurng the traffc load rather than measurng nterference or lnk gan values. TPC strateges whch are based on the legacy 82. MAC layer standard wthout.h modfcatons are dscussed n [8] and []. These approaches are based on sendng RTS/CTS at the maxmum possble power and measurng the C/I at the recever. The data packets are then sent usng the smallest possble power. The TPC algorthm presented n ths paper has been adapted from nvestgatons on Hperlan/2 [2] whch are based on a fuzzy controller that works on a lnk-by-lnk bass. It s dscussed n detal n secton III. A. Infrastructure Algorthm III. TPC ALGORITHMS In the algorthm, the concept of the TPC owner s ntroduced. Ths s a central staton,. e. one of the access ponts whch s connected to the neghbourng access ponts by a wred backbone. The TPC owner s determned by sgnallng on the wred backbone; nsde a gven network, the access pont wth the lowest MAC address. The TPC owner perodcally trggers all access ponts nsde the network to request nterference measurements from ther assocated termnals and report the measurement results back to the TPC owner. Based on the measurement data, the TPC owner calculates the new TX power for each of the moble termnals and forwards the results to the termnals. For the calculaton, the lnk gan matrx G s consdered whch contans all sgnal attenuatons G j between a sender and a recever j. The case of = j dentfes pars of a sender and a recever whch run a communcaton,. e. an access pont and an assocated termnal, so that the receved sgnal s goodput. The case j dentfes nterference between statons between whch no communcaton s gong on. G G 2 G = G 2 G 22 ()..... The values G j are normalsed by dvdng them by the gan G of the lnk whch transports the goodput data: Z j = G j G (2) The C/I observed at termnal ncludes the nose N, whch s also normalsed: N = N G (3) Wth P beng the transmt power at staton, the values C/I can then be calculated as follows: C P = I n j= P jz j P + N From these C/I, the TPC owner determnes the TX power P (n+) for each staton whch s used for the next,.e. the (n + )st teraton of the algorthm. Due to the sgnallng overhead, a sutable compromse for the frequency of the power readjustment has to be found. A hgh frequency causes the system to adapt rapdly to changng condtons, for example due to moblty; however, due to the ncreased sgnallng overhead the network performance can be reduced. A low frequency mnmses ths overhead, but the system mght not adjust the powers adequately accordng to the current channel condtons. P () = P ; P > ; P (n+) = βp (n) ( + C/I (n) P s a startng value whch s arbtrarly specfed. The optmum value for β has to be found by smulaton experments. B. Ad-hoc Algorthm The algorthm dscussed here s based on a fuzzy controller. The algorthm works on a lnk-by-lnk bass, whch means no central staton s needed; the transmt power s adjusted between neghbourng statons whch are connected by a drect lnk. Fuzzy algorthms are n partcular useful where the system whch has to be controlled s non-determnstc, whch s the case for a rado channel due to the unpredctable nterference by neghbourng networks. The fuzzness s appled n a way that there are no sharp lmts to classfy measurement parameters such as the channel qualty as good or bad, or consder the channel load as hgh or low. Instead, the boundares between the classfcatons are fuzzy, whch means that the membershp to a certan classfcaton s determned by a weghtng factor between and. An example s shown n fgure, where the C/I values of the channel are mapped to three fuzzy classes wth the attrbutes bad, medum or good channel qualty. ) (4) (5)

3 Fg.. membershp value bad medum good C/I / db Example fuzzy membershp functon for the channel qualty The recever of a sgnal evaluates the sgnal qualty (C/I) based on the fuzzy controller. The recever then reports the decson of the controller back to the sender,. e. by whch amount the sender should ncrease or reduce the transmt power. The algorthm s mplemented as a fnte state machne whch changes between the states durng the ongong controllng process: When startng the controller algorthm, t begns n the ntalsaton state where each staton transmts wth full power. The average load of the sender s transmt queue and the C/I wthn a specfed tme nterval are measured and taken as reference values for the further control process. After ths tme nterval has expred, the controller swtches to the testng state. In the testng state, the TX power s reduced to check f the neghbourng networks behave n a cooperatve way by reducng ther TX power. Durng ths process, the queue load and the C/I are observed to determne the lnk qualty. After the mnmum possble TX power s reached, the algorthm swtches to the controllng state. In the controllng state, the fuzzy control takes place. As nput values for the controller, the C/I and the load of the transmt queue are consdered. The output value,. e. the TX power, s computed accordng to the nference. If the amount of change of the TX power s close to, meanng that the controller decded to change t by a small amount, t s consdered that the power control has reached a stable state. The algorthm then changes to the montorng state. In the montorng state, the C/I s observed. If t exceeds a certan threshold for longer than a specfed tme nterval, the system returns to the controllng state. The fuzzy controllng process s performed n the controllng state. The fuzzy controller works accordng to the followng prncple: In the fuzzfcaton, sharp nput values,. e. the C/I of a connecton and the load of the sender s transmt queue, are mapped to fuzzy output values such as channel s bad, channel s good or queue load s low. From the fuzzy nput values, fuzzy output values are computed usng nferences. Such an nference can be for example: IF the queue load s hgh AND the channel qualty s poor THEN ncrease the transmt power. Due to the fuzzy nature of the processed values, there s no sharp IF THEN, however. Instead, weghtng factors are determned by whch the measurement value such as the queue load belongs to the attrbute hgh or low. These weghtng factors are termed as a degree of membershp. The output value s calculated accordng to these membershp degrees. In the defuzzfcaton, the result of the nference such as ncrease the TX power s mapped to a sharp output value,. e. a factor by whch the transmt power should be ncreased or reduced. IV. SIMULATION ENVIRONMENT The smulator mplements the IEEE 82.a protocol stack whch s extended by the TPC support,. e. the algorthms descrbed above and the sgnallng requred by the IEEE 82.h standard. An arrangement of statons where a par of two statons form a network s consdered as shown n fg. 2. In case of an network, the statons nsde a par form a BSS, n case of an network they form an IBSS. Between a par of statons, there s a bdrectonal data flow wth the same constant btrate n each drecton. The same arrangement of statons s used for the and the algorthm. In case of the operaton, the whte squares n fg. 2 denote access ponts, the black squares denote moble termnals. m 2.5m Fg. 2. m Testng scenaro (n case of network) Access Pont moble termnal data flow (bdrectonal) Both the traffc load for each network and the number of networks can be vared. The smulatons are run for two cases: The number of statons s fxed to 8, they are grouped nto 9 networks. The offered traffc load for each connecton s ncreased from 6 kbt/s to 2 kbt/s at constant btrate. Ths experment shows at whch amount of traffc load the saturaton of the network s reached, whch s the case f the achevable troughput becomes smaller than the offered traffc load. It s expected that due to the hgher spatal reuse n case of TPC, the saturaton throughput wll ncrease. In the second scenaro, an ncreasng number of networks s enabled, startng wth 2 networks. Each network ncludes two statons wth bdrectonal symmetrc traffc

4 network Fg. 3. Testng scenaro wth ncreasng number of networks (2 to 9) load as shown n fg. 2. The order n whch the networks are enabled s gven by fg. 3. The offered traffc load for each connecton s set to a fxed value of 2 kbt/s at constant btrate. Ths experment shows how the number of competng statons affects the behavour of the TPC algorthms. The two smulaton cases, ncreasng load and ncreasng number of statons, are appled both for the and for the algorthm. The antennas at the wreless statons are modeled wth sotropc radaton characterstcs. The receved sgnal power P r s calculated accordng to the equaton P recv = P transm k r α (6) where P transm s the transmtted power, k s a constant factor, r s the sender recever dstance and α s the dampng factor. The averaged values gven n the graphs are the mean values over all connectons nsde the consdered network scenaro. The smulaton parameters are set as follows: PHY mode: 6 Mbt/s recever nose: 95 dbm ntal transmt power: 5 mw (7dBm) data packet sze: 5 byte mnmum contenton wndow CW mn : 5 maxmum contenton wndow CW max : 23 dampng factor α: 3.5 V. RESULTS Fgure 4 shows the acheved throughput per connecton as a functon of the offered traffc load. It can be observed that the system wthout power control runs nto saturaton for offered traffc loads hgher than 2 kbt/s: up to ths value, the full load can be transferred, beyond ths value the acheved throughput remans slghtly above 2 kbt/s n case that no power control s used. When applyng power control, the acheved throughput s sgnfcantly ncreased, where the algorthm performs better at offered traffc loads of 8 or 2 kbt/s. When consderng the delay, t can be seen from fg. 5 that power control mproves the performance both for small and for bg loads. For hgh loads, the algorthm performs avg. acheved throughput per connecton / kbt/s --> Fg. 4. average delay / ms --> Average acheved throughput per connecton for varyng load Fg. 5. Average delay for varyng load better than the algorthm. The power control reduces the delay by at least 5% n comparson to the non-powercontrolled case. The reason for the reducton of the delay s as follows: By ntroducng power control, the statons reduce ther nterference range. Ths means that the amount of tme s reduced where the channel s busy. A staton whch attempts to transmt a number of packets can do more frequently than n the case wthout power control. In case of hgh traffc loads, ths means that each packet spends less tme nsde the transmt queue. Fgures 6 and 7 show the behavour of the system when the number of networks resp. statons s vared. It can be observed that the power control works more effectvely wth an ncreasng number of statons. In case of a small number of networks, they are located closely to each other, so that even wth the applcaton of power control, they do not get out of each other s nterference range whch reduces the effect of the power control. For a larger number of networks, power control can reduce the amount of overlappng nterference ranges so that the non-overlappng networks can perform transmssons smultaneously. Fgures 8 and 9 depct the average transmt power to whch each of the statons s adjusted. Ths measurement s n partcular meanngful consderng non-ieee 82. systems

5 avg. acheved throughput per connecton / kbt/s --> Fg. 6. Average acheved throughput per connecton for varyng number of statons average transmt power / dbm --> Fg. 8. Average TX power for varyng load average delay / ms --> Fg. 7. Average delay for varyng number of statons average transmt power / dbm --> Fg. 9. Average TX power for varyng number of statons whch share the frequency band wth the 82. systems. In contrast to the stuaton wthout power control, where each staton transmts at 7 dbm (5 mw), the TX power can be reduced here to values between and dbm. From fgure 8 t can be seen that the TX power becomes hgher wth ncreasng load snce there s more background nterference. VI. CONCLUSIONS AND OUTLOOK The examples gven n ths paper show that the proposed TPC algorthms mprove the QoS propertes of an 82. LAN network. The delay and the throughput are sgnfcantly mproved, whereas the algorthm works better than the algorthm for hgh traffc loads. The reducton of the transmt power also gves benefts for non-wlan rado systems by reducng the nose floor and the resultng nterference. In future work, the transmt power control s tested n cooperaton wth lnk adaptaton and dynamc frequency selecton. Also, the nfluence of the RTS/CTS handshakng n combnaton wth TPC on the hdden staton problem wll be nvestgated. REFERENCES [] IEEE Standard : Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons. [2] IEEE Standard 82.a-999: Hgh Speed Physcal Layer n the 5 GHz Band. [3] IEEE Standard 82.h-23: Spectrum and Transmt Power Management Extensons n the 5 GHz Band n Europe. [4] A. Könsgen, C. Görg: Performace Evaluaton of Dynamc Frequency Selecton Strateges n IEEE 82.h Based Networks. Proc. st Regonal Conference on ICT and E-Paradgms. Colombo, Sr Lanka, 24. [5] J. Rohwer, C. T. Abdallah, A. El-Osery: Power Control Algorthms n Wreless Communcatons. Proc. SPIE (AeroSense), Vol. 474, Orlando, Florda, USA, 22. [6] J. Zander: Transmtter Power Control for Co-Channel Interference Management n Cellular Rado Systems. Electrum 27, S-64 4 Stockholm- Ksta, Sweden. [7] S. A. Grandh, R. Vjayan, D. J. Goodman, J. Zander: Centralzed Power Control n Cellular Rado Systems. IEEE Trans. Veh. Tech., Vo. 42, No. 4, November 993. [8] E. Jung, N. H. Vadya: A Power Control MAC Protcol for Ad Hoc Networks. Proc. 8th Annual Internatonal Conference on Moble Computng and Networkng. Atlanta, Georga, USA, 22. [9] S. Agarwal, R. H. Katz, S. V. Krshnamurthy, S. K. Dao: Dstrbuted Power Control n Ad-hoc Wreless Networks. Proc. PIMRC 2. [] S.-J. Park, R. Svakumar: Load-Senstve Transmsson Power Control n Wreless Ad-hoc Networks. Proc. Globecomm 22. [] J. Monks, V. Bharghavan, W. Hwu: Transmsson Power Control for Multple Access Wreless Packet Networks. Proc. 25th Annual IEEE Conference on Local Computer Networks, 2. [2] J. Peetz: Multhop-Ad-hoc-Kommunkaton mt dynamscher Frequenzwahl, Lestungssteuerung und Ratenanpassung für drahtlose Netze m 5 GHz Band. (Multhop Ad-Hoc Communcaton wth Dynamc Frequency Selecton, Power Control and Rate Adaptaton for Wreless Networks n the 5 GHz Band). ABMT Vol. 34, Wssenschaftsverlag Manz, Germany, 23.