CDMA dynamic reverse link power control with variable quality of service Sunjeev Kumar gupta, Ranjit kumar karma Department Of Electronics and Computer Kathmandu Engineering College, Kathmandu, Nepal ABSTRACT For wireless communication systems, In this paper, we propose new dynamicallyiterative power control algorithms have been power control with QOS for different substreamsproposed to minimize the . For this, we study first the embedded trellistransmitter power while maintaining reliable coded modulation for UEP on which puncturedcommunication between mobiles and base convolutional codes have been applied for reliabilitystations. A digital cellular radio code- of a substreams increased through migration fromdivision multiple-access (CDMA) system can only high rate to low rate code. But we found difficultiessupport a finite number of users before the to provide trellis equivalent of variable rateinterference plus noise power density, I0, received at convolution code by varying constellation size sothe cellular base station causes an unacceptable proper solution is to perform UEP by using a fixedframe-error rate. Once the maximum interference rate trellis code & varying power. This makeslevel is reached, new arrivals should be blocked. In beneficial to transmit the minimum power necessarya power-controlled CDMA system, the base station to support the given QOS for a substream as thiscan direct mobiles to reduce their power and data creates least interference to other users.rate to reduce interference and allow more users onthe system. In current IS-95 systems, forward link 2. NECESSITY OF POWER CONTROLpower control is far less powerful than reverse linkpower control. Thus, this paper presents an All users in CDMA share the same RF bandalgorithm to focus on the current IS-95 reverse link through the use of PN codes, each user looks likepower control but in a more general sense, it random noise to other users. So power of eachpresents a systematic approach to the designing of a individual user therefore must be carefullypower control unit. In this paper, we present a controlled so that no one user is unnecessarilypower control algorithm, which simultaneously interfering with others who are sharing the sameminimizes interference & also provides variable band. Under no power control the MS nearer to theQOS contracts for different traffic types in a CDMA BS transmits higher power than the MS far from thesystem by assigning different power levels to each BS transmits lower power. This makes greatertraffic type. enjoyment to the MS nearer to the BS than others. This is the classic near-far problem in SSMA1. INTRODUCTION system. Power control is implemented to overcome A substream of the individual user (concept near-far problem & to maximize capacity by thesimilar to internet protocol defines flow headers to action of controlled transmitted power from eachsupport variable QOS across different applications) user such that received power of each user is equalconsist of one media type (audio or video).The to one other.substream abstraction enhances network efficiencyby only the appropriating more resources.Substrems are variable rate & multiplexed into oneaggregate stream for each user. Sum of substreambit rates for any user don‟t exceed total bit rate ofthat user‟s stream . Each stream then undergoeschannel coding, modulation & power control beforebeing assigned a spreading code & transmitted.Different substream consists of audio, video & dataall have different unequal error protection so thathigher efficiency is made for protecting mustsignificant bits than to least significant bits. Fig.1 Power Controlled System
One problem that has to be immediately identical to reverse path loss. So better Closed Loopsolved in power control is initial mobile transmit Power Control (CLPC) is forwarded to compensatepower which can‟t be controlled by the BS.So the for power fluctuation due to fast reyleigh fadingbest solution is the MS to attempt to transmit a involving both BS & MS. CLPC continues measuresseries of access probe i.e. a series of transmissions the link quality along with OLPC & its contributionof progressively higher power. This process is in reverse link (uplink) is as follow:continued until the BS acknowledgment & step size BS continuously monitors Eb/No onfor the access probe correction is specified by reverse link.system parameter PWR_Step. If Eb/No is too high (exceeding certain Knowing received power & ERP of BS, threshold) then BS commands MS toMS would know how much it needs to transmit decrease it‟s transmit power & vice versa.power to compensate path loss . But in reality, The power control commands are in theMS neither know ERP of BS nor received power form of power control bits & amount ofcontributed by the neighboring BS, So generic power up & down per PCB is normallyassumptions of initial power transmission of MS in +1dB or -1dB.decibels: Ptinitial=-Pr – 73 + NOM_ PWR + INIT_PWR. Since, CLPC is combated Rayleigh fading, MSWhere NOM_PWR & INIT_PWR are the response to these PC commands must be very fastadjustments factors. These adjustment factors are .So Power Control Bits (PCBS) are directly sentbroadcasted by MS in access parameter message. over traffic channel by robbing some bits from traffic channel. Fig.3 PCBS are multiplexed directly onto baseband system at 19.2 Kbps The PCBS are integrated into traffic channel by robbing selected bits from baseband Fig 2 Initial transmit power stream. The stream of PCBS at 800 bps is Power Control Sub channel (PCS). Since the rate of PCB2.1. POWER CONTROL PROCESS transmission is 800 bps, a PCB is sent once every (1/800) second or 1.25 ms. Since PCB is sent every After initial power transmission, two methods 1.25 ms, each traffic channel frame is divided intoopen loop & closed loop power control in (20 ms/1.25 ms) or 16 segments called Powerproceeded. After a call is established & as MS Control Groups (PCGS). Since each PCG is 1.25 msmoves around within cell, path loss between MS & in duration & baseband is at a rate of 19.2 Kbps thenBS will continue to change, so received power at each PCG contains (19.2 *1000)*(1.25*1000) ORMS will change & open loop power control will 24 bits.continue to monitor MS received power Pr & adjust In a closed loop section, for example BSMS transmit power. measures Eb/No in PCG7, decide in PCG8 for Pr= -Pr -73 + NOM_PWR + INIT_PWR + (sum inserting 0 or 1 & transmit decided 0 or 1 duringof all access probe correction). PCG9 on forward traffic channel. This process isOpen Loop Power Control (OLPC) is used to repeated for every power control group in the framecompensate for slow-varying & log-normal . The PCG can be inserted in any one of 1 st 16shadowing effects but inadequate to compensate fast positions. The exact location of PCB in PCG isRayleigh fading coz of frequency dependency & determined by decibel value of four most significantworks under assumption of forward path loss is bits of decimator output.
Fig 4 Closed loop power control using PCBSClosed loop power control has inner loop & outerloop. Fig 5 Schematic CLPCInner loop decides the power up & down decisionby threshold decision. Outer loop makesdynamically adjusted to maintain an acceptanceFER. This CLPC is also assisted by the soft handoffprocess.3. PROPOSED CONTROL SYSTEM Fig 6 Reverse link PC functions carried out by BS. The reverse link power control with the On the MS side, it receives forward linkmultiple co-ordinations of substreams leading the signal. It recovers PCB & based on PCB, makes avariable quality of service scheme is shown a below. decision (closed loop decision) to power up/downThe fig. 6 shows high level schematic of the system by (1 dB).This correction is combined with openconsidered. The subsystems for each user are loop terms & combined result is fed to transmitterstatically multiplexed into one stream (How this is so that it can transmit at the power (proper) level.done in accordance with the substreams‟ differentdelay bounds will not be described in thispaper).The stream then undergoes channel coding,modulation & power control before being assigned acode & transmitted. BS demodulates & estimateFER of reverse link, this information on reverse linkframe quality is fed into threshold which adjusts(Eb/No) based on received frame quality. The PCBare multiplied onto forward traffic channel &transmitted to MS .
- If feasible, how do we allocate power to each substream? - Hoe do we decide if we can admit a new stream without violating the reliability guarantees for streams in progress? In this paper we don‟t consider the reliability requirement of a substream by its desired E/I (remains for future work) Let K= No of substreams. (Eb/No)= E/I required by substream I, i= 1, 2……K. βi = 1, if substream I is transmitting during the current time slot 0, otherwise xi = Power assigned to substream I given that it is transmitting during current time slot P = Total Power N = Spreading Code Processing Gain σi2 = Intercell interference experienced by stream i. The E/I experienced by substream „i‟ is given by the expression Nxi E/I= ………Eq. (1) ( i k xk ) k i We wish to minimize total power subject to constraints that E/N for every substream is satisfied i.e. minimize k P k xk ……….Eq. (2) k 1 such that Nxi ( i k xk ) ≥ E N b oi ..........Eq. (3) k i For i=1....k xi ≥0 and (Eb ) >0 ............Eq. (4) N The Eq. (4) can be expressed in the form of linear program matrix asFig.7 Reverse link Power Control functions carried 1 N - Eb No 1 2 - Eb N o1 k out by MS. A= - Eb No 2 1 1 N - E b N o2 k 4. POWER CONTROL ALGORITHM Before PC, each of the substrams has its Eb - No k 1 - Eb No 2 k 1 N own desired reliability requirement, we wise to .........Eq. (5)address three issues : - How to determine if the set of requirement isfeasible or not?
xi 1 Eb 2 No 1 K.S. Gilhousen et al, “On the capacity of a cellular CDMA system,”IEEE Trans. VehicularX= , b= , Technology, vol. 40, no. 2, pp. 303-312, x k 2 Eb 1 No k May1991. Theodore S. Rapport,” Wireless Communication, Principles & Practice,”2nd Edition, Published inc = 1 k .........Eq. (6) New Delhi, 2005.The above equations are modified as: Samuel C. Yang,” CDMA RF SignalMinimize cx such that Engineering,” London, ISBN 0-89006-991- Ax ≥ b, x ≥ 0 .......Eq. (7) 3.1998.The Eq. (7) is solved optimally for finding closed Qualcomm Inc., “Compatibility Standard forform optimal solution to the system. Dual-Mode Wideband Spread Spectrum CellularA feasible solution to the system Ax=b, x ≥ 0 is System,” TIA/EIA/IS- 95, July 1993.obtained by considering following assumptions: Google search as www.power control algorithm xi = i i i2 P ......Eq. (8) class based power control algorithm in cdma.html.Whereα= E N , i=1....k b N E o i ......Eq. (9) b N oiAnd total transmitted power k k k k2 P= k 1 ......Eq. (10) k 1 k k k 1From Eq. (10), 0≤ P≤ ∞ if and only if k k k < 1 ......Eq. (11) k 1If Eq.(11) holds true, then x ≥ 0 and Eq. (8) & (9)represent a finite.Now from Eq.(1) & (11), under comparison , βk isconstant integer so, x<<1≈ 0 provides uniquesolution to the system [Ax ≈ 0].5. CONCLUSION Dynamically PC algorithm performs thetask of closed loop power control as well as QOSimprovement Substream concept enhances networkefficiency as well as combating of both near farproblem & self -jamming/anti-jamming problem. Itreduces the MS transmission power & the capacityof the system enhances but system becomes morecomplex in the case of substream division.6. REFERENCESResearch documents from University of California at Berkeley, Berkeley, CA94720 published by Louis C. Yun and David G. Messerschmitt.