Power Control and RAKE Receiver - Iowa State University

IS-95 Standard-Power Control and RAKE ReceiverRobert VoleskyIowa State University EE 521Robert Volesky 29 Nov 001

Overview• IS-95 standard for cellular• Mobile propagation environment• Power control in IS-95• RAKE receiver in IS-95Iowa State University EE 521Robert Volesky 29 Nov 002

IS-95 Standard• Direct Sequence CDMA• Developed by Qualcomm to replace IS-54 TDMAstandard in early 1990’s• TIA (Telecommunications Industry Association)TR45.5 committee• Features– 42-bit maximal-length sequence– Universal frequency reuse, cells, and sectoring (N=1)– RAKE receivers– Low average power, longer battery lifeIowa State University EE 521Robert Volesky 29 Nov 003

IS-95 StandardBandwidthChip RateFrequency band uplinkFrequency band downlinkFrame lengthSoft handoverPower controlNumber of RAKE fingersSpreading codes1.25 MHz1.2288 Mc/s869-894 MHz1930-1980 MHz824-849 MHz1850-1910 MHz20 msYesUplink: open loop + fast closed loopDownlink: slow quality loop4Walsh+Long M-sequenceIowa State University EE 521Robert Volesky 29 Nov 004

Power Control in IS-95• Need for power control• Reverse link (Mobile-to-Base)– Open loop–Closed loop• Forward link (Base-to-Mobile)Iowa State University EE 521Robert Volesky 29 Nov 006

Need for Power Control in IS-95• All CDMA signals interfere with one another– Near-orthogonal codes make other users appear asinterference noise at the receiver– High transmitted power levels of mobiles near the baseraise noise level at base (Near-far problem)• Multipath fading channel• Prolong battery life– Smallest level possible to maintain quality– Maximize capacity and increase talk-timeIowa State University EE 521Robert Volesky 29 Nov 007

Reverse Link Open Loop Power Control• Used during mobile’s access attempts• Determine power level to transmit “accessprobe” based on total power received frombase station(s)• Update power level if probe unsuccessful• Wait random time before transmitting next“access probe”Iowa State University EE 521Robert Volesky 29 Nov 008

Reverse Link Open Loop Power ControlIowa State University EE 521Robert Volesky 29 Nov 009

Reverse Link Closed Loop Power Control• Begins once access is given to mobile• Separation of forward and reverse link by 45 MHz– Reverse channel characteristics are not stronglycorrelated to forward channel characteristics• Tight power control needed– Power level updated at 800 bps (1.25ms)– Attempts to maintain E b /I t at base– Power control subchannel uses puncturing techniquesto transmit ‘1’ or ‘0’ to mobile to raise or lower itspower levelIowa State University EE 521Robert Volesky 29 Nov 0010

Reverse Link Closed Loop Power ControlIowa State University EE 521Robert Volesky 29 Nov 0011

Forward Link Power Control• Slow quality loop– Not as critical to have tight power control onforward link• Based on frame error rates at mobileIowa State University EE 521Robert Volesky 29 Nov 0012

Forward Link Power ControlIowa State University EE 521Robert Volesky 29 Nov 0013

Time Diversity and RAKEReceiver• Time diversity combines replicas of transmittedsignal to improve signal quality• In DS-CDMA, multipath creates identical versionsof transmitted signal• Due to low correlation of time delayed signals(appear as noise), IS-95 combines the delayedsignals together using a RAKE receiver• Necessary to have an adaptive RAKE receiver– Continuously estimates the power and delay ofmultipath signalsIowa State University EE 521Robert Volesky 29 Nov 0014

RAKE ReceiverIowa State University EE 521Robert Volesky 29 Nov 0015

IS-95 RAKE Receiver• Chipping rate of 1.2288 Mcps allows amultipath resolution of 0.814 μs• Forward link – 3-finger RAKE receiver• Reverse link – 4-finger RAKE receiver• “Searcher Receiver” programmed to finddelay and amplitudes of multipath signals• Used for communication with more thanone base station in soft handoffIowa State University EE 521Robert Volesky 29 Nov 0016

Conclusions• Use of RAKE receiver together with tightpower control gave Qualcomm a solution tosome of the problems faced by DS-CDMA• IS-95 became a successful cellular standardIowa State University EE 521Robert Volesky 29 Nov 0017

References• [1] QUALCOMM Incorporated. Answers to Questions for all Presenters. 1992.• [2] Steele, Raymond. “The Evolution of Personal Communications.” IEEE Personal CommunicationsMagazine. Second Quarter 1994, Volume 1, Number 2. Online. Accessed on 10 May 2000.http://www.comsoc.org/pubs/surveys/steele/steele-orig.html• [3] Prasad, Ramjee and Tero Ojanperä. “An Overview of CDMA Evolution toward Wideband CDMA.” IEEECommunications Surveys. 1998 Online. Accessed on 7 May 2000.http://www.comsoc.org/pubs/surveys/4q98issue/prasad.html• [4] Rappaport, Theodore S. Wireless Communications: Principles and Practice. Upper Saddle River, NJ:Prentice Hall. 1996.• [5] QUALCOMM Incorporated. “Presentation of CDMA to the 1 st CTIA Wideband/Spread Spectrum OpenForum.” San Diego. 17 March 1992.• [6] Larson, L.; Asbeck, P.; Hanington, G.; Chen, E.; Jayamaran, A.; Langridge, R.; Xuejun Zhang. “Device andcircuit approaches for improved wireless communications transmitters.”IEEE Personal Communications, Volume: 6 Issue: 5, Oct. 1999. Pages 18–23.• [7] “Power Control Methods: The Mobile Environment.” Online. Accessed on 20 Nov 2000.http://www.amug.org/~ahmrphd/PowerControl.html• [8] Garg, Vijay K. IS-95 CDMA and cdma2000. Upper Saddle River, NJ: Prentice Hall. 2000.• [9] Nikolai, D.; Kammeyer, K.-D. “Noncoherent RAKE-receiver with optimum weighted combining andimproved closed-loop power control.” Spread Spectrum Techniques and Applications Proceedings, 1996,Volume: 1, 1996. Pages: 239 –243.• [10] Donegan, Patrick. “IS-95 CDMA Becomes a World Standard.” CDMA Spectrum. CDMA One, June1997. Online. Accessed on 10 May 2000. http://www.cdg.org/library/spectrum/Jun97/article4_int.htmlIowa State University EE 521Robert Volesky 29 Nov 0018

Questions?Iowa State University EE 521Robert Volesky 29 Nov 0019