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Understanding CDMA Power Control and SDMA Techniques in Wireless Communication

This lecture explores the intricacies of Code Division Multiple Access (CDMA) and Space Division Multiple Access (SDMA) methods in wireless communication systems. Key topics include the conversion of narrowband signals to wideband using Direct Sequence Spread Spectrum (DS-SS), the role of unique PN codes for user identification, and the significance of power control mechanisms to mitigate near/far problems and interference from adjacent cells. The discussions also highlight the performance improvements achieved through advanced antenna technologies and multi-beam adaptive systems in SDMA.

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Understanding CDMA Power Control and SDMA Techniques in Wireless Communication

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  1. Multiple Access Methods • CDMA : Code Division Multiple Access • PN sequence converts narrowband signal to wideband signal  Direct Sequence-Spread Spectrum (DS-SS) • Many users with unique PN codes share same RF channel • As # users  system BW efficiency  but also the PN cross-correlation noise floor  • Perfect power control need to keep PN noise from single user from dominating base station Rx for all other users  near/far • Strongest mobile (near to base) can “capture” base Rx • Much more significant problem than ACI in other MA methods b/c one mobile can affect all users in a given cell ECE 4730: Lecture #23

  2. CDMA Power Control • Power control done by each cell base station (not MSC)  • Want each mobile to contribute  same level of power @ base Rx regardless of distance from base • ** Mobiles in adjacent cells controlled by different base stations ** • Source of additional PN noise • Mobiles at adjacent cell edge are using max Tx power ! • Book discusses/advocates not using 1 cell frequency reuse (N= 1!!) to combat this problem • Can’t have macroscopic reverse link diversity (soft-handoff) if N > 1 !! ECE 4730: Lecture #23

  3. CDMA Power Control • All CDMA operators (Sprint PCS, Verizon Wireless, etc.) use N = 1 along with significant overlap with adjacent cells (needed for soft handoff!!) • Power control in overlap area done by multiple base stations with logic rules in mobile Rx • PU & PU & PD = PD  PU : power up PD : power down • Done to minimize effect of PN noise from mobiles in adjacent cells ECE 4730: Lecture #23

  4. CDMA Cell Overlap • Significant cell overlap can lead to : • More base stations/area  $$  • Large number of base stations + multipath signals serving mobile unit • If # serving signals > 3 (finger # in RAKE Rx) then major source of forward link interference at mobile Rx • Also # blocked calls  if multiple base stations serving same mobile • Adjacent cell interference on forward link (BS to MS) • Not Gaussian (AWGN)  not “noiselike” • No forward link power control coordination between BSs • Reduced forward-link capacity compared to reverse-link capacity ECE 4730: Lecture #23

  5. Multiple Access Methods • CDMA Features:  1)  CDMA Channel • Large number of users share same channel • Assigned individual PN identifying codes • Utilization limited by interference from other users (same cell and adjacent cell) • Gradual performance decline as # users increase  soft limit • Other users produce Rx noise after despreading 2) Spread Signal BW >>> Information BW • Frequency diversity reduces effect of multipath fading • No equalization required • Low battery drain b/c low peak power (Tx power is spread) ECE 4730: Lecture #23

  6. Multiple Access Methods • CDMA Features: 3) Macroscopic Spatial Diversity • MSC monitors multiple base stations for same mobile signal • Choose best version at any instant in time to pass to PSTN • “Soft” handoff • Can be signficant interference source on forward link if # base stations + multipath > finger # in RAKE Rx 4) Time Diversity As Well ! • High chip rate  different multipath signals will be independent • RAKE Rx diversity combines delayed multipath signals to improve SNR at mobile ECE 4730: Lecture #23

  7. Multiple Access Methods • CDMA Features: 5) Power Control • Essential for minimizing near/far problem within a cell • Must be perfect to achieve capacity advantages • Forward and reverse link power control needed • Forward link power control in 3G CDMA standards 6) Adjacent Cell Mobiles • Co-channel interference source for neighboring cells • Power not controlled by neighboring base station! • Max. interference source on boundary between cells • Some frequency reuse is usually required to minimize interference ECE 4730: Lecture #23

  8. Multiple Access Methods • SDMA : Space Division Multiple Access • Direct radiated energy to each individual users in space • Use adaptive multi-beam antenna arrays • Multi-beam  serve different groups of users by location • Adaptive  must follow mobile units • Cell Sectoring  primitive non-adaptive form of SDMA ECE 4730: Lecture #23

  9. SDMA Omni Pattern 3-Sector Pattern Adaptive Spot Beams ECE 4730: Lecture #23

  10. Multiple Access Methods • How does SDMA improve performance? • Reverse link performance usually most critical • Limited mobile Tx power + battery life • Perfect power control not possible  near/far problem & ACI • CCI at base Rx from co-channel Tx (tall BS antennas) • Use narrow Rx (not Tx) antenna beam to focus in on mobile users • Increases received energy @ base from mobile • Decreases ACI & CCI from all other directions by significant amount (10-15 dB) • Acts as spatial “filter” • Less Tx power required from mobile  longer battery life! ECE 4730: Lecture #23

  11. SDMA • Antenna + DSP Technology • Antenna array (many individual antenna elements) required to have : • Multiple beams with focused capability • Adaptive  change pattern width & direction vs. time • Requires significant DSP solutions • Only suitable for base station b/c of size (array) and computational load (DSP) constraints • Best suited for TDMA & CDMA systems • SDMA antenna technology is currently being used for 2G/3G systems (CDMA & TDMA) • Further increase capacity of existing systems ECE 4730: Lecture #23

  12. Mutiple Access Methods 1G 2G 2G+ 2G - Obsolete GSM-GPRS 2G - Obsolete 2G 2.5G 3G EDGE 3G ECE 4730: Lecture #23

  13. Cellular System Capacity • Cellular System Capacity • Channel capacity (C)  max. # channels or users (@ specific GOS) provided by a system in a fixed frequency band & specific geographic area • Depends primarily on: • Frequency reuse (N) and channel BW (Bc) • Radio capacity (m)  measure of the overall spectrum efficiency of a system • Depends on : • Bc : channel BW (not coherence MRC BW) • C/I : carrier-to-interference ratio ECE 4730: Lecture #23

  14. Cellular System Capacity • Radio Capacity (m) • For hexagonal geometry : • Allows comparisons between systems & standards with different Bt, Bc, and (C/I)minspecifications !! • Digital systems can tolerate lower(C/I)minthan analog & still produce better voice quality (radio channels/cell) where n : propagation constant Bt : total spectrum ECE 4730: Lecture #23

  15. Cellular System Capacity • TDMA Capacity • Improved performance at (C/I)min~ 10 dB !! (vs. 18 dB for AMPS) • Error correction, speech coding, & MAHO improve link budget and performance at lower (C/I) • Lower (C/I) more interference  smaller cluster size (N)  larger frequency reuse/area  larger capacity • TDMA vs. AMPS Table 9.3, pg. 474 ECE 4730: Lecture #23

  16. Cellular System Capacity ECE 4730: Lecture #23

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