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Multiple Access Methods

Multiple Access Methods. CDMA : C ode D ivision M ultiple A ccess PN sequence converts narrowband signal to wideband signal  Direct Sequence-Spread Spectrum (DS-SS) Many users with unique PN codes share same RF channel

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Multiple Access Methods

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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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