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Multiple Access Methods

Multiple Access Methods. Primary Goal : maximize the # users sharing limited frequency spectrum while at the same time providing quality communications with reasonable system costs. For any type of cellular system the service quality  as the # users  ACI and CCI limit user capacity

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Multiple Access Methods

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  1. Multiple Access Methods Primary Goal : maximize the # users sharing limited frequency spectrum while at the same time providing quality communications with reasonable system costs. • For any type of cellular system the service quality  as the # users  • ACI and CCI limit user capacity • User capacity is related to QoS and revenue $$ for service provider ECE 4730: Lecture #22

  2. Frequency Separation FVC RVC f Multiple Access Methods • Duplexing • “Simultaneous” 2-way communication • FDD : Frequency Division Duplexing • Two separate SX frequency channels for each user • Duplexer : device that allows Tx & Rx to operate simultaneously using same antenna • Desire large frequency separation for good Tx/Rx isolation • Frequency separation is a fixed or constant value • Does NOT depend on channel # ECE 4730: Lecture #22

  3. Time Separation FVC RVC t Multiple Access Methods • TDD : Time Division Duplexing • Two separate time slots (forward & reverse for each user on same frequency channel • Time separation must be small (~ 10 msec) for real-time services (e.g. voice) so that service appears to be continuous • Simpler mobile unit Tx/Rx since duplexer not needed • Antenna is shared in time by Tx/Rx • Duplexer is complicated & expensive component • TDD/FDD hybrid  many users share FVC frequency using TDD and different RVC frequency also using TDD  GSM ECE 4730: Lecture #22

  4. Multiple Access Methods • Multiple Access (MA) Methods • FDMA, TDMA, & CDMA  3 major types • Many MA schemes are hybrids of these basic types • Narrowband vs. Wideband • Signal BW (Bs) < channel BW (Bc)  narrowband • Large # of narrow frequency channels  FDMA (e.g. AMPS) • Bs >> Bc wideband • Multipath fading only affects small % of signal frequency content • no equalization needed • Large # users share same channel  CDMA (Sprint PCS) ECE 4730: Lecture #22

  5. Multiple Access Methods ECE 4730: Lecture #22

  6. Multiple Access Methods • FDMA : Frequency Division Multiple Access • 1 user/channel with FDD channel pair  • FDMA Features :  1) FDMA channel • Carries only one user at a time • Relatively narrow ~ 30 kHz for AMPS • Idle when not in use  wasted resource 2) Base station and mobile transmit simultaneously and continuously 3) Narrowband system usually • Only way to support large # of users in limited spectrum  4) Normally use Analog FM for mobile communications ECE 4730: Lecture #22

  7. Multiple Access Methods • FDMA : Frequency Division Multiple Access  • FDMA Features (continued) :  5) FDMA Mobile Systems • Low complexity compared to TDMA or CDMA • Fewer overhead bits in digital systems • Less timing and synchronization 6) Higher Cell Site Costs (relative to TDMA) • Lower channel utilization (1 channel/user) • Rejection of ACI requires expensive BPF in base station • Duplexers needed so Tx and Rx can use antenna simultaneously 7) Adjacent Channel Interference • Significant interference source  need good BPF @ base station • Power amplifiers are non-linear and cause : • spectral regeneration of modulation sidebands •  increased ACI ECE 4730: Lecture #22

  8. Multiple Access Methods • FDMA : Frequency Division Multiple Access  • Non-linear power amplifiers • Spectral broadening  increased ACI • Non-linear Tx emission byproducts  intermodulation (IM) • Same-system interference (ACI) • Out of band interference  system -to-system • U.S. AMPS  1G System • Narrowband analog FM • FDMA/FDD • 45 MHz separation between FVC & RVC   ECE 4730: Lecture #22

  9. Multiple Access Methods • FDMA : Frequency Division Multiple Access  • U.S. AMPS  1G System • # FDMA/FDD channels = • Bt : total spectrum allocation • Bs : signal BW (specification) • Bg : guard band which provides separation to minimize ACI to other provider • For AMPS Bt = 12.5 MHz Bs = 30 kHz Bg= 10 kHz & Nc = 416  (21 control and 395 voice channels) ECE 4730: Lecture #22

  10. Multiple Access Methods • TDMA : Time Division Multiple Access   • Divide radio channel into N time slots for one “frame” of data to support multiple users/channel ECE 4730: Lecture #22

  11. Multiple Access Methods • TDMA : Time Division Multiple Access   • TDD  mobile & base share (50/50) same channel • Not widely used • Used in some digital cordless phone (Table 9.1) • FDD  different channels with multiple users for forward & reverse links • Most widely used • Several time slot delay between forward & reverse link! • Mobile unit not simultaneously using Tx/Rx •  no duplexer required • Simple Tx/Rx design ECE 4730: Lecture #22

  12. Multiple Access Methods • TDMA : Time Division Multiple Access • Frame structure • Depends on wireless standard (GSM, USDC, etc.) • Preamble • Control, setup, MIN, & synchronization (timing) • Not all standards have this! (e.g. GSM) • Guard bits  need for proper timing • Efficiency  % of information data bits as opposed to overhead, control, & timing bits hf • Doesn’t include coding  actual efficiency (info. data) < hf ECE 4730: Lecture #22

  13. Multiple Access Methods • TDMA : Time Division Multiple Access • Frame Efficiency where boh= # overhead bits/frame bT = # total bits/frame • # TDMA channels = where m = max. # users/channel ECE 4730: Lecture #22

  14. Multiple Access Methods • TDMA : Time Division Multiple Access  • TDMA Features:  1) TDMA channel • Multiple users share same channel • Non-overlapping time slots in frame structure • User data communication done in periodic bursts • must use digital modulation 2) Lower battery consumption than FDMA • Mobile Tx is off most of the time (e.g. ~ 7/8 for GSM) 3) Mobile Assisted Handoffs • Mobile Rx monitors multiple base stations during idle time slots • Relay information back to MSC for handoff execution 4) Tx and Rx of mobile unit never on at same time • No duplexer required • Simple Tx/Rx switch  less costly mobile architecture ECE 4730: Lecture #22

  15. Multiple Access Methods • TDMA : Time Division Multiple Access  • TDMA Features:  5) Higher data rates relative to FDMA • Bs is sometimes > Bc • Equalization often required • Training bit overhead 6) Timing is critical • Synchronization between base and mobile andbetween Tx and Rx • Timing data bits required in each slot (many slots/frame) • Guard bits to separate users (limit multipath impact) 7) Bandwidth on demand: • Allocate multiple time slots per frame for a higher rate user ECE 4730: Lecture #22

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