Transcript ECE4730_L22
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 Same system interference (ACI, CCI, etc.) limit cellular capacity Cellular capacity is related to QoS and revenue $$ for service provider ECE 4730: Lecture #22 1 Multiple Access Methods Duplexing “Simultaneous” 2-way communication FDD : Frequency Division Duplexing Two separate SX frequency channels for each user Frequency Separation FVC RVC f 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 2 Multiple Access Methods TDD : Time Division Duplexing Two separate time slots (forward & reverse for each user on same frequency channel FVC Time Separation RVC t 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 3 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, Verizon) ECE 4730: Lecture #22 4 Multiple Access Methods ECE 4730: Lecture #22 5 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 6 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 7 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 8 Multiple Access Methods FDMA : Frequency Division Multiple Access U.S. AMPS 1G System » # FDMA/FDD channels = Bt 2Bg Bs Nc 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 9 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 10 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 11 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 needed for proper timing Efficiency % of information data bits overhead, control, & timing bits hf as opposed to » Doesn’t include coding actual efficiency (info. data) < hf ECE 4730: Lecture #22 12 Multiple Access Methods TDMA : Time Division Multiple Access Frame Efficiency boh 100% h f 1 bT where boh = # overhead bits/frame bT = # total bits/frame # TDMA channels = m Bt 2 Bg Bs where m = max. # users/channel ECE 4730: Lecture #22 13 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 14 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 and between 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 15