Multiple access techniques

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1 Multiple access techniques Narrowband and wideband systems FDMA TDMA CDMA /FHMA SDMA Random-access techniques Summary Wireless Systems 2015 Narrowband and wideband systems Coherence BW B coh 1/σ τ σ τ - rms delay spread σ 2 τ 2 2 = E(τ ) E (τ) Received Power Power-delay proile (PDP) Narrowband: B S << B coh and T S >> σ τ lat ading channel -mostly operated in FDD (ull duplex) and well suited or FDMA and OFDMA Wideband: B S > B coh and T S < σ τ requency selective ading requires equalization, but total Rx power not much aected, well suited or sharing by TDMA or CDMA; also better suited or TDD (hal duplex) 2 1

Real PDP /indoor/ 3 FDMA, TDMA, and CDMA N channels in band Users_operate simultanously on individual channels User_1 User_2 User_3 N time slots (time rame) t t t t t t Users_share the channel

2 Real PDP /indoor/ 3 FDMA, TDMA, and CDMA N channels in band Users_operate simultanously on individual channels User_1 User_2 User_3 N time slots (time rame) t t t t t t Users_share the channel in dierent time slots User_1 User_2 User_3 User_1 User_2 User_3 N orthogonal codes in band C1 C2 C3 User_1 User_2 User_3 Users_operate simultanously and share the channel using individual codes 4 2

3 FDMA Each channel accommodates one link Continuous transmission Usually narrowband system with FDD or TDD T S >> σ τ Demands narrowband ilters to combat adjacent channel intererence Guards between channels required Vulnerable to IMD and HD FDD needs duplexers Seldom used as true FDMA, rather a base line or all other MA schemes 5 TDMA System bandwidth B = M t, Time rame = N t Each carrier shared by several users FDMA/TDMA Data transmission occurs in bursts Higher data rates than in true FDMA Synchronization critical hardware overhead T S < σ τ requires equalization Dierent time slots or up/down link even with FDD Idle slots useul or hand-o procedures More time slots can be assigned to a user 6 3

4 TDMA example GSM: up-link /down-link 25 MHz band channel BW = 200kHz shared by 8 users N = 25 MHz / 200kHz 8 = 1000 users, but because o req. reuse actor N < 250 users/cell Data rate: 270 kbps, 156 bits/slot T slot = 156/270k = 577 µs T rame = 8 T slot = ms Trail bits Sync. bits Inormation data bits Guard bits t Trail: 6bits, training: 26bits, data: 2 58 bits, guard: 8 bits Time slot: = 156bits with = 40 overhead bits Frame eiciency = 1- (8 40)/(8 156) = 0.74 = 74% 7 CDMA Direct sequence CDMA 1 bit period BB data Code sequence (chip) Data encoded Coding sequences or dierent users are orthogonal (e.g. Walsh, Barker), signals overlap in requency band and in time. User 1 User 2 Encoding ( spectrum spreading ) Resistant to ading Signal 1 Signal 2 Decoding or User 1 ( spectrum despreading by correlation) Signal 1 Signal 2 Noise alike 8 4

5 CDMA (cont d) Channel sharing /FDD/TDD possible FDMA as base line Wideband B SS /B BB = R chip /R BB T chip << σ τ requires equalization Sot capacity limit, N then SNR or all users, ends-up with sel jamming Near-ar problem requires strict power control 9 CDMA capacity estimation CDMA up-link, one cell K users, same power P R at BS rom each Interering power: (K-1)P R spectral density: (K-1) P R /B SS Received signal energy per bit: E b = P R /R Necessary E b /N 0 = (P R /R)/[(K-1) P R /B SS ] = (B SS /R)/(K-1) K = (B SS /R)/(E b /N 0 ) + 1 Example: E b /N 0 = 5 (7dB), R = 10 kb/s and B SS = 1.25 MHz K = 26 users/cell in 25 MHz band 25/ = 520 users (ideally) 10 5

6 FHMA Frequency-hopping MA BB data 3 FH also spreads spectrum Code sequence o a user Frequency Synthesizer 2 1 t 1 t 2 time FHMA more immune to strong intererers than DS CDMA, since it is similar to FDMA Example: Bluetooth, 1600 hops/sec among 79 channels with 1MHz spacing 11 FH/DS MA DS CDMA is narrowband but makes hops in pseudo-random ashion. Near-ar problem is avoided. Not suited or hand-o scenarios diicult to synchronize with BS 12 6

7 SDMA SDMA always put on top o the other MA techniques SDMA relies on smart antennas improves range improves t C/I ratio allows denser requency reuse, thus higher capacity BS 13 Random access Packet Radio Many subscribers attempt to access a single channel in an uncoordinated (or minimally coordinated manner) Collision rom simultaneous transmissions are prevented by BS using hand shaking protocol Easy to implement but suer rom delays and low spectral eiciency Useul e.g. in satellite systems Example: ALOHA contention technique send any time, then wait or ACK/NACK signal; when ailed, retransmit the packet 14 7

8 Throughput o Packet Radio R No o packets per second = λτ Ave packet duration To avoid channel overloading R < 1 Normalized throughput: T = R Pr( no collision ) (no collision) = (successul transmission) npackets are generated during time τwhen the channel can be busy. n λτ (λτ) e Pr( n) = n! Pr( no collision ) = Pr( n) = e Poisson distribution n= 0 λτ 15 Throughput o Packet Radio (cont d) Packet B Packet C Pr( no collision ) = e 2λτ τ t 0 Packet A Vulnerable time: (t 0 -τ, t 0 +τ) 2τ (pure ALOHA) (double packet duration) τ time T = λτ e 2λτ Users can transmit at any time T = λτ e λτ Pure ALOHA Slotted ALOHA Users can only transmit within well deined time rames (slots which it the packet time) so the vulnerable time is τ. Slotted ALOHA achieves more throughput (less collisions) but suers rom more delay (has to wait). Other systems: CSMA (listen to the channel) - more 16 8

9 Summary Base line to all systems is FDMA FDMA itsel not lexible and narrow-band when stand-alone Other MA techniques put on top o FDMA, systems can be wideband, special case OFDMA Require synchronization and equalization - hardware eort Support by FDD or TDD 17 9

Multiple Access Techniques

Multiple Access Techniques Instructor: Prof. Dr. Noor M. Khan Department of Electrical Engineering, Faculty of Engineering, Mohammad Ali Jinnah University, Islamabad Campus, Islamabad, PAKISTAN Ph: +92