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Multiple Access Techniques Content Multiple Access Techniques Dr. Francis LAU Dr. Francis CM Lau, Associate Professor, EIE, PolyU 1 Content • Introduction • Frequency Division Multiple Access • Time Division Multiple Access • Code Division Multiple Access Dr. Francis CM Lau, Associate Professor, EIE, PolyU 2 1 Introduction • multiple access – techniques allowing users to share simultaneously a finite amount of radio spectrum • duplexing – two-way communications to occur simultaneously Dr. Francis CM Lau, Associate Professor, EIE, PolyU 3 Introduction • frequency division duplexing (FDD) – frequency separation between each forward and reverse channel is constant throughout the system, regardless of the particular channel being used Dr. Francis CM Lau, Associate Professor, EIE, PolyU 4 2 Introduction • time division duplexing (TDD) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 5 Introduction • narrowband systems – signal bandwidth is comparable to the coherence bandwidth of the channel – radio spectrum divided into a large number of narrowband channels – usually operated using FDD • frequency separation as large as possible to minimize interference – TDD also possible Dr. Francis CM Lau, Associate Professor, EIE, PolyU 6 3 Introduction • wideband systems – signal bandwidth is much larger than the coherence bandwidth of the channel • what type of fading occurs? – TDMA allocates time slots to many users on the channel and allows only one user to access the channel at any one time – CDMA allows all users to access the channel at the same time – work with both FDD and TDD Dr. Francis CM Lau, Associate Professor, EIE, PolyU 7 Introduction Dr. Francis CM Lau, Associate Professor, EIE, PolyU 8 4 Frequency Division Multiple Access (FDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 9 FDMA • each user is assigned a unique frequency band or channel • no other user can share the same channel during the period of the call • in FDD systems, a channel consists of a frequency pair is assigned – one frequency for forward channel – one frequency for reverse channel Dr. Francis CM Lau, Associate Professor, EIE, PolyU 10 5 FDMA • each FDMA channel has a relatively narrow bandwidth because only one user is being supported • symbol time is large compared to the average delay spread – what type of fading occurs? • lower complexity and lower data rate compared with TDMA • fewer bits needed for overhead compared with TDMA •… Dr. Francis CM Lau, Associate Professor, EIE, PolyU 11 FDMA • nonlinear effects – many channels share the same antenna at the base station – power amplifiers or power combiners are nonlinear when operating at or near saturation for maximum power efficiency – nonlinearities cause intermodulation (IM) which can interfere with other channels Dr. Francis CM Lau, Associate Professor, EIE, PolyU 12 6 Example 9.1 • Find the intermodulation frequencies generated if a base station transmits two carrier frequencies at 1930MHz and 1932MHz that are amplified by a saturated clipping amplifier. If the mobile radio band is allocated from 1920MHz to 1940MHz, designate the IM frequencies that lies inside and outside the band. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 13 Solution 9.1 • Intermodulation distortion products occurs at frequencies mf1+nf2 for all integer values of m and n. Some of the possible IM frequencies that are produced by a nonlinear device are –(2n+1)f1 –2nf2, (2n+2)f1 –(2n+1)f2, (2n+1)f2 –2nf1, (2n+2)f2 –(2n+1)f1 required signals Dr. Francis CM Lau, Associate Professor, EIE, PolyU 14 7 Time Division Multiple Access (TDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 15 TDMA • divide the radio spectrum into time slots • only one user is allowed to either transmit or receive in each time slot • N time slots comprise a frame • data transmitted in a buffer-and-burst method • noncontinuous transmission Dr. Francis CM Lau, Associate Professor, EIE, PolyU 16 8 TDMA Dr. Francis CM Lau, Associate Professor, EIE, PolyU 17 TDMA • mobile assisted handoff (MAHO) can be performed by a subscriber by listening on an idle slot in the TDMA frame • possible to allocate different number of time slots per frame to different users (e.g. GPRS) • higher transmission rate gives rise to a signal bandwidth larger than the coherence bandwidth of the channel – What type of fading occurs? • larger overheads compared with FDMA •… Dr. Francis CM Lau, Associate Professor, EIE, PolyU 18 9 TDMA • Efficiency – frame efficiency: percentage of bits per frame that contain transmitted data • information rate/transmission rate Dr. Francis CM Lau, Associate Professor, EIE, PolyU 19 Example 9.3 • Consider GSM, which is a TDMA/FDD system that uses 25MHz for the forward link, which is broken into radio channels of 200kHz. If 8 speech channels are supported on a single radio channel, and if no guard band is assumed, find the number of simultaneously users that can be accommodated in GSM. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 20 10 Solution 9.3 • number of simultaneously users that can be accommodated in GSM N = (25MHz/200kHz) x 8 = 1000 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 21 Example 9.4 • If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data are transmitted at 270.833 kbps in the channel, find • (a) the time duration of a bit, • (b) the time duration of a slot, • (c) the time duration of a frame, • (d) how long must a user occupying a single time slot wait between two successive transmissions. Dr. Francis CM Lau, Associate Professor, EIE, PolyU 22 11 Solution 9.4 • If GSM uses a frame structure where each frame consists of eight time slots, and each time slot contains 156.25 bits, and data are transmitted at 270.833 kbps in the channel, find • (a) the time duration of a bit Tb = 1/270.833 kbps = 3.692 µs • (b) the time duration of a slot Ts = 156.25 Tb = 0.577ms • (c) the time duration of a frame Tf = 8 Ts = 4.615 ms • (d) a user needs to wait one frame duration, i.e., 4.615 ms, between two successive transmissions Dr. Francis CM Lau, Associate Professor, EIE, PolyU 23 Example 9.5 • If a normal GSM time slot consists of six trailing bits, 8.25 guard bits, 26 training bits, and two traffic bursts of 58 bits of data, find the frame efficiency Dr. Francis CM Lau, Associate Professor, EIE, PolyU 24 12 Solution 9.5 • If a normal GSM time slot consists of six trailing bits, 8.25 guard bits, 26 training bits, and two traffic bursts of 58 bits of data, find the frame efficiency • no. of data bits per time slot = 2 x 58 = 116 • equivalent no. of bits per time slot = 2 x 58 + 6 + 8.25 + 26 = 156.25 • frame efficiency = 116/156.25 = 74.24% Dr. Francis CM Lau, Associate Professor, EIE, PolyU 25 Code Division Multiple Access (CDMA) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 26 13 CDMA Binary Phase 2P cos[ω0t + θd (t)] data modulator 2Pc(t)cos[ω0t +θd (t)] c(t) transmitted 2P cosω0t signal BPSK DS-SS transmitter • BPSK spreading accomplished by multiplying sd(t) by a function c(t)= ±1 representing the spreading waveform Dr. Francis CM Lau, Associate Professor, EIE, PolyU 27 CDMA Binary Phase 2P cos[ω0t + θd (t)] data modulator 2Pc(t)cos[ω0t +θd (t)] c(t) 2P cosω0t Dr. Francis CM Lau, Associate Professor, EIE, PolyU 28 14 BPSK DS-SS Power spectral density of data-modulated carrier 1 S ( f ) = PT{sinc 2 [( f − f )T + sinc 2 [( f + f )T} d 2 0 0 (two-sided psd of a BPSK carrier) Dr. Francis CM Lau, Associate Professor, EIE, PolyU 29 BPSK DS-SS psd of data- and spreading code-modulated carrier spreading • st(t) is also a BPSK carrier with T replaced by Tc code chip • Tc = T/3 ⇒ bandwidth of the transmitted signal spread by a factor of 3 ⇒ level of the psd reduced by a factor of 3 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 30 15 BPSK DS-SS distortionless channel ] 2Pc(t − Td )cos[ω0t + θd (t − Td ) + φ transmission delay + interference interference Data and/or Bandpass Estimated phase Gaussian noise filter data demodulator signal component ˆ Despreading mixer c(t −Td ) ˆ ] 2Pc(t − Td )c(t − Td )cos[ω0t + θd (t − Td ) + φ receiver's best estimate of the transmission delay BPSK DS-SS receiver • despreading: re-modulation or correlation of the received signal with the delayed spreading waveform Dr. Francis CM Lau, Associate Professor, EIE, PolyU 31 Spreading Codes • pseudorandom (PN) codes • m-sequence • Gold codes •Walsh Codes Dr. Francis CM Lau, Associate Professor, EIE, PolyU 32 16 Hadamard matrix Mn • n x n matrix – n = even integer •elements are±1 • one row of the matrix contains all ones • other rows contain n/2 no. of “+1” and n/2 no. of “– 1” • any row differs from the other row in exactly n/2 positions Dr. Francis CM Lau, Associate Professor, EIE, PolyU 33 Hadamard matrix Mn 1 1 M M M = n n M 2 = 2n 1 −1 M n M n 1 1 1 1 −1 −1 −1 −1 1 −1 1 −1 −1 1 −1 1 M = ; M = 4 1 1 −1 −1 4 −1 −1 1 1 1 −1 −1 1 −1 1 1 −1 Dr. Francis CM Lau, Associate Professor, EIE, PolyU 34 17 Walsh-Hadamard Codes • rows of the Hadamard matrix used as code words 1 1 1 1 code 1 1 −1 1 −1 code 2 • mutually orthogonal M = 4 1 1 −1 −1 code 3 1 −1 −1 1 code 4 – e.g. row 1 and 2, 1.1+1.(–1)+1.1+1.(–1) = 0 – breaks down in the presence of multipath Dr.
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