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)
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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
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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
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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
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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.
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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
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7 Time Division Multiple Access (TDMA)
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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
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8 TDMA
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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
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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.
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10 Solution 9.3
• number of simultaneously users that can be accommodated in GSM N = (25MHz/200kHz) x 8 = 1000
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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.
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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
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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
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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%
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Code Division Multiple Access (CDMA)
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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
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CDMA
Binary Phase 2P cos[ω0t + θd (t)] data modulator 2Pc(t)cos[ω0t +θd (t)]
c(t) 2P cosω0t
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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)
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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
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Spreading Codes
• pseudorandom (PN) codes • m-sequence • Gold codes •Walsh Codes
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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
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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
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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. Francis CM Lau, Associate Professor, EIE, PolyU 35
CDMA
• narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal • all users use the same carrier frequency and may transmit simultaneously, TDD or FDD may be used • unlike FDMA or TDMA, CDMA has a soft capacity limit – increasing the no. of users raises the noise level, more errors occur
Dr. Francis CM Lau, Associate Professor, EIE, PolyU 36
18 CDMA
• near-far problem – mitigated using power control • spread spectrum bandwidth much greater than the coherence bandwidth – what type of fading? • frequency reuse factor in CDMA cellular system is 1 – all cells use the same spectrum • soft handoff •… Dr. Francis CM Lau, Associate Professor, EIE, PolyU 37
Summary
• multiple access techniques – frequency division multiple access – time division multiple access – code division multiple access
Dr. Francis CM Lau, Associate Professor, EIE, PolyU 38
19 Summary
• Reading – Rappaport T. S., Wireless Communications: Principles and Practice, Prentice Hall PTR, Sections 9.1-9.4.2, 2002. •Problems – Rappaport T. S., Wireless Communications: Principles and Practice, Prentice Hall PTR, Problems 9.1-9.5, 2002.
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