Multiple Access Techniques
Dr. Francis LAU
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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Content
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Introduction
Frequency Division Multiple Access
Time Division Multiple Access
Code Division Multiple Access
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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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
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2
Introduction
• time division duplexing (TDD)
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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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
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Introduction
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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Frequency Division Multiple
Access (FDMA)
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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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
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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
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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
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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
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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Time Division Multiple Access
(TDMA)
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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TDMA
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
• …
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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
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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.
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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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.
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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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
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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Code Division Multiple Access
(CDMA)
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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CDMA
Binary
data
Phase
modulator
2 P cos[ω 0 t + θ d (t )]
2 P c(t ) cos[ω 0 t + θ d (t )]
c(t)
2 P cos ω0 t
transmitted
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
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CDMA
Binary
data
Phase
modulator
2 P cos ω 0 t
2 P cos[ω 0 t + θ d (t )]
2 Pc(t ) cos[ω 0 t + θ d (t )]
c(t)
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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BPSK DS-SS
Power spectral density of data-modulated carrier
1
S d ( f ) = PT sinc 2 [( f − f 0 )T + sinc 2 [( f + f 0 )T
2
(two-sided psd of a BPSK carrier)
{
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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BPSK DS-SS
distortionless channel
2 Pc (t − Td ) cos[ω 0 t + θ d (t − Td ) + φ ]
+ interference
interference
and/or
Gaussian noise
Bandpass
filter
c(t − Tˆd ) Despreading mixer
receiver's best
estimate of the
transmission delay
transmission delay
Data
phase
demodulator
Estimated
data
signal component
2 Pc (t − Td )c (t − Tˆd ) cos[ω 0 t + θ d (t − Td ) + φ]
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
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Spreading Codes
•
•
•
•
pseudorandom (PN) codes
m-sequence
Gold codes
Walsh Codes
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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
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Hadamard matrix Mn
1 1 
M2 = 

1 − 1
1 1 1 1 
1 − 1 1 − 1
;
M4 = 
1 1 − 1 − 1


1 − 1 − 1 1 
M
M 2n =  n
M n
Mn
M n 
− 1 − 1 − 1 − 1
− 1 1 − 1 1 

M4 = 
− 1 − 1 1 1 


− 1 1 1 − 1
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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Walsh-Hadamard Codes
• rows of the Hadamard matrix used as code
1 1 1 1  code 1
words
1 − 1 1 − 1
 code 2
• mutually orthogonal M = 
 code 3
4
1 1 −1 −1


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
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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
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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
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Summary
• multiple access techniques
– frequency division multiple access
– time division multiple access
– code division multiple access
Dr. Francis CM Lau, Associate Professor, EIE, PolyU
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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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