II. Medium Access & Cellular
Standards
TDMA/FDMA/CDMA
Multiple Access in Wireless Communications
Medium Access
Mechanisms to allow many users to simultaneously share
a finite amount of wireless communication channels
Narrowband Systems
The bandwidth of a single communication channel is
smaller than the expected coherence bandwidth
Wideband Systems
The bandwidth of a single communication channel is
much larger than the expected coherence bandwidth
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Frequency Division Multiple Access (FDMA)
Power
Number of Channels in
supported in an FDMA
System (N)
Frequency
N
Bt  2Bguard
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Bc
Bguard
Bc
Bguard
Bt
Bt : Total Spectrum Allocation
Bguard: Guard Band at edge of Allocated Bandwidth
Bc : Channel Bandwidth
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Time Division Multiple Access (TDMA)
Efficiency of TDMA:
A measure of the
percentage of transmitted
data that contains
information as opposed to
providing overhead for the
access scheme

bOH 
ηf   1 
  100%
bT 

Number of Channels in
supported in an TDMA
System (N) with m Slots
per TDMA Frame
© Tallal Elshabrawy
One TDMA Frame
Preamble
Slot 1
Slot 2
Trail Bits Sync Bits
bOH
bT
Trail Bits
Information Message
Slot 3
------------
Information Data
Slot N
Guard Bits
: Total Number of Overhead Bits per TDMA Frame
: Total Number of Bits per TDMA Frame
 Bt  2Bguard 
N  m

Bc


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TDMA vs FDMA: Channel Bandwidth
The channel bandwidth in FDMA systems is
smaller than that in TDMA systems


FDMA systems are less susceptible to frequency
selective fading
FDMA systems have a larger number of carriers
and therefore might suffer from higher costs
because of the need for a carrier (i.e., oscillator)
per frequency channel
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TDMA vs FDMA: Transmission Mode
FDMA supports continuous transmission
TDMA features discontinuous transmission






TDMA must use digital communications while FDMA could
support analog and digital communications
TDMA provides an opportunity to regulate battery
consumption by turning off the transmitter when not in use
TDMA Enables MAHO to simplify handoffs as mobile units
may listen to transmissions from other base stations during
idle times
The FDMA mobile unit uses duplexers to allow for
simultaneous transmission and reception
TDMA uses different timeslots for transmission and
reception and therefore duplexers need could be avoided
TDMA requires synchronization and guard time overhead
bits
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TDMA vs FDMA: Dynamic Capacity Allocation
TDMA opens an avenue for Dynamic capacity
allocation by allocating different number of
timeslots per frame to different users
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Code Division Multiple Access
(CDMA): Basic Concepts
TSymbol
Signal Spreading
S(f)
Data
f
TChip
Data
Spreading
Code
Received
Signal
Spreading
Code
S(f)
Transmitted
Signal
f
 Signal Spreading: Transmission bandwidth significantly
exceeds information bandwidth
Each User is assigned a unique spreading Code.
 Processing Gain: Number of chips per data symbol.
Processing gain reflects the ratio between the transmission
and information bandwidths.
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Code Division Multiple Access
(CDMA): Basic Concepts
S(f)
Signal De-Spreading Received
Received
Signal
Spreading
Code
Spreading
Code at Tx
De-Spread
Signal
Signal
f
TChip
TChip
Spreading
Code at Rx
TSymbol
De-Spread
Signal
S(f)
f
 Signal De-Spreading: Multiplying the received signal by
the spreading code
De-spreading of the received signal with the same
spreading code that was used for spreading restores the
original data
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Code Division Multiple Access
(CDMA): Basic Concepts
S(f)
Signal De-Spreading Received
Received
Signal
Spreading
Code
Spreading
Code at Tx
De-Spread
Signal
Signal
f
TChip
Spreading
Code at Rx
TSymbol
De-Spread
Signal
S(f)
f
 Signal De-Spreading: Multiplying the received signal by
the spreading code
De-spreading of the received signal with a different
spreading code than that was used for spreading does not
restore the original data and maintains bandwidth
characteristics of spread signal
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Code Division Multiple Access
(CDMA): Basic Concepts
Symbol Detection: De-spreading using the same
spreading code that was used for spreading
TSymbol
De-Spread
Signal T
Symbol

4 -4 -4
4
0
Symbol Detection: De-spreading using a different
spreading code than that used for spreading
TSymbol
De-Spread
Signal T
Symbol

0
0
0
0
0
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CDMA Operation
Transmitter for User 1
m1(t)
m1(t)c1(t)
Wireless
Channel
Receiver for User 1
m1(t)+
m2(t)c1(t)c2(t)
m1(t)+e1(t)
TSymbol
m’1(t)

0
m1(t)c1(t)+
m2(t)c2(t)
c1(t)
Transmitter for User 2
m2(t)
m2(t)c2(t)
c1(t)
Receiver for User 2
m2(t)+
m1(t)c1(t)c2(t)
TSymbol

c2(t)
m2(t)+e2(t)
m’2(t)
0
Important Note:
The value of ei(t) depends on the cross
correlation properties between c1 & c2
ei(t)=0 if c1 & c2 are orthogonal
© Tallal Elshabrawy
c2(t)
mi(t):
ci(t):
ei(t):
m’i(t):
Information Message of User i
Spreading code of user i
Interference sensed at receiver of user I
Message detected at receiver
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CDMA in Military Applications
The CDMA concept has been introduced as early
as 1970s in military applications to elude
jamming signals
Spectral
density
Jamming
signal
Spectral
density
signal
signal
De-spreading
frequency
© Tallal Elshabrawy
frequency
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CDMA in Wireless Communications
BW= BS
BW= GBS
BW= GBS
BW= BS
Data
Symbol
Symbol
Detection
Spreading Code
Signal Spreading
© Tallal Elshabrawy
Interference
Spreading Code
Communication
Channel
Signal De-spreading
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Spreading Code Requirements
 Good CDMA spreading codes should be characterized by
relatively low cross-correlation properties to minimize
multiple access interference (MAI).
 Good CDMA spreading codes should be characterized by
low autocorrelation properties to minimize inter-symbol
interference due to multi-path channels
 Ideally it is desirable to have both correlation functions to
approach zero
© Tallal Elshabrawy
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Spreading Codes: Walsh-Hadamard Codes


H2n
Walsh functions provide orthogonal spreading codes
Walsh matrices constructed recursively as follows:
Hn

Hn
Hn 
where H1  1

Hn 
c1
1 1 
H2  

1

1


c1
c2
1 1 1 1 
1 1 1 1

H4  
1 1 1 1


1 1 1 1 
c2
c3
c4
© Tallal Elshabrawy
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Orthogonal Variable Spreading Factor (OVSF) using
Walsh Codes
c11
c21
SF = 1
SF = 2
c22
SF = 4
c41
c42
c43
c44
SF = 8
SF = 16
OVSF TREE
 Available system bandwidth determines the value of Tchip
 TSymbol=SF x Tchip Bit rate is inversely proportional to SF
 OVSF permits users to be allocated different SF (i.e., bit
rates)
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Orthogonal Variable Spreading Factor (OVSF) using
Walsh Codes
c11
c21
SF = 1
SF = 2
c22
SF = 4
c41
c42
c43
c44
SF = 8
SF = 16
OVSF TREE



If a user is allocated a certain code, then all codes that branch from
such code cannot be allocated to any other user
c21 is orthogonal to c22, c43, c44
c21 is NOT orthogonal to c41, c42
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Characteristics of Walsh Codes
 Walsh codes are orthogonal presuming perfect
synchronization
 Walsh codes suffer from poor auto-correlation
properties for time offsets that is greater than zero
 Walsh codes suffer from poor cross-correlation
properties when codes are not perfectly
synchronized (i.e., for time offsets greater than
zero)
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Spreading Codes: Maximal Length Sequences
 Theoretically A randomly chosen sequence should
have good auto-correlation properties
 For CDMA communications, we need to construct
spreading codes that have properties of random
sequences and can be generated simply at both
transmitter and receiver (Pseudorandom
sequences)
 Feedback shift register with appropriate feedback
taps can be used to generate pseudorandom
sequence
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Spreading Codes: Maximal Length Sequences
g2
g0
R0
R1
g3
R2
output
g(x) = x3 + x2 + 1
The coefficients of a
primitive generator polynomial
determine the feedback taps




Time
R0 R1 R2
0
1 0 0
1
0 1 0
2
1 0 1
3
1 1 0
4
1 1 1
5
0 1 1
6
0 0 1
7
1 0 0
Sequence repeats
from here onwards
The registers R0 R1 R2 can assume 23 possible states
State 0 0 0 will result in all zeros output sequence
Maximal length sequence is possible if R0 R1 R2 passes through all
23-1 states before repeating
Maximal length sequences are achievable using coefficients of
primitive polynomials to determine feedback taps
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Maximal Length Sequence Properties
 For a generator with m registers the sequence length is
2m-1
 Each maximal length sequence has 2m-1 ones and 2m-1-1
zeros
 For 2m-1 initial states of registers, we may construct 2m-1
sequences that are cyclic shifts of each other.
 The cross-correlation between maximal length sequences
generated by the same generator is 1/(2m-1) (i.e., they are
not perfectly orthogonal)
 Maximal length sequences have good auto-correlation
properties
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Generating Spreading Codes from Maximal Length
Sequences
 A Single Maximal Sequence Generator:
 Assign different shifts of same sequence to different
users
 If transmitters are uncoordinated, they might not know
each other’s timing and could reuse the same
sequence
 Multiple
Maximal Sequence Generator:
 Different primitive polynomials to determine the
feedback taps of each generator
 Sequences from different generators of same length do
not necessarily have good cross-correlation properties
 There is a limited number of generators (i.e. primitive
polynomials) for each sequence length
© Tallal Elshabrawy
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Spreading Codes: Gold Codes
 Sum two maximal–length sequences of the same length
but using different generators
Example of Gold Code Generator of length 27-1
Sequence 1 Generator: x7+x3+1
Sequence 2 Generator: x7+x5+x4+x3+x2+x+1
© Tallal Elshabrawy
R0
R1
R2
R3
R4
R5
R6
R0
R1
R2
R3
R4
R5
R6
Gold
Sequence
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Gold Sequence Properties
 For each starting state of the first generator, there
are 2m-1 potential starting states of the second
generator
 Gold was able to show that for particular choices
of generator polynomials, Gold sequences could
have good cross-correlation properties
 The auto-correlation of Gold codes is proportional
to 2/sqrt(2m-1)
© Tallal Elshabrawy
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Diversity in CDMA Systems
w1
r(t-tpd-τ1)
τ1
TSymbol=X
m(t)
s(t)
TChip=X/G





∑
τ2
τ1 τ2
c(t)
w2
r(t-tpd-τ2)
τNp
FrequencySelective
Channel
wNp
r(t-tpd-τNp)
τNp
Multi-Path resistant
RAKE Receiver can collect energy spread by the small-scale channel
Suitable for bursty applications
No need for frequency planning (frequency reuse of one)
Soft blocking and soft handoff
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