ii. medium access & cellular standards. tdma/fdma/cdma
TRANSCRIPT
II. Medium Access & Cellular Standards
TDMA/FDMA/CDMA
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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
Power
BcBguard Bguard
Bt
Bt : Total Spectrum AllocationBguard: Guard Band at edge of Allocated BandwidthBc : Channel Bandwidth
t guard
c
B 2BN
B
Number of Channels in supported in an FDMA System (N)
Frequency Division Multiple Access (FDMA)
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Efficiency of TDMA:A measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme
OHf
T
bη 1 100%
b
Preamble Information Message Trail Bits
One TDMA Frame
Slot 1 Slot 2 Slot NSlot 3 ------------
Trail Bits Sync Bits Guard BitsInformation Data
Preamble Information Message Trail Bits
One TDMA Frame
Slot 1 Slot 2 Slot NSlot 3 ------------
Trail Bits Sync Bits Guard BitsInformation Data
bOH : Total Number of Overhead Bits per TDMA FramebT : Total Number of Bits per TDMA Frame
t guard
c
B 2BN m
B
Number of Channels in supported in an TDMA System (N) with m Slots per TDMA Frame
Time Division Multiple Access (TDMA)
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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
TDMA vs FDMA: Channel Bandwidth
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FDMA supports continuous transmissionTDMA 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
TDMA vs FDMA: Transmission Mode
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TDMA opens an avenue for Dynamic capacity allocation by allocating different number of timeslots per frame to different users
TDMA vs FDMA: Dynamic Capacity Allocation
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Code Division Multiple Access (CDMA): Basic Concepts
Signal Spreading: Transmission bandwidth significantly exceeds information bandwidthEach 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.
DataSignal Spreading
Data
Spreading Code
Received Signal
Spreading Code
Transmitted Signal
S(f)
f
S(f)
f
TSymbol
TChip
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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
De-Spread Signal
Signal De-Spreading
Spreading Code
De-Spread Signal
Spreading Code at Rx
Received Signal
S(f)
f
S(f)
f
TSymbol
TChip
Received Signal TChip
Spreading Code at Tx
Code Division Multiple Access (CDMA): Basic Concepts
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Signal De-Spreading: Multiplying the received signal by the spreading codeDe-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
De-Spread Signal
Signal De-Spreading
Spreading Code
De-Spread Signal
Spreading Code at Rx
Received Signal
S(f)
S(f)
f
TSymbol
TChip
Received Signal
Spreading Code at Tx
f
Code Division Multiple Access (CDMA): Basic Concepts
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De-Spread Signal
Symbol Detection: De-spreading using the same spreading code that was used for spreading
SymbolT
0
TSymbol
4 -4 -4 4
De-Spread Signal
SymbolT
0
TSymbol
0 0 0 0
Symbol Detection: De-spreading using a different spreading code than that used for
spreading
Code Division Multiple Access (CDMA): Basic Concepts
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CDMA Operation
m1(t)
c1(t)
m1(t)c1(t)
m2(t)
c2(t)
m2(t)c2(t)
Transmitter for User 1
Transmitter for User 2
SymbolT
0
Receiver for User 1
WirelessChannel
m1(t)c1(t)+m2(t)c2(t)
SymbolT
0
Receiver for User 2
c1(t)
c2(t)
m1(t)+m2(t)c1(t)c2(t)
m2(t)+m1(t)c1(t)c2(t)
m1(t)+e1(t)
m2(t)+e2(t)
m’1(t)
m’2(t)
mi(t): Information Message of User ici(t): Spreading code of user iei(t): Interference sensed at receiver of user Im’i(t): Message detected at receiver
Important Note:The value of ei(t) depends on the cross correlation properties between c1 & c2
ei(t)=0 if c1 & c2 are orthogonal
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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
frequency
Spectral density
frequency
Spectral densityJamming
signal
signal
signal
De-spreading
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Data Symbol
Spreading Code
Communication Channel
Signal Spreading
Interference Spreading Code
Symbol Detection
Signal De-spreading
BW= BS BW= GBS BW= GBS BW= BS
CDMA in Wireless Communications
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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
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Spreading Codes: Walsh-Hadamard Codes
Walsh functions provide orthogonal spreading codes Walsh matrices constructed recursively as follows:
n n2n 1
n n
H HH where H 1
H H
2
1 1H
1 1
4
1 1 1 1
1 1 1 1H
1 1 1 1
1 1 1 1
c1
c2
c1
c2
c3
c4
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Orthogonal Variable Spreading Factor (OVSF) using Walsh Codes
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)
SF = 1
SF = 2
SF = 4
SF = 8
SF = 16
OVSF TREE
c11
c21 c22
c41c42 c43 c44
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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
SF = 1
SF = 2
SF = 4
SF = 8
SF = 16
OVSF TREE
c11
c21 c22
c41c42 c43 c44
Orthogonal Variable Spreading Factor (OVSF) using Walsh Codes
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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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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
R0 R1 R2
g(x) = x3 + x2 + 1
g0g2 g3
The coefficients of a primitive generator polynomialdetermine the feedback taps
Time R0 R1 R2
0 1 0 01 0 1 02 1 0 13 1 1 0 4 1 1 1 5 0 1 1 6 0 0 1 7 1 0 0
Sequence repeatsfrom here onwards
output
Spreading Codes: Maximal Length Sequences
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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
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Spreading Codes: Gold Codes
Sum two maximal–length sequences of the same length but using different generators
R0 R1 R2 R3 R4 R5 R6
R0 R1 R2 R3 R4 R5 R6
Example of Gold Code Generator of length 27-1Sequence 1 Generator: x7+x3+1Sequence 2 Generator: x7+x5+x4+x3+x2+x+1
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)
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Diversity in CDMA Systems
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
TSymbol=X
Frequency- Selective Channel
τ2
∑
TChip=X/G
τ1 τ2 τNp
τNp
m(t)
c(t)
s(t)
r(t-tpd-τ2)
r(t-tpd-τNp)
w1
w2
wNp
r(t-tpd-τ1)
τ1