II. Medium Access & Cellular Standards. TDMA/FDMA/CDMA.

II. Medium Access & Cellular Standards

TDMA/FDMA/CDMA

© Tallal Elshabrawy 3

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


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)


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)


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


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


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


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


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



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



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



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


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


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


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


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


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


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


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)


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


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


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


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


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


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)


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

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