1. Wireless and mobile computing
Multiple Access Techniques for Wireless Communication
Lecture-4
Instructor : Mazhar Hussain

2. FDMA : Frequency division Multiple Access
TDMA : Time division Multiple Access
CDMA : Code division Multiple Access
SDMA : Space division Multiple Access
PDMA : Polarization division Multiple Access

3. Introduction
Multiple access schemes are used to allow many users to share simultaneously a finite amount of radio spectrum
Sharing of spectrum is required to increase capacity
For high quality communication this sharing of spectrum should not degrade performance of the system
high performance
duplexing generally required
frequency domain
time domain

4. Duplexing What is Duplexing?
to talk and listen simultaneously is called duplexing
Classification of communication systems according to their connectivity
Simplex
Half-duplex
Duplex A B A B A B

5. Duplexing Contd… Duplexing may be done using
frequency domain technique
time domain technique

6. Frequency division duplexing (FDD)
two bands of frequencies for every user
forward band ( for traffic from Base station to mobile unit)
reverse band (for traffic from mobile unit to Base station)
duplexer needed
frequency separation between forward band and reverse band is constant throughout the system
reverse channel
forward channel
frequency separation/split f

7. Time division duplexing (TDD)
uses different time slots for forward and reverse link
forward time slot
reverse time slot
no duplexer is required (a simple switch can be used)
Communication is not full-duplex
reverse channel
forward channel t
time separation/split

8. Trade-offs b/w FDD and TDD
Provides individual radio frequencies to each user hence, transceiver should work on two frequency bands
Frequency allocation must be carefully coordinated with Out-of-band users
Duplexer needed
TDD
Single frequency hence simple transceiver
Duplexer not needed, a switch can do the job
There is time latency, communication is not full-duplex

9. Multiple Access Techniques in Wireless Communication System
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Space division multiple access (SDMA)
grouped as:
narrowband systems
wideband systems

10. Narrowband systems
Bandwidth of the signal is narrow compared with the coherence bandwidth of the channel
In NB systems available radio spectrum is divided into large number of narrowband channels usually FDD (large frequency split)
Narrowband FDMA
Narrowband TDMA

11. Narrowband systems Narrowband FDMA
a user is assigned a particular channel which is not shared by other users
if FDD is used then each channel has a forward and reverse link (called FDMA/FDD)
Narrowband TDMA
Allows users to share the same channel but allocates a unique time slot to each user
FDMA/FDD
FDMA/TDD
TDMA/FDD
TDMA/TDD

12. Narrowband systems
FDMA/FDD
FDMA/TDD
TDMA/FDD
TDMA/TDD

13. Logical separation FDMA/FDD
forward channel
user 1
reverse channel
... f
forward channel
user n
reverse channel t

14. Logical separation FDMA/TDD
user 1
forward channel
reverse channel
... f
user n
forward channel
reverse channel t

15. Logical separation TDMA/FDD
forward
channel
forward
channel
...
user 1
user n f
reverse
channel
reverse
channel t

16. Logical separation TDMA/TDD
user 1
user n
...
forward
channel
reverse
channel
forward
channel
reverse
channel f t

17. Wideband systems
The transmission BW of a single channel is much larger than the coherence bandwidth of the channel
users are allowed to transmit in a large part of the spectrum
large number of transmitters on one channel
TDMA techniques allocates time slots to different transmitters
CMA techniques allows the transmitters to access the channel at the same time

18. Wideband systems FDD or TDD multiplexing techniques TDMA/FDD TDMA/TDD
CDMA/FDD
CDMA/TDD

19. Logical separation CDMA/FDD
user 1
forward channel
reverse channel
...
code
user n
forward channel
reverse channel f

20. Logical separation CDMA/TDD
user 1
forward channel
reverse channel
...
code
user n
forward channel
reverse channel t

21. Multiple Access Techniques
FDMA
time
power
frequency
TDMA
time
power
frequency
CDMA
time
power
frequency

22. Multiple Access Techniques in use
Advanced Mobile Phone System (AMPS) FDMA/FDD
Global System for Mobile (GSM) TDMA/FDD
US Digital Cellular (USDC) TDMA/FDD
Digital European Cordless Telephone (DECT) FDMA/TDD
US Narrowband Spread Spectrum (IS-95) CDMA/FDD
Cellular System

23. Frequency division multiple access FDMA
one phone circuit per channel
idle time causes wasting of resources
simultaneously and continuously transmitting
usually implemented in narrowband systems
Complexity of FDMA mobile systems is lower compared to TDMA
FDMA uses duplexers
for example: AMPS is a FDMA system with bandwidth of 30 kHz

24. FDMA compared to TDMA fewer bits for synchronization
fewer bits for framing
higher costs for duplexer used in base station and subscriber units
FDMA requires RF filtering to minimize adjacent channel interference

25. Nonlinear Effects in FDMA
many channels - same antenna
for maximum power efficiency operate near saturation
near saturation power amplifiers are nonlinear
nonlinearities causes signal spreading
intermodulation frequencies

26. Nonlinear Effects in FDMA
IM are undesired harmonics
interference with other channels in the FDMA system
interference outside the mobile radio band: adjacent-channel interference
RF filters needed - higher costs

27. Number of channels in a FDMA system
Bt - 2Bguard N= Bc
N … number of channels
Bt … total spectrum allocation
Bguard … guard band
Bc … channel bandwidth

28. Example: Advanced Mobile Phone System
AMPS
FDMA/FDD
analog cellular system
12.5 MHz per simplex band - Bt
Bguard = 10 kHz ; Bc = 30 kHz
12.5E6 - 2*(10E3) N=
= 416 channels
30E3

29. Time Division Multiple Access
Time slots
one user per slot
Buffer and burst method
Non-continuous transmission
Advantage:
Total bandwidth is utilized
Disadvantage:
Strict Burst Timing is required at the earth station

30. Repeating Frame Structure
One TDMA Frame
Preamble Information Message Trail Bits
Slot 1 Slot 2 Slot … Slot N
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.

31. Features of TDMA a single carrier frequency for several users
transmission in bursts
handoff process much simpler (can listen when idle)
Low battery consumption
Bandwidth can be supplied on demand
FDD : switch instead of duplexer
high synchronization overhead

32. Number of channels in a TDMA system
m*(Btot - 2*Bguard) N= Bc
N … number of TDMA channel slots
m … number of TDMA users per radio channel
Btot … total spectrum allocation
Bguard … Guard Band
Bc … channel bandwidth

33. Example: Global System for Mobile (GSM)
TDMA/FDD
forward link at Btot = 25 MHz
radio channels of Bc = 200 kHz
if m = 8 speech channels supported, and
if no guard band is assumed :
8*25E6 N=
= 1000 simultaneous users
200E3

34. Efficiency of TDMA
percentage of transmitted data that contain information
frame efficiency f
usually end user efficiency < f ,
because of source and channel coding
How get f ?

35. Repeating Frame Structure
One TDMA Frame
Preamble Information Message Trail Bits
Slot 1 Slot 2 Slot … Slot N
Trail Bits Sync. Bits Information Data Guard Bits
The frame is cyclically repeated over time.

36. bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
Efficiency of TDMA
bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg
bOH … number of overhead bits
Nr … number of reference bursts per frame
br … reference bits per reference burst
Nt … number of traffic bursts per frame
bp … overhead bits per preamble in each slot
bg … equivalent bits in each guard time interval

37. Efficiency of TDMA bT = Tf * R bT … total number of bits per frame
Tf … frame duration
R … channel bit rate

38. Efficiency of TDMA f … frame efficiency f = (1-bOH/bT)*100%
bOH … number of overhead bits per frame
bT … total number of bits per frame

39. Spread Spectrum Multiple Access (SSMA)
SSMA uses Signals have transmission BW that is several orders of magnitude greater than the minimum required BW
A Pseudo-noise sequence converts a narrow band signal to a wideband noise-like signal before transmission
SSMA not BW efficient when used by a single user
Many users can share the same BW without interfering with one another
Type of SSMA techniques:
frequency hoped multiple access (FHMA)
Direct sequence multiple access (DS) or Code division multiple access (CDMA)

40. Pseudo-noise (PN) sequence
PN code sets can be generated from linear feedback shift registers
Example:
Modulo 2 adder
Stage 1
Stage 2
Stage 3
Register
Output
Shift Register
Clock

41. Correlation matrix of 31-bit PN sequence

42. Frequency Hopped Multiple Access (FHMA)
It is digital multiple access system
Carrier frequencies of individual users varied in pseudorandom fashion with in a wideband channel
Digital data broken into uniform sized bursts which are transmitted on different carriers
Instantaneous BW of any one transmission burst is much smaller than the total spread BW
Locally generated PN code is used to synchronize the receiver frequency with that of transmitter.
Provides a High level of security
Fast frequency hopping system
Slow frequency hopping

43. FHMA
Erasures can occur when two or more users transmit on the same channel at the same time
FH(Frequency hopped) signal is immune to fading so error control coding can be combined to guard against erasures .

44. CDMA
Narrowband signals is multiplied by a very large bandwidth signal called the spreading signal.
The spreading signal is pseudo noise code sequence that has a chip rate which is orders of magnitudes greater than data rate of the message
All users use the same carrier frequency and transmit simultaneously
Each user has its own pseudo random code word which is approximately orthogonal to other codewords

45. CDMA Receiver performs time co-relation
All other codewords appear as noise
Receiver needs to know the code word used by transmitter
Many users same frequency, TDD or FDD
Soft capacity(capacity increases linearly)
Self-jamming
Near far problem

46. Packet Radio
Many subscribers attempt to access a single channel in an uncoordinated or minimally coordinated manner
Collisions from simultaneous transmissions are detected at the BS
ACK and NACK is broadcasted by BS (its like perfect feedback)
In case of NACK the signal retransmitted
PR easy to implement
Induces delays

47. Packet Radio Protocols
Vulnerable period Vp: time interval during which the packets are susceptible to collisions with transmission from other users
If packet length in time is t
then a packet will suffer collision if other terminals transmit packets during the period t1 to t1+2t
Packet A
Packet B t1
t1+2t

48. Packet Radio contd … Subscribers use contention techniques
ALOHA protocol example of contention techniques
Advantage: ability to serve many users without any overhead

49. Pure ALOHA
Uncoordinated stations are contesting for the use of a single shared channel
Algorithm
Stations transmit whenever they have something to send
Colliding frames will be destroyed
If destroyed, the sender waits a random amount of time and sends it again
Maximum channel utilization is 18.4%
81.6% of the total available bandwidth is essentially wasted due to losses from packet collisions

50. In pure ALOHA, frames are transmitted at completely arbitrary times

51. Slotted ALOHA A method for doubling the capacity of pure ALOHA systems
Divide up time into discrete interval slots
Stations share the same time by synchronizing with a master
Master periodically broadcasts a synchronization pulse (clock tick packet) to all stations
Stations are only allowed to send their packets immediately after receiving a clock tick
Each interval corresponding to one frame time
Maximum channel utilization is 36.8%

52. Packets overlap completely or not at al

53. ALOHA Protocols Advantages Disadvantages
Simple (due to distributed control)
Flexible to fluctuations in the number of hosts
Fair
Disadvantages
Low channel efficiency with a large number of hosts
Not good for real-time traffic (e.g., voice)
Cannot support priority traffic

54. Questions/Comments?