Wireless Communication 171004 Arjav A. Bavarva Dept. of Electronics and Communication.

Wireless

Communication

171004

Arjav A. BavarvaDept. of Electronics and Communication

Multiple access techniques


Important terms

Simplex channel

Half duplex channel

Full duplex or duplex channel (Using frequency and time)

Forward channel

Reverse channel


Frequency division duplexing (FDD)

• two bands of frequencies for every user• forward band• reverse band• duplexer needed• frequency separation between forward band

and reverse band is constant

frequency seperation

reverse channel forward channel

fArjav A. Bavarva

Dept. of Electronics and Communication

Time division duplexing (TDD)

• uses time for forward and reverse link• multiple users share a single radio channel• forward time slot• reverse time slot• no duplexer is required

time seperationt

forward channelreverse channel


FDMAFrequency Division Multiple Access


General Specification of FDMA

• Rx: 869-894MHz Tx: 824-849MHz• 832 Channels spaced 30kHz apart• DQPSK modulation scheme• 48.6kbps bit rate• Used in analog cellular phone systems (i.e. AMPS)• Uses Frequency Division Duplexing (FDD)• ISI (Inter symbol Interference) is low


FDMA Operation

• Number of FDMA Channels

• In the U.S. each cellular carrier is allocated 416 channels where:

2

- total spectrum

- guard band

- channel bandwidth

f guard

c

f

guard

c

N

12.5

10

30

12.5 2 10416

30

f

guard

c

MHz

kHz

kHz

MHz kHzN

kHz

Arjav A. Bavarva


Advantages of FDMA

• If channel is not in use, it sits idle• Channel bandwidth is relatively narrow (30kHz)• Simple algorithmically and lower complexity• Fairly efficient when the traffic is uniformly constant• Capacity increase can be obtained by reducing the

information bit rate and using efficient digital code• No need for network timing – fewer bits required for

synchronization and framing• No restriction regarding the type of baseband or type

of modulationArjav A. Bavarva


Disadvantages to using FDMA

• The presence of guard bands• It increases cost as duplexer is compulsory• Maximum bit rate per channel is fixed • Many channels use same antenna, causes

nonlinear effect and signal will spread in frequency domain (Inter modulation frequency IM ).

• Requires right RF filtering to minimize adjacent channel interference


TDMATime Division Multiple Access


General Specification of TDMA

• Rx: 869-894MHz Tx: 824-849MHz• 832 Channels spaced 30kHz apart

(3 users/channel)• DQPSK modulation scheme• 48.6kbps bit rate• Interim Standard (IS) – 54• Digital AMPS (Advanced Mobile Phone System)• Uses Time Division Duplexing (TDD) usually


TDMA Operation

• Efficiency of TDMA frame: - overhead bits per frame

- number of reference bursts per frame

- number of traffic bursts per frame

- number of overhead bits per reference burst

- number of overhead bits per preamble p

OH

r

t

r

p

b

N

N

b

b

er slot

- number of equivalent bits in each guard time interval

- frame duration

- bit rate of the radio-frequency channel

1 100%

g

f

rf

OH r r t p t r g

total f rf

OHf

total

b

T

R

b N b N b N N b

b T R

b

b


Time Division Multiple Access• time slots• one user per slot• buffer and burst method• Non continuous transmission• digital data• digital modulation


Slot 1 Slot 2 Slot 3 … Slot N

Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.


Features of TDMA

• A single carrier frequency for several users• Transmission in buffer and burst• Low battery consumption• Handoff process much simpler as it is

discontinuous process• FDD : switch instead of duplexer• Very high transmission rate• High synchronization overhead• Guard slots necessaryArjav A. Bavarva


Number of channels in a TDMA system

• N … number of channels• m … number of TDMA users per radio channel• Btot … total spectrum allocation• Bguard … Guard Band• Bc … channel bandwidth

N= m*(Btot - 2*Bguard)Bc


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 :

N= 8*25E6200E3 = 1000 simultaneous users


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 ?


Efficiency of TDMA

• 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

intervall

bOH = Nr*br + Nt*bp + Nt*bg + Nr*bg


Efficiency of TDMA

bT = Tf * R

• bT … total number of bits per frame• Tf … frame duration• R … channel bit rate


Efficiency of TDMA

• f … frame efficiency• bOH … number of overhead bits per frame• bT … total number of bits per frame

f = (1-bOH/bT)*100%


Advantages of TDMA

• Flexible bit rate• No frequency guard band required• No need for precise narrowband filters• Easy for mobile or base stations to initiate and

execute hands off• Extended battery life• BW can be supplied on demand


Disadvantages to using TDMA

• Requires network-wide timing synchronization• Requires signal processing for matched

filtering and correlation detection• Multipath distortion


CDMACode Division Multiple Access


General Specification of CDMA

• Rx: 869-894MHz Tx: 824-849MHz• 20 Channels spaced 1250kHz apart

(798 users/channel)• QPSK/(Offset) OQPSK modulation scheme• 1.2288Mbps bit rate• IS-95 standard• Operates at both 800 and 1900 MHz frequency bands


5: DataLink Layer 5a-38

CDMA Encode/Decode


5: DataLink Layer 5a-39

CDMA: two-sender interference

sender 1

sender 2

uses sender 1 codeto receive sender 1 dataArjav A. Bavarva


Advantages of CDMA

• Many users of CDMA use the same frequency, TDD or FDD may be used

• Multipath fading may be substantially reduced because of large signal bandwidth

• No absolute limit on the number of users• Easy addition of more users• Impossible for hackers to decipher the code sent• Better signal quality• Soft hand off is performed• RAKE receiver can be used to improve reception.


Disadvantages to using CDMA

• Power control needs to be implemented at the base station.

• Rapidly sampling the Radio Signal Strength Indicator (RSSI) level for each mobile and then sending power control command over a forward radio link.

• Near- Far- problem arises


Narrowband systems

• Large number of narrowband channels• Usually FDD• Narrowband FDMA• Narrowband TDMA• FDMA/FDD• FDMA/TDD• TDMA/FDD• TDMA/TDD


Logical separation FDMA/FDD

f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...


Logical separation FDMA/TDD

f

t

user 1

user n

forward channel reverse channel


...


Logical separation TDMA/FDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...


Logical separation TDMA/TDD

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...


Wideband systems

• large number of transmitters on one channel• TDMA techniques• CDMA techniques• FDD or TDD multiplexing techniques• TDMA/FDD• TDMA/TDD• CDMA/FDD• CDMA/TDD


Logical separation CDMA/FDD

code

f

user 1

user n



...


Logical separation CDMA/TDD

code

t

user 1

user n



...


Multiple Access Techniques in use

Multiple Access

TechniqueAdvanced 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


•There are three different types of CSMA protocols :- (i) 1-Persistent CSMA (ii) Non-Persistent CSMA (iii) P-Persistent CSMA

CSMA

1-Persistant Non- Persistent P-Persistent

CSMA


Drawback of 1-persistent•The propagation delay time greatly affects this protocol. Let us suppose, just after the station 1 begins its transmission, station 2 also become ready to send its data and sense the channel. If the station 1 signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision.

•Even if propagation delay time is zero, collision will still occur. If two stations become ready in the middle of third station’s transmission both stations will wait until the transmission of first station ends and both will begin their transmission exactly simultaneously. This will also result in collision.

CONTINUOUSLY SENSES

SENSES & TRANSMIT

TIME

BUSY CHANNELIDLE CHANNEL

CHANNEL?

STATION CAN TRANSMIT

IDLE

BUSY


channel Wait randomly

idle

Busy

Station can transmit

Sense & transmit

Sense Sense

Wait(Random time)

wait

time

Busy channel


CONTINUOUSLY SENSES

PROBABILITIY OUTCOME DOES NOT ALLOW TRANSMISSION

Time slot Time slot Time slot

TRANSMIT

TIME

IDLE

CHANNEL? WAIT A SLOT

PROBABILITY OUTCOME?

CHANNEL?

Acts as though collision has occurred & start again

<_PBUSY

STATION CAN START

BUSY

IDLE

>P


References

T. S. Rappaport, “Wireless Communication”, Prentice hall

Wireless Communication 171004 Arjav A. Bavarva Dept. of Electronics and Communication.

Documents

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