Chapter 3 - Wireless Communication

7/29/2019 Chapter 3 - Wireless Communication

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TOPIC 3:-

MULTIPLE ACCESS TECHNIQUES

EP 602:- WIRELESS COMMUNICATION


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AT THE END THIS TOPIC STUDENT SHOULD BE ABLE TO:-

3.1 Understand the Multiple Access Techniques3.1.1 Define:

a) Frequency Division Multiple Access (FDMA)

b) Time Division Multiple Access (TDMA)

c) Code Division Multiple Access (CDMA)

d) Orthogonal Frequency Division Multiple Access (OFDMA)

3.1.2 Describe structure of Multiple Access Techniques

3.1.3 State the advantages and disadvantages of Multiple Access

Techniques


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MULTIPLE ACCESS TECHNIQUESa) Frequency Division Multiple Access (FDMA)

Frequency Division Multiple Access (FDMA) is common in the first

generation of mobile communication systems so called analogue systems.

The analog communications technique that uses a common channel forcommunication among multiple users allocating unique time slots to different

users.

The process of dividing bandwidth into multiple frequencies which can beshared out among base transceiver stations (BTS)within the network.

A method allowing multiple carriers to share a single satellite transponder orrange of frequencies. The transponder bandwidth is divided into sub-channels,each of which is allocated to a particular earth station (carrier). The earthstations transmit continuously and the transponder conveys several carrierssimultaneously at different frequencies.


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b) Time Division Multiple Access (TDMA)

Time Division Multiple Access (TDMA), it makes use of the samefrequency spectrum but allows more users on the same band of

frequencies by dividing the time into slots and shares the channel

between users by assigning them different time slots.

TDMA is a technology used in digital cellular telephonecommunication that divides each cellular channel into three time

slots in order to increase the amount of data that can be carried.

In TDMA, the available spectrum is divided in time into Time Slots.

Each subscriber requiring resources is allocated a unit of time (timeslot) during which they can transmit or receive data.

The TDMA system is used in many second generation (2G) systems

such as GSM and TDMA/D-AMPS.


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c) Code Division Multiple Access (CDMA)

CDMA is a digital technique for sharing the frequency spectrum. It isa spread spectrum technology that employs codes to separate users

in the same frequency spectrum.

In CDMA, all subscribers share the same frequency at the same time

within a cell, so there is a need to distinguish between the differentcalls or sessions.


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d) Orthogonal Frequency Division Multiple Access (OFDMA)

Orthogonal Frequency Division Multiplexing (OFDM) is a technique

for transmitting large amounts of digital data over a radio wave.

Orthogonal frequency division multiplexing (OFDM) is a multi-carriertransmission technique, which divides the available spectrum intomany subcarriers, each one being modulated by a low data ratestream.

The technology works by splitting the radio signal into multiplesmaller sub-signals that are then transmitted simultaneously atdifferent frequencies to the receiver.

OFDM is a broadband multicarrier modulation method that offerssuperior performance and benefits over older, more traditionalsingle-carrier modulation methods because it is a better fit withtodays high-speed data requirements and operation in the UHF andmicrowave spectrum.

O
http://www.webopedia.com/TERM/O/OFDM.htmlhttp://www.webopedia.com/TERM/O/OFDM.html


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3.1.2 Describe structure of Multiple Access Technique

The available spectrum in FDMA is divided into physical channels

of equal bandwidth as Figure below:-

Frequency Division Multiple Access

B) FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)


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30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHz

30 KHzFrequency

FDMA Frequency Division Multiple Access


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One physical channel is allocated per subscriber.

In pure FDMA systems, different speech/data/signaling (per

subscriber) transmissions may be transmitted at the same time on

different frequencies.

The physical channel allocated to the subscriber is used during the

entire duration of the call and is unavailable for other subscribersduring that time.

The physical channel is released at the end of the call and is then

available for the next subscriber.

In summary, in FDMA, narrow bandwidth is used for continuous

transmission and reception, there is orthogonality in frequency within

the cell, and no synchronization in time is needed.


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Time Division Multiple Access

B) TIME DIVISION MULTIPLE ACCESS (TDMA)


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Time Division Multiple Access

Fre

quency

Time

200 KHz

200 KHz

200 KHz

200 KHz

One timeslot = 0.577 ms One TDMA frame = 8 timeslots


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TDMA systems must carefully synchronize the transmission times

of all the users to ensure that they are received in the correct time

slot and do not cause interference.

Since this cannot be perfectly controlled in a mobile environment,

each time slot must have a guard-time, which reduces the

probability that users will interfere, but decreases the spectral

efficiency.


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C) CODE DIVISION MULTIPLE ACCESS (CDMA)

Code Division Multiple Access


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In CDMA each user is assigned a unique code sequence

(spreading code), which it uses to encode its data signal.

The receiver, knowing the code sequence of the user, decodes the

received signal and recovers the original data.

The bandwidth of the coded data signal is chosen to be much

larger than the bandwidth of the original data signal, that is, theencoding process enlarges (spreads) the spectrum of the data

signal.

CDMA is based on spread-spectrum modulation.

If multiple users transmit a spread-spectrum signal at the same

time, the receiver will still be able to distinguish between users,

provided that each user has a unique code that has a sufficiently

low cross-correlation with the other codes.


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OFDM can be viewed as either a modulation technique or amultiplex technique.

1. Modulation technique

Viewed by the relation between input and output signals

2. Multiplex technique

Viewed by the output signal which is the linear sum of themodulated signals

22afewtw

Input Signal

OFDM System

Output Signal

S/P

D) ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS (OFDM)


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D) ORTHOGONAL FREQUENCY DIVISION MULTIPLE ACCESS (OFDM)


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Advantages of Multiple Access Techniques:-

Advantages of FDMA

1. Simplest2. Best suited for analog links

3. Each station has its own frequency band, separated by guard

bands

4. Receivers tune to the right frequency

5. Cost-effective technology for upgrading a current analog system

to digital

6. Number of frequencies is limited

reduce transmitter power; reuse frequencies in non-adjacent

cells example: voice channel = 30 KHz

833 channels in 25 MHz band

with hexagonal cells, partition into 118 channels each

but with N cells in a city, can get 118N calls => win if N > 7


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Disadvantages of FDMA

1. User FDMA has a predefined time slot. That means, users roamingfrom one cell to another are not allotted a time slot.

2. Multipath distortion A signal coming from a tower to a handset

might come from any one of several directions. The signal have

bounced off several different building before arriving which cancause interference.


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Advantages of TDMA

1. TDMA can easily adapt to transmission of data as well as voice

communication.

2. TDMA provides users with an extended battery life, since it

transmits only portion of the time during conversations.

3. Users can be given different amounts of bandwidth

4. Mobiles can use idle times to determine best base station

5. Can switch off power when not transmitting

Disadvantages of TDMA

1. Users has a predefined time slot. When moving from one cell site

to other, if all the time slots in this cell are full the user might be

disconnected.

2. TDMA is subjected to multipath distortion. To overcome this

distortion, a time limit can be used on the system. Once the time

limit is expired, the signal is ignored.

3. Synchronization overhead

4. Greater problems with multipath interference on wireless links


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Advantages of CDMA

1. Users separated both by time and frequency

2. Send at a different frequency at each time slot (frequency

hopping) or, convert a single bit to a code (direct sequence)3. Receiver can decipher bit by inverse process.

Disadvantages of CDMA

1. Immune from narrowband noise

2. No need for all stations to synchronize3. No hard limit on capacity of a cell

4. All cells can use all frequencies

5. Implementation complexity

6. Need for power control

to avoid capture

7. Need for a large contiguous frequency band (for direct sequence)


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Advantages of OFDMA

1. Multi-user Diversity

broadband signals experience frequency selective fading

OFDMA allows different users to transmit over different portions of

the broadband spectrum (traffic channel)

Different users perceive different channel qualities, a deep faded

channel for one user may still be favorable to others

Multi-user Diversity


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2. Efficient use of Spectrum

3. Very easy and efficient in dealing with multi-path

4. Robust again narrow-band interference

5. Bit Error Rate performance is better only in Fading environment


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Disadvantages of OFDM

1. Synchronization

Tight Synchronization between users are required for FFT in

receiver

Pilot signals are used for synchronizations

2. Co-channel interference

Dealing with this is more complex in OFDM than in CDMA Dynamic channel allocation with advanced coordination among

adjacent base stations.

3. Sensitive to frequency offset and phase noise

4. Peak-to-average problem reduces the power efficiency of RF

amplifier at the transmitter


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AT THE END THIS TOPIC STUDENT SHOULD BE

ABLE TO:-

3.2 Difference between Multiple Access Techniques.

3.2.1 Compare between TDMA,FDMA,CDMA andOFDMA

3.2.3 Relate between TDMA,FDMA,CDMA and

OFDMA with respect to wireless communication

standard.


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3.2.1 Compare between TDMA,FDMA,CDMA and OFDMA

TDMA

Time division multiple access (TDMA) is a channel access method forshared medium (usually radio) networks. It allows several users toshare the same frequency channel by dividing the signal into differenttimeslots. The users transmit in rapid succession, one after the other,each using his own timeslot. This allows multiple stations to share thesame transmission medium (e.g. radio frequency channel) while usingonly the part of its bandwidth they require.

TDMA is used in the digital 2G cellular systems such as GlobalSystem for Mobile Communications (GSM), IS-136, Personal DigitalCellular (PDC) and iDEN, and in the Digital Enhanced CordlessTelecommunications (DECT) standard for portable phones. It is alsoused extensively in satellite systems, and combat-net radio systems.

TDMA is a type of Time-division multiplexing, with the special pointthat instead of having one transmitter connected to one receiver, thereare multiple transmitters. In the case of the uplink from a mobile phoneto a base station this becomes particularly difficult because the mobile

phone can move around and vary the timing advance required tomake its transmission match the gap in transmission from its peers


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FDMA

Frequency Division Multiple Access or FDMA is an accesstechnology that is used by radio systems to share the radio

spectrum. The terminology multiple access implies the sharing ofthe resource amongst users, and the frequency division describeshow the sharing is done: by allocating users with different carrierfrequencies of the radio spectrum.

This technique relies upon sharing of the available radio spectrum bythe communications signals that must pass through that spectrum.

The terminology multiple access indicates how the radio spectrumresource is intended to be used: by enabling more than onecommunications signal to pass within a particular band; and thefrequency division indicates how the sharing is accomplished: byallocating individual frequencies for each communications signalwithin the band.

In an FDMA scheme, the given Radio Frequency (RF) bandwidth isdivided into adjacent frequency segments. Each segment is providedwith bandwidth to enable an associated communications signal topass through a transmission environment with an acceptable level ofinterference from communications signals in adjacent frequencysegments


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CDMA

Code division multiple access (CDMA) is a form of multiplexing and amethod of multiple access that divides up a radio channel not by time

(as in time division multiple access), nor by frequency (as in

frequency-division multiple access), but instead by using different

pseudo-random code sequences for each user. CDMA is a form of

"spread-spectrum" signaling, since the modulated coded signal has amuch higher bandwidth than the data being communicated.

CDMA has been used in many communications and navigation

systems, including the Global Positioning System and in the

OmniTRACS satellite system for transportation logistics


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OFDM

Most broadband systems are subject to multipath

transmission.

Conventional solution to multipath is an equalizer in the

receiver

high data rates equalizers too complicated

Simple way of dealing with multipath

relatively simple DSP algorithms


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Orthogonal Frequency Division Multiple Access (OFDMA)

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Chapter 3 - Wireless Communication