15. Multiple Access Techniques

| University of Dar es Salaam | Department of Electronics and Telecommunications Engineering |

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Multiple Access Techniques

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| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |



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Goal: Allow many users to ‘simultaneously’

share a communications resource.

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• FDMA: frequency-division multiple access

• TDMA: time-division multiple access

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• CDMA: code-division multiple access (Spread Spectrum)

• SDMA: space-division multiple access

• PDMA: polarization-division multiple access (not applied to mobile radio)


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The safe transfer of users across the highway is possible:

– Several lanes separating the traffic (space division multiplexing)

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Parallel conversations may take place at the same time and same place if people talk in different languages (code division multiplexing)

multiplexing)

– Use of the same medium at different points in time (time division multiplexing)


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Terminology: Multiplexing vs. Multiple Access

• Multiplexing: separation of radio access medium

between available dimensions (i.e., freq., time, code, space)

• Multiple access: methods that are used to enable

multiple users to connect to/from the same BS

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multiple users to connect to/from the same BS

5

o Commonly, no difference is made

between these two terms.

o Sometimes, this may lead to a

confusion if the terminology is not

properly used.

TDMTDMA

Multiple-access


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Frequency Division Multiple Access (FDMA)

Subdivide the given

frequency band into

sub-bands (frequency

channels).

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channels).

A frequency channel is

then allocated for the

exclusive use by one

particular user.


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• Used in AMPS.

• Simple to implement.

• Narrowband channels � No ISI � little or no equalisation required.

• Continuous transmission. Less overhead.

• Idle times wasted. Capacity not shared.

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• Idle times wasted. Capacity not shared.

• Need guard bandwidth (guard BW lowers utilisation efficiency).

• Receivers tune to desired frequency (filter based � bulky, expensive).

• Number of frequencies is limited.

• No. of channels = (Total BW – Guard BW ) / Message BW

• FDMA alone is not used in current digital systems. Typically combined

with another technique.


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Time Division Multiple Access (TDMA)

Divide the time axis into

time slots. A time slot is

then allocated for the

exclusive use by one

particular user.

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particular user.

Time slots are allocated

within a single frequency

i.e. terminals transmit at

different times, but occupy

the entire bandwidth.


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Time Division Multiple Access (TDMA)

• Used in GSM and IS-136.

• Users can be given different amounts of bandwidth.

• Guard time required.

• Need time synchronisation (signal processing required).

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• Need time synchronisation (signal processing required).

• Mobiles can use idle times for mobile assisted handover.

• Power consumption lower: can switch off power when not transmitting.

• Multipath destroys slot orthogonality.

Frame efficiency = [1 – (frame overhead / frame time)] * 100%

No of channels = [No of slots per channel * (Total BW – Guard BW)] / Slot BW


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TDMA Practicalities

• One TDMA frame:

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– Preamble contains address and sync information used by base and mobile

– Guard times allow sync of receivers between different frames

– Users are assigned a position in each frame


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TDMA Slot structure

– Header: guard time for receiver synch. between slots

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– Header: guard time for receiver synch. between slots

– Synch: used to establish bit synch (also for equalizer training)

– Control: used for handshaking, control, and supervisory messages

– Info Bits: coded/uncoded information bits, may include pilot symbols

/ sequences for channel measurement and equalizer training

– CRC: cyclic redundancy check code (for error detection)

– Guard Time: prevents overlap at base of slots arriving from different

terminals.


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Code Division Multiple Access (CDMA)

The name code-division stems

from the fact that the users are

assigned different codes.

Each user is given a separate

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Each user is given a separate

code that works like a key. The

user having the particular key will

receive the desired signal.

Users share the entire frequency

band simultaneously.


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Spreading Factor

• The main parameter of importance in the discussion of spread spectrum systems is the processing gain - the ratio of the transmission bandwidth to the information bandwidth:

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• It is also referred to as the spreading factor. It determines– the number of users that can be allowed in a system,

– the amount of multi-path effect reduction,

– the difficulty to jam or detect a signal etc.

• For spread spectrum systems it is advantageous to have a processing gain as high as possible!


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‘Near-far’ problem

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• Softer capacity � graceful degradation.

• Multipath fading problem reduced.

• Near-far problem: efficient power control techniques required.

• Need large contiguous freq. band as all users use entire BW.

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• Need large contiguous freq. band as all users use entire BW.

• More Complex.

• No. of channels =


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Orthogonal Codes

The codes used can either be orthogonal or nearly orthogonal.

• Orthogonal : the signals do not interfere with one-another.

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• Orthogonal : the signals do not interfere with one-another.– requires precise time synchronization among users.

i.e. the signals must all arrive at the same time.

• The forward channel for IS-95 uses orthogonal codes.– Called Walsh codes.

– Implemented with a Hadamard matrix.


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PN Codes

• For the reverse link, orthogonal codes are not practical.– Difficult to make all user signals arrive at the base station at

precisely the same time.

• Instead, we would like a code that:

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• Instead, we would like a code that:– Has low cross-correlation, even if not synchronized.

• Means ability to separate users (less interference)

– Has low autocorrelation• Ability to separate multipath components.

– Is easy to generate• Less expensive TX and RX hardware.

• PN codes satisfy these requirements.– PN = pseudorandom noise


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Spread Spectrum Methods

• Frequency Hopped Spread Spectrum (FHSS)

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• Direct-Sequence Spread Spectrum (DSSS)

• Hybrid


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Frequency Hopped Spread Spectrum (FHSS)

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Frequency Hopped Spread Spectrum (FHSS)

• The frequency band is divided into several “frequency-slots”

– At each instant of time, the transmitter sends a signal in just one of the frequency-slots.

• After a short period of time, a new frequency-slot is used.

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• After a short period of time, a new frequency-slot is used.

– Changed according to a PN “hopping pattern”.

– Carrier doesn’t dwell in a particular frequency slot long enough to experience or cause significant interference.

• Several users can share the same band.

– Occasionally there will be collisions.

– However, the collisions are infrequent.


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Uses of FHSS

• FHSS is secure

– Hard to track the signal without knowing the PN hopping pattern.

– Difficult to jam.

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• FHSS is used for uncoordinated communications in unlicensed bands.

– Peer-to-peer communications rather than multiple-access.

– Wireless LANs

– Simple cordless telephones.

– Military communications.


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Direct-Sequence Spread-Spectrum (DSSS)

• Chosen as the multiple access method for 3G cellular systems.

• Each user is assigned unique “spreading” sequence of N “chips”.– N = the processing gain = the amount of bandwidth increase.

– PN generators are used to create the spreading sequence.

– Each data bit is multiplied by the signature sequence.

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– Each data bit is multiplied by the signature sequence.

– The result is a wideband signal.

• The receiver is designed to detect only the signature sequence of the desired user.

• All users can use the system bandwidth at all times � possible interference between users can be very large, and channel estimation and data detection is difficult for large system loads.


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DSSS Transmitter

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DSSS Receiver

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DSSS Performance in Multipath

• In DSSS, multiple paths are rejected.– Useful for mobile environments.

• A RAKE receiver can be used to exploit information from both signals.

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signals.

Soft-Handover

• By using a RAKE receiver, DSSS can provide soft handover in cellular systems.– Two base stations can talk to the same mobile at the same time.

– The signals from the two base stations are treated like two distinct multipath components.

– Greatly improves performance at cell boundaries.


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Multiple Access Capability of DSSS

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Multiple Access Capability of DSSS

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‘Near-Far’ Problem Example

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‘Near-Far’ Problem Example

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Space Division Multiple Access (SDMA)

Users given separate space segments for accessing a particular facility.

Highly directional antenna used.

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Desired Signal Direction

Highly directional antenna used.

– The receiver selects the beam that provides the greatest signal enhancement and interference reduction.

– A smart antenna can adjust its antenna pattern.


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Hybrid Schemes

• Combine the multiple access techniques.

• Example:

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– Combine space and frequency: space frequency division multiple access, that is both space and frequency are utilised.

– Combine space with time: space time division multiple access, that is utilising space as well as time to increase the number of users, to give them multiple access facility.


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Multiple Access Control and Assignment Classification

• Aim: to utilise resources efficiently – maximum throughput, minimum delay

– assignment and control are important for this

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• Fixed assignment– pre-assignment: assign beforehand

– demand assignment: allocated based on demand

• Random access – users want to access medium at any time, anywhere.

– random access can be controlled access.


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Wideband Systems

• Conventionally, different radio resources have been

assigned to the neighboring cells

• Nevertheless, the trend has changed recently (last decade)

• Nowadays, wideband networks tend to use reuse = 1

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• Nowadays, wideband networks tend to use reuse = 1

• That is, all radio resources are re-used in each individual cell


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Wideband Systems

• This has been the case not only in CDMA based

technologies (like WCDMA and HSPA), but also in recent

OFDMA based technologies (like LTE and LTE Advanced).

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| TE 412 Introduction to Wireless Communications | Christine Mwase | 10/03/2014 |S-72.3216 RC Systems I (5 cr) Lectures 9-10, Autumn

2013

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• OFDM divides the available spectrum into a large number of narrowband sub-carriers

• Particularity: at the peak of each sub-carrier, all other sub-carriers have zero amplitude

• Avoid sub-carrier interference


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Duplexing

• Separation of uplink and downlink traffic

• Two forms:

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• Two forms:

– Separate frequencies for the uplink and downlink:

FDD

– Separate time slots for the uplink and downlink:

TDD


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Frequency Division Duplex (FDD)

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Time Division Duplex (TDD)

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Duplexing: Uplink / Downlink

•Uplink (from MSs to BS)

• Many transmitters (MSs) send data simultaneously to one receiver (BS)

•Downlink (from BS to MSs)

• One transmitter (BS) sends data to many receivers (MSs)

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to one receiver (BS)

• Transmissions depart from different sources• Users are not synchronized

by definition

• Most of the interference is coming from the MSs in adjacent cells

receivers (MSs)

• Transmissions depart from a single source• Users are synchronized to

each other by definition

• Most of the interference is coming from the BSs in adjacent cells

S-72.3216 RC Systems I (5 cr) Lectures 9-10, Autumn

2013

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UL and DL use the same frequency band, but at different time periods

Only one band (sometimes called as ”unpaired band”) is needed for TDD systems

UL and DL use different frequency bands, and may transmit info at the same time

Two bands needed (sometimes called as ”paired bands”), one for DL and the other one for UL

Time Domain Duplexing Frequency Domain Duplexing

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for TDD systems

Asymmetric DL and UL time division possible (different DL/UL data rate limits can be defined by system configuration)

Good time synchronization between cells and users needed (otherwise, UL and DL may seriously interfere each other)

Interference from distant BSs may become a problem

for DL and the other one for UL

Guard distance between DL and UL bands required (usually, from tens to hundreds of MHz)

Symmetric DL/UL bands applied

Synchronization between cells is not needed, but users in same cell are usually synchronized

Interference between DL and UL transmissions is small


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System FDD TDD Comment

W-CDMA X O Some frequency bands were granted for

TDD but most of the bands are not used by

TDD version of this technology. FDD

dominating

HSPA X O FDD dominating

Mobile WiMAX O X FDD version not widely deployed

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Mobile WiMAX O X FDD version not widely deployed

LTE X O FDD version expected to be first deployed

TD-SCDMA X

GSM/GPRS/EDGE X

S-72.3216 RC Systems I (5 cr) Lectures 9-10, Autumn

2013

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X = main stream version O = not widely deployed or expected to be widely deployed

15. Multiple Access Techniques

Documents

timedivision multiple

multiple access multiplexing

space multiple access

multiple users

access techniquesgoal

telecommunications engineering

access techniques fdma

communications resource