By

SANJANA PRASAD 1601RL01

PhD-RESEARCH SCHOLAR

PSG COLLEGE OF TECHNOLOGY

EQUAL GAIN COMBINING AND ORTHOGONALITY RESTORING

COMBINING

INTRODUCTION OVERVIEW OF MAXIMAL RATIO

COMBININGEQUAL GAIN COMBININGORTHOGONALITY RESTORING COMBININGSUMMARY REFERENCES

TOPICS

Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access scheme used in OFDM-based telecommunication systems,

system support multiple users at the same time over same frequency band.

Spreads each user symbol in the frequency domain.Each user symbol is carried over multiple parallel subcarriers,

but it is phase shifted (typically 0 or 180 degrees) according to a code value.

The code values differ per subcarrier and per user. The receiver combines all subcarrier signals, by weighing these

to compensate varying signal strengths and undo the code shift. Receiver =>separate signals of different users, because these

have different (e.g. orthogonal) code values.

INTRODUCTION

Combines multipath components to maximize SNR

Characterized by Equalisation GainWhen SNR is high ,BER curves converged to

BER floorBER Floor-dependent on the Spreading

Factor.worst for SF=16;As SF>16=>BER floor

reducedCombining multipath components impairs the

orthogonality of the codesTo avoid the impairment of

orthogonality ,EGC is preferred.

OVERVIEW OF MAXIMAL RATIO COMBINING

Spreading codes-orthogonalOrthogonality destroyed if differently delayed

multipath components received Attempts to correct the channel induced

phase rotations , leaving faded magnitude uncorrected

Equalisation gain

EQUAL GAIN COMBINING

3 SIGNAL COMPONENTS

Single userBER curves

approached theoretical ones

Overall EGC slightly worse than MRC

Spreading factor>4No improvement in

the case of MRC

Number of users-increases

Spreading factor=4Performance –

degradedBetter than MRC

Multi-users

When all users transmittingSpreading factor=1Multi-user interference increases significantly Eroding the benefits of frequency diversity and

performance becomes worse than OFDM,corresponding to SF=1

Drawback Of MRC & EGC:Due to lack of orthogonality they inflict MUI

Frequency selective fading –destroys orthogonality of different users spreading codes

Aim Of Orthogonality Restoring Combining(ORC)or Zero Forcing Equalisation:Orthogonality of different users-maintained if

we cancel effect of channel transfer function by estimating it and reversing its effects.

Uses equalisation gain

ORTHOGONALITY RESTORING COMBINING

2 components in the decision variable

SIMULATION RESULTS FOR VARIOUS SF WHEN ORC-APPLIED IN MC –CDMA SCHEME

MC-CDMA system using various spreading factors ,performed worse than single user OFDM scheme,SF=1Average per user γb<25dbAs SF increased->performance degraded;orc scheme had

already removed the frequency selectivity of the channel transfer function

Source of performance degradation-noise enhancement-at lower SNR values

Fading severe-> ng/Hg =>high; amplifies both signal and noise

SF increased-{Hg}-encountered deep frequency domain fades-excessive noise enhancement

Less significant at high SNR

Controlled EqualisationProposed by Yee and Linnartz

To combat noise enhancementUses a subset of subcarriers for the ORC

scheme ,associated with the channel transfer factors that are above a certain threshold.

OFDM and MC-CDMA : A Primer by Lajos Hanzo and Thomas Keller

Overview Of Multicarrier CDMA By Shinsuke Hara And Ramjee Prasad

REFERENCES