equal gain combining and orthogonality restoring combining
TRANSCRIPT
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