Combining techniques in OFDM

1. By
SANJANA PRASAD
1601RL01
PhD-RESEARCH SCHOLAR
PSG COLLEGE OF TECHNOLOGY

2.  INTRODUCTION
 OVERVIEW OF MAXIMAL RATIO COMBINING
 EQUAL GAIN COMBINING
 ORTHOGONALITY RESTORING COMBINING
 SUMMARY
 REFERENCES

3.  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.

6.  Combines multipath components to maximize SNR
 Characterized by Equalisation Gain
 When SNR is high ,BER curves converged to BER
floor
 BER Floor-dependent on the Spreading Factor.
 worst for SF=16;As SF>16=>BER floor reduced
 Combining multipath components impairs the
orthogonality of the codes
 To avoid the impairment of orthogonality ,EGC is
preferred.

7.  Spreading codes-orthogonal
 Orthogonality destroyed if differently delayed
multipath components received
 Attempts to correct the channel induced phase
rotations , leaving faded magnitude uncorrected
 Equalisation gain

8.  3 SIGNAL COMPONENTS

10. Single user
 BER curves approached
theoretical ones
 Overall EGC slightly
worse than MRC
 Spreading factor>4
 No improvement in the
case of MRC
 Number of users-
increases
 Spreading factor=4
 Performance – degraded
 Better than MRC
Multi-users

12.  When all users transmitting
 Spreading factor=1
 Multi-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

13.  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

14. 2 components in the decision variable

16.  MC-CDMA system using various spreading factors
,performed worse than single user OFDM scheme,
 SF=1
 Average per user γb<25db
 As 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

17.  Controlled Equalisation
 Proposed by Yee and Linnartz
 To combat noise enhancement
 Uses a subset of subcarriers for the ORC scheme
,associated with the channel transfer factors that are
above a certain threshold.

18.  OFDM and MC-CDMA : A Primer by Lajos Hanzo and
Thomas Keller
 Overview Of Multicarrier CDMA By Shinsuke Hara
And Ramjee Prasad