Transcript Equalisation Architectures for OFDM and 3G

Equalisation Architectures
for OFDM and 3G
Steve McLaughlin
Yushan Li
David G .M. Cruickshank
IDCOM, University of Edinburgh
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© 2004 Mobile VCE
LGE Paris 2005
Outline
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
Introduction and Motivation

Chip-Level Equalisation for WCDMA

FDE in SC Systems

Chip-Level FDE for WCDMA

Joint structure for Channel Estimation, ChipLevel FDE and Parallel Interference
Cancellation

Conclusions
LGE Paris 2005
Motivation
 Concerned with algorithmic issues which will enable
Multimode behaviour
 Consider UMTS and OFDM systems
 Frequency domain equalisation approach
 Unfortunately, a CP-based FDE is not compatible for the
current UMTS system and the overhead introduced by a
CP will reduce the spectral efficiency.
 Suggest some solutions to overcome this while
minimising complexity
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Chip-Level Equalisation for WCDMA
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RAKE: The performance is dominated by the
MAI and this results in saturation at a fairly
high error rate.
MUD: The possibility to perform multiuser
detection in mobile handsets is limited by its
high complexity
Symbol-Level Equalisation: Not suitable for
long code WCDMA.
Chip-Level Equalisation: Achieves good
compromise between performance and
complexity
LGE Paris 2005
FDE in SC Systems
1. Transform the received signal from the time
domain to the frequency domain (FFT)
2. Adjust each discrete frequency bins and
make the spectrum flat. Single tap equalizer in
the frequency domain -> Simple structure
3. Transform the equalized signal back to the
time domain (IFFT)
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SC-FDE vs. Time Domain Equalisation:
Complexity
Computationally simpler,
especially for channels with
severe delay spread (11~20
chips)
For severe channel
spreading, complexity of
frequency domain
processing grows slowly
than time domain
processing.
Details may be found in:
Falconer, et al, “Frequency domain equalization for single-carrier broadband
wireless systems”, IEEE Comm. Magazine, April 2002
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SC-FDE in Multimode Receivers
Merit:
Employing a similar architecture as in OFDM
systems, SC-FDE and OFDM can easily be
configured to coexist, thus makes the
multimode receiver simpler while a connection
to UMTS is required
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Issue & Solution
Issue: Unfortunately, a CP-based FDE is not
compatible for current single-carrier systems
with no CP. It is desirable to design a receiver
without changing the format of the transmitted
signal.
Solutions: A number of solutions have been
proposed for OFDM systems or single-carrier
systems without CP. They aim at compensating
the effect of the missing CP.
– cyclic reconstruction.
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Solutions
1. D. Kim and G. Stüber, "Residual ISI cancellation for OFDM
with applications to HDTV broadcasting", IEEE Journal on
Select Areas in Communications, Vol. 16, No. 8, Aug. 1998,
pp. 1590-1599. (RISIC Algorithm)
2. C. Park and G. Im, "Efficient DMT/OFDM transmission with
insufficient cyclic prefix", IEEE Communications Letters,
Vol. 8, Issue 9, Sept. 2004, pp. 576-578.
3. H. Won and G. Im, "Iterative cyclic prefix reconstruction
and channel estimation for a STBC OFDM system", IEEE
Communications Letters, Vol. 9, Issue 4, Apr. 2005, pp. 307309.
4. Y. Li, S. McLaughlin and D.G.M. Cruickshank, "Bandwidth
efficient single carrier systems with frequency domain
equalization", Electronics Letters, Vol. 41, No. 15, July 2005,
pp. 857-858.
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RISIC Algorithm
Introduction: In the RISIC algorithm, the
missing CP is regarded as bursty distortion in
a time domain block and the amount of
distortion is diminished in an iterative process
with hard decisions being made in the
frequency domain.
Performance degrades in channel with deep
nulls since the hard decision will cause noise
enhancement.
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Existing CP reconstruction methods
Tail
Cancellation
FFT
Equalizer
BPSK
Demodulation
DeInterleaver
SISO
Decoder
RISIC
CP
Reconstruction
IFFT
IFFT
Soft
Mapper
Extended RISIC
Interleaver
RISIC Scheme and Extended RISIC Scheme
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Chip-Level FDE for WCDMA
 In principle, proposed cyclic reconstruction
schemes can all be extended for single-carrier
WCDMA systems in order to deploy FDE at chip
level. However, some of them suffer from high
computational complexity, especially in the case
of the application to WCDMA.
 Solutions particularly proposed for WCDMA
1. “Overlap-Cut” Method
2. “FDE based on Self cyclic reconstruction”
3. “FDE based on Slot Segmentation”
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Overlap-Cut Method
1. Applying a conventional FDE on a single
carrier system without CP gives errors that
are significantly larger at the edges of the
block.
2. Samples at the beginning and the end of
each equalized blocks are discarded.
3. Processing blocks are overlap with each
other.
Ref: M. Vollmer, M. Haardt and J. Gotze, "Comparative study of joint
detection techniques for {TD-CDMA} based mobile radio systems", IEEE
Journal on Select Areas in Communications, Vol. 19, No. 8, Aug. 2001, pp.
1461-1475.
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FDE based on Self cyclic reconstruction

The algorithm exploits the relationship
between the required cyclic part and the
transmitted signal itself. The estimated cyclic
part is then added to the received block
signal to enable frequency domain
equalization. This can be viewed as a cyclic
reconstruction process.
Ref: Y. Li, S. McLaughlin, D.G.M. Cruickshank, "UMTS FDD frequency
domain equalization based on self cyclic reconstruction", IEEE International
Conference on Communications, Vol.3, May 2005, pp. 2122-2126.
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FDE based on Slot Segmentation


By exploiting the frame and slot structures of
the UMTS downlink, the pilots within one slot
(for FDD mode) are used for cyclic
reconstruction in a FDE.
Furthermore, one slot signal is split into
multiple segments for the sake of combating
channel variance within one slot.
Ref: Y. Li, S. McLaughlin, D.G.M. Cruickshank, "UMTS FDD frequency
domain equalization based on slot segmentation", Proceedings of the 61st
IEEE Vehicular Technology Conference, May 2005, Stockholm, Sweden.
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Joint Channel Estimation, Chip-Level FDE and Parallel
Interference Cancellation structure for WCDMA
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
Accurate channel estimation for a practical
mobile communication system is important!

Time-multiplexed pilots: require extra
bandwidth and hence reduce bandwidth
efficiency.

Code-multiplexed pilots: no bandwidth
spreading is necessary.
LGE Paris 2005
Correlation Method
 In practice, the correlation method (CM) is a
simple technique for channel estimation in
WCDMA.
 The distorted autocorrelation property due to
channel impairments degrades its performance.
 A high power code-multiplexed pilot sequence
is required for better channel estimates.
 Unfortunately, high power pilot channel  high
MAI to the data channels.
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IFDCE: Iterative Frequency Domain Channel
Estimation
 The IFDCE method reconstructs the sum of
data channels and the code-multiplexed pilot
channel. The reconstructed composite signal is
being treated as a virtual pilot signal.
 Channel estimation is performed in the
frequency domain.
 The received WCDMA signal is equalised
before spreading at chip level in the frequency
domain.
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Procedures …
Step 1. Correlation method is used to deliver
initial channel estimates.
Step 2. A RAKE receiver then operates on the
received signal and the composite estimated
signal is despread and hard detected.
Step 3. K users' transmitted symbols are respread
and rescrambled. The scrambled codemultiplexed pilots are added to form an
estimated composite signal.
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Procedures
Step 4. The estimated composite signal and the
initial channel estimates are used for cyclic
reconstruction.
Step 5. The reconstructed composite signal,
being treated as a virtual pilot signal, is
converted to the frequency domain and used
for channel estimation.
Step 6. The result from Step 5 is converted to the
time domain and only the first “L” values
(Channel is assumed to span L chips) are kept
to form a new channel estimate.
Step 7. Frequency domain equalisation.
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Proposed IFDCE Structure
Correlation
Method
1
CR: Cyclic Reconstruction
hˆ i
ri n
~
xi n
Descramble
Despread
Hard Decision
RAKE
+

~
ri n
+
CR
2
FFT
IFDCE
FFT
hˆ iIFDCE
FDE
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IFFT
Scrambled codemultiplexed pilot
+
+
xˆi n

Descramble
Despread
Hard Decision
LGE Paris 2005
Spread
Scramble
bˆi(,mk)
Parallel Interference Cancellation

Why PIC?
1. Since the proposed iterative channel
estimation requires user symbol detection and
interference reconstruction, a PIC is combined
into the iterative structure in order to further
enhance the system performance.
2. The integration of PIC is with only a slight
increase in computational complexity. This is
exactly why the PIC is introduced into the
iterative structure.
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IFDCE + PIC Scheme
~
ri n
+

FFT
Detector for the
Desired User k
Detector for the
Interfering User 0
Detector for the
Interfering User K-1
Scrambled codemultiplexed pilot
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Regenerate
Transmitted Signal
IFDCE
Reconstruct
Composite
Interference from
User 0 to User K-1
(excluding User u)
and the Parallel
Pilot Channel
LGE Paris 2005
Equalizer
& IFFT
Simulations
WCDMA Systems
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Carrier Frequency = 2 GHz
Chip Rate = 3.84 Mchips/s
Spreading Factor = 64
10 Active Users
Pilot Channel Power = 10% Whole Power
Block Size = 1024 chips (For CE and FDE)
UMTS Vehicular A Test Channel
Mobile Speed = 30 km/h
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Simulation Results
3.8 dB
Close to the ideal case
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Simulation Results
Close to the
single user case
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Discussions
1. The iterative channel estimator can provide
the PIC with better channel estimates,
hence better performance.
2. The iterative cycle can be implemented
efficiently in the frequency domain with
complexity of O(NlogN) where N is the
block size.
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Conclusions
 To design a FDE for the current WCDMA system
is very attractive. OFDM has become a strong
candidate for the fourth generation systems and
hence a WCDMA receiver adopting FDE will be
compatible with the current FFT based receiver
structures.
 By adopting FDE for single carrier WCDMA, a
multi-mode receiver can be programmed to
switch to a particular system more conveniently.
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Thank you!
Steve McLaughlin
IDCOM, University of Edinburgh
Email: [email protected]
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© 2004 Mobile VCE
LGE Paris 2005