On Receiver Algorithms for OFDM Systems
This thesis addresses the problem of designing a receiver for an
Orthogonal FrequencyDivisionMultiplexing (OFDM)communication system operating
over fading channels, taking into account the effects of hardware imperfections.
We discuss solutions for efficiently estimating the parameters of a channel
model and for compensating oscillator phase noise and analog to digital
converter (ADC) quantisation errors at the receiver.
The thesis consists of a general introduction to the topics of digital
communication (OFDM, channelmodelling, estimation) and of threemain parts
addressing the issues of phase noise compensation, channel estimation and ADC
quantisation noise cancellation, respectively.
In part I, after characterising the oscillator phase noise using
aWiener process model, we analyse the effects of phase noise on an OFDM system
operating over fading channels and provide an analytical bit error rate (BER)
result. We then discuss the problem of estimating and reducing the effects of
phase noise. Our solution is based on a two step iterative algorithm that
estimates and removes the phase noise-induced common phase error (CPE) and
intercarrier interference (ICI). The method is compared to existing algorithms
and shows improvement at the cost of additional complexity.
In part II, we propose a channel estimation algorithm for a coded multiband-OFDM
system operating over the UWB channel. The algorithm is based on the classic
Wiener filter-based MMSE channel estimator, which is extended to a decision
directed iterative algorithm, taking into account the reliability of the
information feedback. Different feedback methods with different orders of
complexity are proposed and compared. The performance of the receiver algorithms
are presented for the standardMB-OFDM UWB system which uses a punctured
convolutional code as well as for a modified system which replaces the channel
code for a rate and complexity equivalent turbo code. The results show that
introducing the reliability measure in the estimation procedure improves the
error rate performance of the system. Moreover, for an equivalent complexity,
simulation showed an improvement of the performance using a turbo code over a
In part III, we discuss the effect of ADC quantisation of the received signal
for an OFDM system using a memoryless uniform quantiser. After providing an
analysis of the effect of quantisation of an OFDM signal on the bit error rate
and the channel capacity for AWGN and fading channels, we discuss a compensation
algorithm. The algorithm is based on a decision feedback method with a feedback
loop describing the quantisation error with a power-series type non-linearity.
The algorithm is tested on a perfectly known channel as well as with a channel
estimated with the algorithm in part II and proved to be able to effectively
cancel the effects of quantisation on the received OFDM signal, as well as being
robust to channel estimation error.
Intercarrier Interference (ICI)
orthogonal frequency division multiplexing (OFDM)
Room EC, Hörsalsvägen 11, Chalmers Univ. Tech.
Opponent: Prof. Mikael Sternad, Signals and Systems, Uppsala University, Sweden