Advanced signal and receiver design for next generation OFDM systems
Creators: name: Rosati, Stefano <1982>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2011
Subjects: ING-INF/03 Telecomunicazioni

description: This thesis deal with the design of advanced OFDM systems. Both waveform and receiver design have  been treated. The main scope of the  Thesis is to study, create, and propose, ideas and novel design solutions able to cope with the weaknesses  and crucial aspects of modern OFDM systems.


Starting from the the transmitter side, the problem represented by low resilience  to non-linear distortion has been assessed. A novel technique that considerably reduces the Peak-to-Average Power Ratio (PAPR) yielding a quasi constant signal envelope in the time domain (PAPR close to 1 dB) has been proposed.The proposed technique, named Rotation Invariant Subcarrier Mapping (RISM),is a novel scheme for subcarriers data mapping,where the symbols belonging to the modulation alphabet are not anchored, but maintain some degrees of freedom. In other words, a bit tuple is not mapped on a single point, rather it is mapped onto a geometrical locus,
which is totally or partially rotation invariant.
The final positions of the transmitted complex symbols are chosen by an iterative optimization process in order to minimize the PAPR of the resulting OFDM symbol.
Numerical results confirm that RISM makes OFDM usable even in severe non-linear channels.

Another well known problem which has been tackled is the vulnerability to synchronization errors.
Indeed in OFDM system an accurate recovery of carrier frequency and symbol timing is crucial for the proper demodulation of the received packets.
In general,  timing and frequency synchronization is performed in two separate phases called PRE-FFT and POST-FFT synchronization.
Regarding the PRE-FFT phase, a novel joint symbol timing and carrier frequency synchronization algorithm has been presented.
The proposed algorithm is characterized by a very low hardware complexity, and, at the same time, it guarantees very good performance in in both AWGN and multipath channels.
Regarding the POST-FFT phase, a novel approach for both pilot structure and receiver design has been presented.  In particular, a novel pilot pattern has been introduced in order to  minimize  the occurrence of overlaps between two pattern shifted replicas. This allows to  replace conventional pilots with nulls in the frequency domain, introducing the so called Silent Pilots.
As a result, the optimal receiver turns out to be very robust against severe Rayleigh fading multipath and characterized by low complexity.
Performance of this approach has been analytically and numerically evaluated. Comparing the proposed approach  with state of the art alternatives,  in both AWGN and multipath fading channels,  considerable performance improvements have been obtained.

The crucial problem of channel estimation has been thoroughly investigated, with particular emphasis on  the decimation of the Channel Impulse Response (CIR) through the selection of the Most Significant Samples (MSSs).
In this contest our contribution is twofold, from the theoretical side, we derived  lower bounds on the estimation mean-square error (MSE) performance for any MSS selection strategy,from the receiver design we proposed novel MSS selection strategies which have been shown to approach these MSE lower bounds, and outperformed the state-of-the-art alternatives.

Finally, the possibility of using of  Single Carrier Frequency Division Multiple Access (SC-FDMA) in the Broadband Satellite Return Channel has been assessed.
Notably, SC-FDMA is able to improve the physical layer spectral efficiency with respect to single carrier systems, which have been used so far in the Return Channel Satellite (RCS) standards.
However, it requires a strict synchronization and it is also sensitive to phase noise of local radio frequency oscillators.
For this reason, an effective pilot tone arrangement within the SC-FDMA frame, and a novel Joint Multi-User (JMU) estimation method for the SC-FDMA, has been proposed.
As shown by numerical results, the proposed scheme manages to satisfy  strict synchronization requirements and to guarantee a proper demodulation of the received signal.