Broadband wireless systems require high power and spectral efficiencies, and transmission over severely time-dispersive channels. In this research theme we consider the design and implementation of digital transmission techniques with high power and spectral efficiency to be employed in the uplink of mobile systems or in satellite communications. We consider SC-FDE techniques (Single- Carrier with Frequency-Domain Equalization) combined with powerful IB-DFE (Iterative Block Decision Feedback Equalizer) receivers [Ben10]. Our main goal is to design signals with low PAPR (Peak-to-Average Power Ratio) or even quasi-constant envelope and high spectral efficiency and employ amplification techniques based on low-cost, highly efficient grossly NL (NonLinear) amplifiers (e.g., class D and E amplifiers), which are simpler and have higher amplification and output power than quasi-linear amplifiers [Rey06]. Two approaches will be followed.
The first one uses LINC techniques (Linear amplification with Nonlinear Components) [Cox74], where a variable-envelope signal is decomposed as the sum of two constant-envelope signals, which are separately amplified without distortion by two NL amplifiers. Although LINC techniques seem an efficient solution for our problem, they have the following problems:
-The power amplification efficiency reduces with the PAPR of the signals at the LINC input;
-The LINC decomposition is performed on a sampled version of the signals and the oversampling requirements can be very high, mainly due to signals’ zero crossings;
-The two amplifiers must be perfectly matched, since amplifiers’ unbalances lead to performance degradation and out-of-band radiation.
To maximize amplification efficiency we use MM (Magnitude Modulation) techniques [Tom02, Gom09, Gom10] so as to reduce the PAPR of the signals at LINC input. Appropriate MM techniques will be developed so as to avoid zero-crossings and the IB-DFE receiver will be designed taking into account the adopted MM schemes and eventual residual amplifier unbalances. The impact of phase and gain mismatches on performance and out-of-band radiation will be studied to define amplifiers’ matching requirements. The second approach is less orthodox and takes advantage of the following:
-A given constellation can be decomposed as the sum of binary components [Din10];
-The complex envelope of OQPSK (Offset Quadrature Phase Shift Keying) signals at an appropriate frequency can be regarded as BPSK signals [Amo77];
-OQPSK signals can have quasi-constant envelope.
Therefore, a given constellation can be decomposed as the sum of several constant-envelope BPSK (or OQPSK) components that can be separately amplified without distortion by different NL amplifiers, as in LINC transmitters, but without oversampling requirements. One problem with this approach is that the band of constant envelope BPSK/OQPSK signals is much wider than the minimum Nyquist bandwidth. For this reason, we replace each BPSK/OQPSK component by an OQPSK-type signal (which is also the sum of OQPSK terms) specially designed to have quasi-constant envelope and good spectral characteristics [Mon10]. This approach is suitable for somewhat loose spectral restrictions. If not, we separate different frequency channels to the minimum Nyquist band and design FDE receivers able to cope with strong ACI levels (Adjacent Channel Interference) by combining the techniques of [Din05,Luz09] with receivers specially designed for offset modulations [Mar09,Luz12]. With this approach, amplifiers’ unbalances do not lead to out-of-band radiation, however they can lead to performance degradation.
The proposed techniques will be studied both analytically and by simulation, to check the potential and limitations of each one, taking into account achievable performance, spectral efficiency robustness and complexity issues.
The main contributions of this project are the following:
-Design of OQPSK-type signals with quasi-constant envelope and good spectral characteristics;
-Design of MM schemes specially designed to be combined with LINC-type techniques, namely to reduce the oversampling requirements;
-Design MM schemes for offset modulations and different pulse shapes;
-Design of linear amplification schemes for general constellations that employ multiple (more than two) grossly NL amplifiers; -FDE design of MM and OQPSK-type schemes, with or without ACI, eventually taking into account residual amplifiers’ mismatches; -Definition of matching requirements for multi-amplifier techniques;
-Validation and system simulation for most promising techniques;
|Start Date: 01-04-2014|
|End Date: 01-03-2015|
|Team: Rui Miguel Henriques Dias Morgado Dinis, Marco Alexandre Cravo Gomes, Vitor Manuel Mendes da Silva|
|Groups: Radio Systems – Lx, Multimedia Signal Processing – Co|
|Local Coordinator: Rui Miguel Henriques Dias Morgado Dinis|
|Links: Internal Page|