Rudy Lauwereins

Low-complexity linear frequency domain equalization for continuous phase modulation

In this paper, we develop a new low-complexity linear frequency domain equalization (FDE) approach for continuous phase modulated (CPM) signals. As a CPM signal is highly correlated, calculating a linear minimum mean square error (MMSE) channel equalizer requires the inversion of a nondiagonal matrix, even in the frequency domain. In order to regain the FDE advantage of reduced computational complexity, we show that this matrix can be approximated by a block-diagonal matrix without performance loss. Moreover, our MMSE equalizer can be simplified to a low-complexity zero-forcing equalizer. The proposed techniques can be applied to any CPM scheme. To support this theory we present a new polyphase matrix model, valid for any block-based CPM system. Simulation results in a 60 GHz environment show that our reduced-complexity MMSE equalizer significantly outperforms the state of the art linear MMSE receiver for large modulation indices, while it performs only slightly worse for small ones.

Joint Transmit and Receive Analog Beamforming in 60 GHz MIMO Multipath Channels

Analog BeamForming (ABF) with one scalar weight per antenna is an attractive technique for low-cost, low-power 60 GHz multi-antenna wireless communication systems. However, the design of the corresponding joint transmit and receive (Tx/Rx) ABF optimization algorithms is still challenging in the case of multipath channels due to the constraint of having only one scalar weight per antenna. In this paper, we aim at maximizing the average Signal to Noise Ratio (SNR) at the input of the equalizer and analytically derive close-to-optimal Tx/Rx scalar weights. We show that the required Channel State Information (CSI) for joint Tx/Rx ABF weights computation is the inner product between all Tx/Rx channel impulse response pairs. Taking the channel length into account, a training-based estimation strategy of this CSI is proposed. Simulation results carried out in a typical 60 GHz multipath environment show that the proposed scheme outperforms the existing ABF schemes in term of BER performances.

A New Symbol Block Construction for CPM with Frequency Domain Equalization

We present a new symbol block construction which yields a cyclic continuous phase modulated (CPM) signal to enable frequency domain equalization. It is known that in addition to a cyclic prefix, a subblock of data-dependent symbols has to be inserted in each block to cope with the memory in the CPM signal. We propose a new subblock, called intrafix, valid for any CPM scheme. Our intrafix is shorter than what is currently known in the literature, reducing the overhead. Moreover, it can be calculated on a per-block basis, without knowledge of previous blocks. We also prove that there are constraints on the length of both the intrafix and the total block by studying the influence of the modulation index. Simulation results in a 60 GHz environment show that our new block construction satisfies all requirements.

Maximum SINR-Based Beamforming for the MISO OFDM Interference Channel

We address the problem of co-channel interference (CCI) in wireless mesh networks (WMNs). In such networks, multiple nodes communicate concurrently using the same time/frequency resources. This is recognized as the interference channel (IFC) because the CCI is a major impairment in such a scenario. To mitigate the CCI, non-cooperative beamforming techniques can be employed. Non-cooperative beamformers are simpler to implement than their cooperative counter-parts because they require neither synchronization nor the sharing of information data between the transmit nodes. In this paper, we then propose a non-cooperative beamforming scheme to improve the performance of WMNs with frequency-selective channels. We present an iterative algorithm that maximizes the signal-to-interference-and-noise ratio criterion over all subcarriers of the orthogonal frequency division multiplexing system. The performance of this algorithm is evaluated through simulations.

A Flexible Antenna Selection Scheme for 60 GHz Multi-Antenna Systems Using Interleaved ADCs

We present a new scheme for maximizing the signal-to-noise ratio (SNR) in wideband multi-antenna receivers that employ time-interleaved analog-to-digital converters (ADCs). Current wideband receivers interleave a fixed number of slower ADCs into one fast ADC and assign this latter to one antenna according to an antenna selection algorithm. However, this does not always guarantee an optimal trade-off between thermal noise and quantization noise. This results in an overall SNR that is lower than what could be obtained with the same number of ADCs, assigned in a more optimal way. Therefore, we propose to adjust the number of slower ADCs assigned to a certain fast, interleaved ADC dynamically, according to the SNR of every individual antenna. Our proposed algorithm can be implemented at the expense of a very limited hardware complexity increase. The SNR gain of our new scheme can exceed 7 dB, depending on channel conditions and ADC specifications.

Performance analysis of distributed ZF beamforming in the presence of CFO

We study the effects of residual carrier frequency offset (CFO) on the performance of the distributed zero-forcing (ZF) beamformer. Coordinated transmissions, where multiple cells cooperate to simultaneously transmit toward one or multiple receivers, have gained much attention as a means to provide the spectral efficiency and data rate targeted by emerging standards. Such schemes exploit multiple transmitters to create a virtual array of antennas to mitigate the co-channel interference and provide the gains of multi-antenna systems. Here, we focus on a distributed scenario where the transmit nodes share the same data but have only the local knowledge of the channels. Considering the distributed nature of such schemes, time/frequency synchronization among the cooperating transmitters is required to guarantee good performance. However, due to the Doppler effect and the non-idealities inherent to the local oscillator embedded in each wireless transceiver, the carrier frequency at each transmitter deviates from the desired one. Even when the transmitters perform frequency synchronization before transmission, a residual CFO is to be expected that degrades the performance of the system due to the in-phase misalignment of the incoming streams. This paper presents the losses of the signal-to-noise ratio gain analytically and the diversity order semi-numerically of the distributed ZF beamformer for the ideal case and in the presence of a residual CFO. We illustrate our results and their accuracy through simulations.

Training Sequence Versus Cyclic Prefix for CPM with Frequency Domain Equalization

Frequency domain equalization (FDE) of continuous phase modulations (CPM) has been thoroughly investigated lately. To enable this low-complexity FDE, all known techniques use a cyclic prefix (CP). However, using a training sequence (TS) of known symbols instead of a CP offers some advantages: the additional known symbols can be used to improve synchronization and channel estimation, with the same performance of a CP. Nevertheless, using a TS for CPM-FDE is not trivial because the memory in a CPM waveform has to be taken into account to guarantee cyclicity and phase continuity after insertion of the TS into a block of input symbols. In this paper, we therefore propose a technique for constructing a TS for CPMFDE. Simulation results in a 60 GHz environment show that the proposed technique satisfies all requirements. The 60 GHz case is chosen because CPM with FDE has recently been proposed for communications at 60 GHz and the latest IEEE and ECMA standards for these frequencies mandate the use of a TS rather than a CP.

Beamforming techniques for enabling spatial-reuse in MCCA 802.11s networks

We address the problem of co-channel interference (CCI) in wireless mesh networks based on the IEEE802.11s extension. The carrier sensing mechanism deployed in those networks insufficiently addresses the CCI problem, causing the hidden and exposed node problems; consequently degrading the throughput and latency. In this paper, we show how beamforming techniques can be implemented on top of the IEEE802.11s medium access control protocol and, using the information readily available, cancel the interference to mitigate this inefficiency of carrier sense and improve the spatial-reuse gain. In addition, we propose the signal-to-jamming-noise ratio (SJNR) beamformer and show that it significantly improves the spatial-reuse gain compared to the simple zero-forcing (ZF) beamformer and the basic IEEE802.11s access scheme. We derive the ergodic capacity of the ZF beamformer and the basic IEEE802.11s access scheme and simulate the performance of the various schemes. We show that improvements of up to 85% are achieved as function of the scenario simulated and the beamforming technique used and that the SJNR scheme outperforms the standard ZF beamformer.

Novel block constructions using an intrafix for CPM with frequency domain equalization

To enable frequency domain equalization (FDE) for continuous phase modulation (CPM), both cyclicity of individual symbol blocks and phase continuity between different blocks have to be guaranteed. In this letter, we present new block constructions that use a subblock of data-dependent symbols, called intrafix, to satisfy both constraints for different CPM-FDE systems: using either a cyclic prefix or a training sequence (TS), both for precoded and nonprecoded CPM. The known symbols of a TS can be used to improve synchronization and channel estimation. Precoding can be applied to a certain class of CPM schemes to halve the bit error rate.

Spectral regrowth analysis of band-limited offset-QPSK

In this paper, we present an analytical analysis to predict the power spectral density (PSD) at the output of a nonlinear power amplifier (PA). We focus on offset quadrature phase shift keying (OQPSK) waveform band-limited by a square root raised cosine (SRRC) filter. This is one of the waveforms used in wideband code division multiple access (W-CDMA) wireless standard. We show that the PA output PSD obtained by our analytical analysis matches well the simulated PSD. Furthermore, we compare the PA output PSD of QPSK and OQPSK waveforms as a function of the SRRC filter roll-off. We conclude that for small roll-off, both QPSK and OQPSK experience almost the same level of spectral re-growth. As the roll-off increases, OQPSK becomes less sensitive to PA nonlinearity relative to QPSK.