Julian Webber

A generalized approach to block diagonalization for multiuser MIMO downlink

In multiple input multiple output (MIMO) systems, multiuser enhancement is in the implementation phase. Block diagonalization (BD) is known as the safest technique for the multiuser MIMO downlink since it suppresses the inter-user interference (IUI) perfectly. However, BD consumes most of the transmit antenna resource to form many nulls. Thus, it is very difficult to obtain the transmit diversity gain. In the paper, the adaptive selection of the number of layers is considered. Such approaches have been limited to iterative beamforming. Here, we formulate deterministic transmit beamforming as a generalized approach to the BD (GBD), based on a concept of projecting the IUI to a limited vector space. Our proposal can efficiently utilize the extra degrees of freedom, which are obtained in exchange for reducing the number of layers, to provide the diversity gain in the transmit beamforming. The high capability of the GBD is confirmed by computer simulations.

Experimental Evaluation of a Poly-Polarization Multiplexing System with Timing/Frequency Recovery for Satellite Communications

In this paper, we demonstrate the performance of our recently proposed spectrally-efficient poly-polarization multiplexing (PPM) and polarization domain modulation (PDM) techniques in a FPGA-based firmware system in order to validate the simulated results. The paper describes the architecture and implementation of the real-time system on FPGA. We employ a Gardner timing error detector and modified Luise & Reggianninni (L&R) frequency recovery algorithm to correct the sample timing and frequency offsets respectively. As the result of hardware experiments, we have confirmed the software PPM LDPC simulations that the BER of 1e-6 can be achieved at 4.8 dB Eb/N0 and that there was minimal degradation due to quantization as well as timing and frequency offsets.

Performance Evaluation of Enhanced Poly-polarization Multiplexing Scheme for Satellite Communications

This paper proposes an enhanced poly-polarization multiplexing (E-PPM) transmission scheme to further increase the spectrum efficiency of a conventional poly-polarization multiplexing (PPM) scheme. In the E-PPM scheme, one of the important techniques is the mapping of signals. Thus, we optimize the E-PPM signal set with 8 bits/symbol and evaluate its performance. By the computer simulations, the proposed E-PPM scheme using the optimized signal set with the spectrum efficiency of 8 bit/s/Hz reduces the required Eb/N0 by 0.4 dB compared with a conventional PPM scheme.

Integrated Synchronization Scheme for WLAN Systems Employing Multiband Simultaneous Transmission

In this paper, we propose a synchronization scheme to obtain frequency diversity in a WLAN system which simultaneously uses the multiple bands such as 5 GHz, 2.4 GHz and 920 MHz simultaneously. The proposed scheme is to combine the output of correlators after compensation for the difference of the center frequency on each frequency band. The proposed scheme obtains a large frequency diversity gain by joint synchronization processing across multiple bands, and improves the accuracy of synchronization. Simulation results show that the proposed scheme can achieve a better performance of synchronization than a conventional scheme which performs synchronization process only in a band.

Low Latency Relay Processing Scheme for WLAN Systems Employing Multiband Simultaneous Transmission

In this paper, we propose a low latency relay transmission scheme for WLAN which simultaneously uses multiple bands. In order to reduce the latency, the proposed relay transmission scheme forwards a bit sequence which is obtained by truncated decoding. In the proposed relay transmission scheme, a relay node receives a header of a frame on a frequency band. If relay transmission is required, the relay node starts relay transmission on another frequency band while the relay node receives the frame. Furthermore, we also propose an adaptive relay transmission selection (ATRS) scheme which selects the relay transmission scheme according to channel condition between a source node and a relay node. The ATRS scheme estimates an expected latency using FER estimated by mutual information which is obtained from SNR. Thus, the ATRS scheme selects appropriate relay transmission scheme. Simulation results show that the proposed schemes can attain the lowest latency or close to the lowest latency under various conditions.

Channel Access Balancing for Multiband Wireless LAN by Using Alternative Primary Channel

In the current IEEE 802.11 wireless LAN (WLAN), each station (STA) must obtain transmission opportunity (TXOP) on its primary channel. It means that current WLAN hardly utilizes the whole spectral resources in a frequency band if congestion occurs in some WLAN channels or bands. To overcome this problem, a concept of alternative primary channel (APCH) has been proposed. The APCH is a channel on which each STA can obtain TXOP if it is idle for a while, even when the primary channel is busy. This paper proposes a channel access procedure to realize load balancing of channel access among multiple WLAN channels by using APCH. Through computer simulation based on IEEE 802.11ac WLAN and the evaluation methodology of IEEE 802.11 WLAN, it is confirmed that the proposed channel access procedure can realize the load balancing. It is also confirmed that the performance of average area throughput and transmission delay can be improved by using APCH.

Adaptive frequency offset estimation for practical satellite communication channels

We have recently implemented a poly-polarization multiplexing (PPM) system as a hardware prototype in order to demonstrate its high spectral-efficiency in a satellite channel. The Luise and Reggiannini (L&R) algorithm is one of the frequency offset estimation methods suitable for use with the Digital Video Broadcasting - Satellite (DVB-S2) standard, and has also been implemented in our PPM receiver. In order to provide sufficient performance at the lowest SNR of about -2 dB, it is recommended to average the correlation estimates over 2048 frames. In higher SNR regions however, such a large averaging size is unnecessary and by reducing the size, the system can be made more responsive to changes in the channel state. In order to reduce the latency, we propose to measure the average noise power and select an efficient frame averaging length using a look-up-table. We show that the benefits of the proposed adaptive architecture can be extended to short UW lengths through increasing the separation of the UW symbols. Performance results show that the size of the averaging window can be substantially reduced whilst maintaining a target BER.

Performance investigation of reduced complexity bit-flipping using variable thresholds and noise perturbation

The near Shannon capacity approaching low-density parity-check (LDPC) linear block codes are now in widespread use in modern systems including the long term evolution advanced (LTE-A) cellular, 802.11η Wi-Fi and DVB-S2 satellite communications standards. The decoders based on the iterative belief propagation algorithm provide near optimum performance but also have very high computational complexity. Therefore significant research has recently focused on reduced complexity architectures based on the group of so-called bit-flipping algorithms. In the basic bit-flipping algorithm the number of failed parity checks for each bit is computed and the bit with the maximum failed parity checks is inverted. Inverting bits above a certain threshold removes the complexity involved with a maximum-search and, adaptive thresholds on each bit can further reduce the computation overhead. The criterion for the threshold update affects the error and convergence performances. Here, we describe a low-complexity architecture that has two (or more) decoder branches each with a different threshold scaling factor and select the threshold and bits at each iteration from the branch with the lowest syndrome sum. We then investigate the effect of adding a random Uniform or Gaussian noise perturbation to the threshold in order to reduce the average iteration count further in order to provide the opportunity to escape from stuck decoding states.

Reduced-Complexity Single-Carrier E-SDM for Wideband Transmissions

In the case of broadband wireless access systems operating over non-line-of-sight (NLOS) links, the multipath can be severe. This leads to frequency-selective fading channel which causes distortions of the received signal. The single-carrier (SC) modulation method combined with frequency-domain equalization (FDE) at the receiver has been proposed as an alternative to the orthogonal frequency division multiplexing(OFDM) system due to its relative small peak-to-average power ratio (PAPR) achieved at a similar complexity. Furthermore, an eigenbeamspace division multiplexing (E-SDM) technique can be applied to the single-carrier system to improve the spectrum efficiency. This scheme makes use of both spatial and frequency diversity while maintaining a low PAPR. The computation load still remains a challenge for the uplink transmission where the power resources are limited, therefore, in this paper we propose some reduced complexity techniques. The transmission performance of these techniques and the trade-off between complexity and bit error rate (BER) are evaluated in a frequency-selective Rayleigh channel by computer simulations.

A Study on Transmit Beamforming at Source Node in MISO-SISO/MIMO-MIMO AF Relays

A cooperative relay has been studied as a method to improve transmission quality in wireless mesh networks and cellular systems. Multiple antenna elements at a source node enable adaptive beamforming and thus improve total performance of relay transmission. In this paper, we propose simple transmit beamforming at a source node when the source node has two or more antenna elements and knows all channel information in MISO-SISO/MIMO-MIMO amplify-and-forward relays with direct link and evaluate the packet error rate. The simulation results show that the proposed method achieves both low complexity and high performance.