9 research outputs found
Vector Coding Optical Wireless Links
The quasi-static nature of the optical wireless channel means that the channel state information (CSI) can be readily available at the transmitter and receiver prior to data transmission. This implies that electrically band-limited optical wireless communication (OWC) systems can make use of optimal channel partitioning or vector coding based multi-channel modulation (MCM) to achieve high throughput by mitigating the non-linearities arising from the optical and electrical channel. This paper proposes a pulse amplitude modulation (PAM) based DC-biased optical vector coding (DCO-VC) MCM scheme for OWC. The throughput performance of DCO-VC is evaluated and compared to the well known DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) over hybrid (line-of-sight and diffuse) and diffuse (non line-of-sight only) visible light communication (VLC) channels with additive white Gaussian noise. For the completeness of the VLC physical layer, the performance comparison is based on an uncoded and a forward error correction transmission mode using well-known convolutional codes with Viterbi decoder. The results show that the coded DCO-VC outperforms DCO-OFDM system by achieving up to 2 and 3 dB signal to noise ratio gains over hybrid and diffuse VLC channels, respectively
Techniques for improving the performance of frequency-hopped multiple-access communication systems
Imperial Users onl
Coding in 802.11 WLANs
Forward error correction (FEC) coding is widely used in communication systems to correct transmis-
sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical
(PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is
supported. These are implemented in hardware and thus di±cult to change, and the coding rates are
selected with point to point operation in mind.
This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are
better aligned with application requirements. For example, coding to support multicast tra±c rather
than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless
channel; and coding which takes account of delay requirements as well as losses. We consider layering
additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between
higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling.
Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated
with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and
without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies
in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance
is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs
cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation
and FEC would bring few bene¯ts.
Secondly, using experimental measurements taken in an outdoor environment, we model the chan-
nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This
hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure
channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further
consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for
multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding
approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre-
gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC
layer overheads, demonstrate that increases in network throughput of more than 100% are possible
over a wide range of channel conditions, and that the simpler time-sharing approach yields most of
these gains and have minor loss of performance.
Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11
WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of
separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair
coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful
and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring
operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise
goodput (treating packets decoded after the delay deadline as losses)
Space-time coded MIMO-OFDM systems for wireless communications: Signal detection and channel estimation
Master'sMASTER OF ENGINEERIN
The Telecommunications and Data Acquisition Report
Tracking and ground-based navigation; communications, spacecraft-ground; station control and system technology; capabilities for new projects; networks consolidation program; and network sustaining are described
Reduced complexity detection for massive MIMO-OFDM wireless communication systems
PhD ThesisThe aim of this thesis is to analyze the uplink massive multiple-input multipleoutput
with orthogonal frequency-division multiplexing (MIMO-OFDM) communication
systems and to design a receiver that has improved performance
with reduced complexity. First, a novel receiver is proposed for coded massive
MIMO-OFDM systems utilizing log-likelihood ratios (LLRs) derived
from complex ratio distributions to model the approximate effective noise
(AEN) probability density function (PDF) at the output of a zero-forcing
equalizer (ZFE). These LLRs are subsequently used to improve the performance
of the decoding of low-density parity-check (LDPC) codes and turbo
codes. The Neumann large matrix approximation is employed to simplify the
matrix inversion in deriving the PDF.
To verify the PDF of the AEN, Monte-Carlo simulations are used to demonstrate
the close-match fitting between the derived PDF and the experimentally
obtained histogram of the noise in addition to the statistical tests and
the independence verification. In addition, complexity analysis of the LLR
obtained using the newly derived noise PDF is considered. The derived LLR
can be time consuming when the number of receive antennas is very large
in massive MIMO-OFDM systems. Thus, a reduced complexity approximation
is introduced to this LLR using Newton’s interpolation with different
orders and the results are compared to exact simulations. Further simulation
results over time-flat frequency selective multipath fading channels demonstrated
improved performance over equivalent systems using the Gaussian
approximation for the PDF of the noise.
By utilizing the PDF of the AEN, the PDF of the signal-to-noise ratio (SNR)
is obtained. Then, the outage probability, the closed-form capacity and three
approximate expressions for the channel capacity are derived based on that
PDF. The system performance is further investigated by exploiting the PDF
of the AEN to derive the bit error rate (BER) for the massive MIMO-OFDM
system with different M-ary modulations. Then, the pairwise error probability
(PEP) is derived to obtain the upper-bounds for the convolutionally coded
and turbo coded massive MIMO-OFDM systems for different code generators
and receive antennas.
Furthermore, the effect of the fixed point data representation on the performance
of the massive MIMO-OFDM systems is investigated using reduced
detection implementations for MIMO detectors. The motivation for the fixed
point analysis is the need for a reduced complexity detector to be implemented
as an optimum massive MIMO detector with low precision. Different
decomposition schemes are used to build the linear detector based on
the IEEE 754 standard in addition to a user-defined precision for selected
detectors. Simulations are used to demonstrate the behaviour of several matrix
inversion schemes under reduced bit resolution. The numerical results
demonstrate improved performance when using QR-factorization and pivoted
LDLT decomposition schemes at reduced precision.Iraqi Government and the Iraqi
Ministry of Higher Education and Scientific researc
Physical Layer Techniques for Indoor Wireless Visible Light Communications
The growing demand for bandwidth-hungry applications and increasing number of smart interconnected devices has increased the data traffic on radio access networks. Subsequently, the saturating spectral efficiencies in crowded radio frequency spectrum has impelled the researchers to exploit the optical spectrum for communications. In particular, many developments in the visible light communication (VLC) as a combined lighting and communications system have taken place.
Despite abundant optical bandwidth, the data transmission rates and power efficiencies in VLC are partly limited by the electrical channel bandwidth and the type of signalling sets which can be used in this intensity modulated, direct detected system. In order to improve the power and spectral efficiencies, this thesis focuses on physical layer (PHY) techniques. The state-of-the-art single channel modulations (SCM) based on M-PAM, multi-channel modulations (MCM) based on OFDM, and IEEE standardised multi-colour modulations are investigated comprehensively through simulations and theoretical analysis, over representative VLC channels considering the optical properties of front-end devices.
The bit error performances and spectral efficiencies of DC-biased and non DC-biased MCM systems are compared. A new vector coding based MCM is proposed to optimally utilise the channel state information at the transmitter as an alternative to optical OFDM. The throughputs, peak-to-average power ratios and DC-bias requirements of SCM and MCM systems are investigated which show that the lower DC-bias requirements reduce power consumed for the same throughput in SCM systems when compared to MCM systems. A new quad-chromatic colour shift keying (CSK) system is proposed which reduces power requirements and complexity, enhances throughput and realises a four-dimensional signalling to outperform the IEEE standardised tri-chromatic CSK system.
For improved power efficiency and throughput of VLC PHY, use of rate-adaptive binary convolutional coding and Viterbi decoding is proposed along with frequency domain channel equalisation to mitigate temporal dispersion over representative VLC channels