25 research outputs found
Signal Processing Design of Low Probability of Intercept Waveforms
This thesis investigates a modification to Differential Phase Shift Keyed (DPSK) modulation to create a Low Probability of Interception/Exploitation (LPI/LPE) communications signal. A pseudorandom timing offset is applied to each symbol in the communications stream to intentionally create intersymbol interference (ISI) that hinders accurate symbol estimation and bit sequence recovery by a non-cooperative receiver. Two cooperative receiver strategies are proposed to mitigate the ISI due to symbol timing offset: a modified minimum Mean Square Error (MMSE) equalization algorithm and a multiplexed bank of equalizer filters determined by an adaptive Least Mean Square (LMS) algorithm. Both cooperative receivers require some knowledge of the pseudorandom symbol timing dither to successfully demodulate the communications waveform. Numerical Matlab® simulation is used to demonstrate the bit error rate performance of cooperative receivers and notional non-cooperative receivers for binary, 4-ary, and 8-ary DPSK waveforms transmitted through a line-of-sight, additive white Gaussian noise channel. Simulation results suggest that proper selection of pulse shape and probability distribution of symbol timing offsets produces a waveform that is accurately demodulated by the proposed cooperative receivers and significantly degrades non-cooperative receiver symbol estimation accuracy. In typical simulations, non-cooperative receivers required 2-8 dB more signal power than cooperative receivers to achieve a bit error rate of 1.0%. For nearly all reasonable parameter selections, non-cooperative receivers produced bit error rates in excess of 0.1%, even when signal power is unconstrained
The study of sequential decoding techniques for spacecraft telemetry systems Final report, 12 Jan. - 12 Jun. 1968
Convolutional encoding-sequential decoding technique for coherent deep space telemetry link and near earth space mission
High speed energy efficient incoherent optical wireless communications
The growing demand for wireless communication capacity and the overutilisation of the conventional
radio frequency (RF) spectrum have inspired research into using alternative spectrum
regions for communication. Using optical wireless communications (OWC), for example, offers
significant advantages over RF communication in terms of higher bandwidth, lower implementation
costs and energy savings. In OWC systems, the information signal has to be
real and non-negative. Therefore, modifications to the conventional communication algorithms
are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing
(OFDM) promise to deliver a more efficient use of the communication capacity through adaptive
bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM
(DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated
discrete multitone (PAM-DMT) – have been introduced in the literature.
The current work investigates the recently introduced scheme subcarrier-index modulation OFDM
as a potential energy-efficient modulation technique with reduced peak-to-average power ratio
(PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a
linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the
PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently
unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is
proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore,
an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral
efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM,
PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based
modulation approach which is electrically and optically more efficient than any other
OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD)
communication systems.
Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed
communication in OWC. This work presents a generalised approach for analysing nonlinear
distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form
analytical solution for an arbitrary memoryless distortion of the information signal and has
been proven to work for the majority of the known unipolar OFDM-based modulation techniques
- DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM.
The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting
diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm
device is demonstrated. The capabilities of using such devices at practical transmission distances
are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up
to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully
modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in
comparison to DCO-OFDM
Study of sequential decoding
Decoding algorithms for data reduction and transmission through noisy space channels using sequential and hybrid computer
Mitigating PAPR in cooperative wireless networks with frequency selective channels and relay selection
The focus of this thesis is peak-to-average power ratio (PAPR) reduction in cooperative wireless networks which exploit orthogonal frequency division multiplexing in transmission. To reduce the PAPR clipping is employed at the source node. The first contribution focuses
upon an amplify-and-forward (AF) type network with four relay nodes which exploits distributed closed loop extended orthogonal space frequency block coding to improve end-to-end performance. Oversampling and filtering are used at the source node to reduce out-of-band interference and the iterative amplitude reconstruction decoding technique is used at the destination node to mitigate in-band distortion which is introduced by the clipping process. In addition, by exploiting quantized group feedback and phase rotation at two of the relay nodes, the system achieves full cooperative diversity in addition to array gain.
The second contribution area is outage probability analysis in the context of multi-relay selection in a cooperative AF network with frequency selective fading channels. The gains of time domain multi-path fading channels with L paths are modeled with an Erlang distribution.
General closed form expressions for the lower and upper bounds of outage probability are derived for arbitrary channel length L as a function of end-to-end signal to noise ratio. This analysis is then extended for the case when single relay selection from an arbitrary number of relay nodes M is performed. The spatial and temporal cooperative diversity gain is then analysed. In addition, exact form of outage probability for multi-path channel length L = 2 and selecting the best single relay from an arbitrary number of relay nodes M is obtained. Moreover, selecting a pair of relays when L = 2 or 3 is additionally analysed.
Finally, the third contribution context is outage probability analysis of a cooperative AF network with single and two relay pair selection from M available relay nodes together with clipping at the source node, which is explicitly modelled. MATLAB and Maple software based simulations are employed throughout the thesis to support the analytical results and assess the performance of algorithms and methods