20 research outputs found
Digital modems for mobile systems
Digital modems for mobile system
Modem design for digital satellite communications
The thesis is concerned with the design of a phase-shift keying system for a
digital modem, operating over a satellite link. Computer simulation tests and
theoretical analyses are used to assess the proposed design.
The optimum design of both transmitter and receiver filters for the system to be
used in the modem are discussed. Sinusoidal roll-off spectrum with different roll-off
factor and optimum truncation lengths of the sample impulse response are designed
for the proposed scheme to approximate to the theoretical ideal. It has used an EF
bandpass filter to band limit the modulated signal, which forms part of the satellite
channel modelling. The high power amplifier (HPA) at the earth station has been
used in the satellite channel modelling due to its effect in introducing nonlinear AMAM
and AM-PM conversion effects and distortion on the transmitted signal from the
earth station. The satellite transponder is assumed to be operating in a linear mode.
Different phase-shift keying signals such as differentially encoded quaternary
phase-shift keying (DEQPSK), offset quaternary phase-shift keying (OQPSK) and
convolutionally encoded 8PSK (CE8PSK) signals are analysed and discussed in the
thesis, when the high power amplifier (HPA) at the earth station is operating in a
nonlinear mode. Convolutional encoding is discussed when applied to the system
used in the modem, and a Viterbi -algorithm decoder at the receiver has been used, for
CE8PSK signals for a nonlinear satellite channel. A method of feed-forward
synchronisation scheme is designed for carrier recovery in CE8PSK receiver.
The thesis describes a method of baseband linearizing the baseband signal in
order to reduce the nonlinear effects caused by the HPA at the earth station. The
scheme which compensates for the nonlinear effects of the HPA by predistorting the
baseband signal prior to modulation as opposed to correcting the distortion after
modulation, thus reducing the effects of nonlinear distortion introduced by the HPA.
The results of the improvement are presented.
The advanced technology of digital signal processors (DSPs) has been used in the implementation of the demodulation and digital filtering parts of the modem
replacing large parts of conventional circuits. The Viterbi-algorithm decoder for
CE8PSK signals has been implemented using a digital signal processor chip, giving excellent performance and is a cost effective and easy way for future developments
and any modifications,
The results showed that, by using the various studied techniques, as well as the
implementation of digital signal processor chip in parts of the modem, a potentially
more cost effective modem can be obtained
Investigation into PRS-precoded, constant-envelope, continuous-phase digital modulation schemes
Bibliography: leaves 78-79.Partial response signaling ( PRS) has been used successfully to improve the spectral properties of Pulse Amplitude Modulated (PAM) digital transmission systems. This thesis investigation studied the effect of PRS on frequency- and phase-modulated carrier systems, in particular on their spectral performance and their maintenance of constant envelope
M-ary Chirp Modulation for Data Transmission
M-ary chirp modulations, both discontinuous- and continuous-phase, for M-ary data transmission are proposed and examined for their error rate performances in additive, white, Gaussian noise (AWGN) channel. These chirp modulated signals are described and illustrated as a function of time and modulation parameters. M-ary chirp modula tion with discontinuous phase is first proposed and then the M-ary Continuous Phase Chirp Modulation (MCPCM) is considered. General descriptions of these modula tion systems are given and properties of signals representing these modulations are given and illustrated. Optimum algorithms for detection of these signals in AWGN are derived and structures of optimum receivers are identified. Using the minimum Euclidean distance criterion in signal-space; upper bounds on Signal-to-Noise Ratio (SNR) gain relative to Multiple Phase Shift Keying (MPSK) are established for 2-.
*4-, and 8-ary MCPCM systems. It is observed that the maximum likelihood coherent and non-coherent receivers for MCPCM are non-linear and require multiple-symbol observations. Since symbol error probability performance analyses of these receivers are too complex to perform, union upper bounds on their performances are derived and illustrated as a function of SNR, number of observation symbols, and modulation parameters for MCPCM. Optimum 2-, 4-, and 8-ary modulation schemes that mini mize union upper bound on symbol error rates have been determined and illustrated. Our results show that 2-, 4-, and 8-ary optimum coherent MCPCM systems, with 5-symbol observation length, offer 1.6 dB, 3.6 dB, and 8 dB improvements relative to 2-ary, 4-ary, and 8-ary PSK systems, respectively. Also, it is shown that opti mum 2-ary and 4-ary non-coherent MCPCM systems can outperform 2-ary and 4-ary coherent PSK systems, respectively
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Study of continuous-phase four-state modulation for cordless telecommunications. Assessment by simulation of CP-QFSK as an alternative modulation scheme for TDMA digital cordless telecommunications systems operating in indoor applications
One of the major driving elements behind the explosive boom in wireless revolution is the advances in the field of modulation which plays a fundamental role in any communication system, and especially in cellular radio systems. Hence, the elaborate choice of an efficient modulation scheme is of paramount importance in the design and employment of any communications system. Work presented in this thesis is an investigation (study) of the feasibility of whether multilevel FSK modulation scheme would provide a viable alternative modem that can be employed in TDMA cordless communications systems. In the thesis the design and performance analysis of a non-coherent multi-level modem that offers a great deal of bandwidth efficiency and hardware simplicity is studied in detail. Simulation results demonstrate that 2RC pre-modulation filter pulse shaping with a modulation index of 0.3, and pre-detection filter normalized equivalent noise bandwidth of 1.5 are optimum system parameter values. Results reported in chapter 5 signify that an adjacent channel rejection factor of around 40 dB has been achieved at channel spacing of 1.5 times the symbol rate while the DECT system standards stipulated a much lower rejection limit criterion (25-30dB), implying that CP-QFSK modulation out-performs the conventional GMSK as it causes significantly less ACI, thus it is more spectrally efficient in a multi-channel system. However, measured system performance in terms of BER indicates that this system does not coexist well with other interferers as at delay spreads between 100ns to 200ns, which are commonly encountered in such indoor environment, a severe degradation in system performance apparently caused by multi-path fading has been noticed, and there exists a noise floor of about 40 dB, i.e. high irreducible error rate of less than 5.10-3. Implementing MRC diversity combiner and BCH codec has brought in a good gain.Higher Education Ministr
Study of efficient transmission and reception of image-type data using millimeter waves
Evaluation of signal processing and modulation techniques for transmission and reception of image type data via millimeter wave relay satellite
Transmitter based techniques for ISI and MAI mitigation in CDMA-TDD downlink
The third-generation (3G) of mobile communications systems aim to provide enhanced voice,
text and data services to the user. These demands give rise to the complexity and power consumption
of the user equipment (UE) while the objective is smaller, lighter and power efficient
mobiles. This thesis aims to examine ways of reducing the UE receiverâs computational cost
while maintaining a good performance.
One prominent multiple access scheme selected for 3G is code division multiple access. Receiver
based multiuser detection techniques that utilise the knowledge of the downlink channel
by the mobile have been extensively studied in the literature, in order to deal with multiple
access and intersymbol interference. However, these techniques result in high mobile receiver
complexity.
Recently, work has been done on algorithms that transfer the complexity from the UE to the
base station by exploiting the fact that in time division duplex mode the downlink channel can
be known to the transmitter. By linear precoding of the transmitted signal the user equipment
can be simplified to a filter matched to the userâs spreading code. In this thesis the problem
of generic linear precoding is analysed theoretically and a method for analytical calculation
of BER is developed. The most representative of the developed precoding techniques are described
under a common framework, compared and classified as bitwise or blockwise. Bitwise
demonstrate particular advantages in terms of complexity and implementation but lack in performance.
Two novel bitwise algorithms are presented and analysed. They outperform significantly
the existing ones, while maintain a reduced computational cost and realisation simplicity.
The first, named inverse filters, is the Wiener solution of the problem after applying a minimum
mean squared error criterion with power constraints. The second recruits multichannel adaptive
algorithms to achieve the same goal. The base station emulates the actual system in a cell
to converge iteratively to the pre-filters that precode the transmitted signals before transmission.
The advantages and the performance of the proposed techniques, along with a variety of
characteristics are demonstrated by means of Monte Carlo simulations
Spectrally efficient systems for satellite communications
In questa Tesi verranno proposte tre diverse soluzioni il cui scopo è l'incremento dell'efficienza spettrale nei sistemi satellitari. Per prima cosa, considereremo uno scenario multi-utente a multiplazione di frequenza, che è uno degli scenari inclusi nello standard DVB-RCS. Useremo modulazioni a fase continua (CPM) concatenate serialmente ad un codificatore tramite un interleaver.
La già elevata efficienza spettrale delle CPM può essere ulteriormente aumentata sfruttando
la tecnica del "frequency packing". Un tale incremento però rende necessaria un'accurata sincronizzazione affinchÊ il ricevitore funzioni correttamente. Pertanto verranno progettati e testati degli stimatori iterativi di fase e frequenza.
In seguito, ci concentreremo su sistemi a divisione di codice basati sulle CPM. VerrĂ quindi proposta una nuova tecnica di espansione spettrale, progettata ad hoc per le CPM. Mostreremo come ottenere facilmente delle densitĂ spettrali di potenza larghe, piatte e lisce, senza ricorrere a sequenze di spreading, e senza tutte le problematiche di progettazione a queste collegate. Inoltre, ricaveremo degli algoritmi di rivelazione multi-utente subottimi che saranno impiegati per mostrare come il sistema proposto possa
superare le prestazioni di tutti gli altri sistemi in letteratura, sia in termini di probabilitĂ d'errore che di efficienza
spettrale.
Infine, considerando lo scenario previsto dallo standard DVB-S2, proporremo di incrementarne l'efficienza spettrale attraverso la tecnica del "time and frequency packing". Questa tecnica causa l'insorgenza di interferenza intersimbolica e intercanale, che a loro volta provocano l'aumento del numero di simboli pilota necessari per una corretta sincronizzazione di fase e frequenza. SarĂ pertanto introdotto un
nuovo design dei piloti, e saranno derivati opportuni algoritmi di rivelazione ottimi e subottimi a ridotta complessitĂ . Mostreremo come il sistema proposto possa superare le prestazioni dello standard DVB-S2 sia in termini di probabilitĂ d'errore che di efficienza spettrale.In this Thesis we propose three different solutions aiming at increasing the spectral
efficiency in satellite communications. First, we will consider a multi-user scenario with frequency multiplexing, that is
one of the scenarios included in the DVB-RCS standard. We will adopt continuous phase modulations (CPMs) serially concatenated with an outer code through an interleaver. The high spectral efficiency of CPMs can be further increased exploiting the frequency packing technique. Such an improvement does not come for free, in fact an accurate synchronization has to be guaranteed
to allow the detector to work properly. To this purpose, new iterative frequency and phase estimators will be derived and the synchronization accuracy tested.
Then, we will focus on code division
multiple access (CDMA) systems employing CPMs. A brand new spectral spreading technique, especially tailored to CPMs, will be presented. We will show how to easily obtain a large, flat, and smooth power
spectral density, without resorting to spreading sequences and then getting rid of all the design problems that come with. Moreover, we will derive some suboptimal multi-user detectors that will be employed
to show that the proposed system outperforms all the other considered systems, found in the literature, in terms of bit error rate and spectral efficiency.
Finally, considering the DVB-S2 scenario, we will propose to increase the spectral efficiency through time and frequency packing. This technique will cause intersymbol and interchannel interferences to arise, requiring a significant increase in the number of pilots used to carry out frequency and phase synchronization. Therefore, new pilot designs will be introduced, and suited optimal and suboptimal
reduced-complexity algorithms derived. We will show that the proposed systems may outperform the DVB-S2 standard in terms of bit error rate and spectral efficiency