Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER

Channel Estimation for MIMO MC-CDMA Systems

The concepts of MIMO MC-CDMA are not new but the new technologies to improve their functioning are an emerging area of research. In general, most mobile communication systems transmit bits of information in the radio space to the receiver. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. To remove ISI from the signal, there is a need of strong equalizer. In this thesis we have focused on simulating the MIMO MC-CDMA systems in MATLAB and designed the channel estimation for them

Reduction of peak to average power ratio in OFDM and OFDM-CDMA system

High Peak to Average Power Ratio is a main problem in Orthogonal Frequency Division Multiplexing(OFDM) system and Orthogonal Frequency Division Multiplexing -Code Division Multiple Access (OFDM-CDMA) system. Considering a compensated High Power Amplifier(HPA), in order to keep linearity for amplification, Input Back-Off(IBO) of the HP-A, has to be increased for handling a high PAPR. Higher 1130 will make HPA less efficient. Additionally, a signal with high PAPR will suffer clipping when PAPR is larger than 1130 of the HPA. Clipping can cause the distortion of the signal and deteriorate the performance of the system. Reduction of PAPR is a main issue for OFDM and OFDM-CDMA application. In this thesis, some coding schemes are discussed to reduce PAPR. Of all possible code words for transmission, some code words with lower PAPR are chosen for transmission. Some redundancies are added to the end of the original code words to form these lower PAPR code words, though the net bit rate will decrease as a tradeoff. To reduce PAPR block coding scheme and cyclic coding scheme are discussed first. An odd parity bit is added as a redundancy bit. Also the code rate is 3/4. A novel block coding scheme with bit position control is discussed, where the position of the redundancy bit in the original code words is chosen by a feedback selection. For multiuser application, OFDM-CDMA is discussed. Walsh- Hadard (WH) and Complementary (CP) and Gold code sequences are used as the spreading sequences. These coding schemes used in OFDM system are also used before spreading to reduce PAPR in OFDM-CDMA system

Interference-Mitigating Waveform Design for Next-Generation Wireless Systems

A brief historical perspective of the evolution of waveform designs employed in consecutive generations of wireless communications systems is provided, highlighting the range of often conflicting demands on the various waveform characteristics. As the culmination of recent advances in the field the underlying benefits of various Multiple Input Multiple Output (MIMO) schemes are highlighted and exemplified. As an integral part of the appropriate waveform design, cognizance is given to the particular choice of the duplexing scheme used for supporting full-duplex communications and it is demonstrated that Time Division Duplexing (TDD) is substantially outperformed by Frequency Division Duplexing (FDD), unless the TDD scheme is combined with further sophisticated scheduling, MIMOs and/or adaptive modulation/coding. It is also argued that the specific choice of the Direct-Sequence (DS) spreading codes invoked in DS-CDMA predetermines the properties of the system. It is demonstrated that a specifically designed family of spreading codes exhibits a so-called interference-free window (IFW) and hence the resultant system is capable of outperforming its standardised counterpart employing classic Orthogonal Variable Spreading Factor (OVSF) codes under realistic dispersive channel conditions, provided that the interfering multi-user and multipath components arrive within this IFW. This condition may be ensured with the aid of quasisynchronous adaptive timing advance control. However, a limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles, employing a combination of several spreading sequences in the time-frequency and spatial-domain. The paper is concluded by quantifying the achievable user load of a UTRA-like TDD Code Division Multiple Access (CDMA) system employing Loosely Synchronized (LS) spreading codes exhibiting an IFW in comparison to that of its counterpart using OVSF codes. Both system's performance is enhanced using beamforming MIMOs

A low-cost time-hopping impulse radio system for high data rate transmission

We present an efficient, low-cost implementation of time-hopping impulse radio that fulfills the spectral mask mandated by the FCC and is suitable for high-data-rate, short-range communications. Key features are: (i) all-baseband implementation that obviates the need for passband components, (ii) symbol-rate (not chip rate) sampling, A/D conversion, and digital signal processing, (iii) fast acquisition due to novel search algorithms, (iv) spectral shaping that can be adapted to accommodate different spectrum regulations and interference environments. Computer simulations show that this system can provide 110Mbit/s at 7-10m distance, as well as higher data rates at shorter distances under FCC emissions limits. Due to the spreading concept of time-hopping impulse radio, the system can sustain multiple simultaneous users, and can suppress narrowband interference effectively.Comment: To appear in EURASIP Journal on Applied Signal Processing (Special Issue on UWB - State of the Art

Area spectral efficiency of soft-decision space–time–frequency shift-keying-aided slow-frequency-hopping multiple access

Slow-frequency-hopping multiple access (SFHMA) can provide inherent frequency diversity and beneficially randomize the effects of cochannel interference. It may also be advantageously combined with our novel space-time–frequency shift keying (STFSK) scheme. The proposed system’s area spectral efficiency is investigated in various cellular frequency reuse structures. Furthermore, it is compared to both classic Gaussian minimum shift keying (GMSK)-aided SFHMA and GMSK-assisted time- division/frequency-division multiple access (TD/FDMA). The more sophisticated third-generation wideband code-division multiple access (WCDMA) and the fourth-generation Long Term Evolution (LTE) systems were also included in our comparisons. We demonstrate that the area spectral efficiency of the STFSK-aided SFHMA system is higher than the GMSK-aided SFHMA and TD/FDMA systems, as well as WCDMA, but it is only 60% of the LTE system

Simulación de una cadena de comunicaciones DS-CDMA - Simulació d’una cadena de comunicacions DS-CDMA

Català: En aquest projecte s'ha analitzat e implementat un sistema basat amb DSSS-CDMA amb un receptor comú y diversos transmissors sobre una plataforma modular en Matlab, essent aquesta una eina de validació teòrica. S'ha primat aquesta per sobre d'una implementació en DSP principalment pel cost ecònomic de les plaques DSP. Així, s'ha decidit fer una implementació en Matlab amb les restriccions pròpies d'una placa DSP. El principal objectiu del projecte es la validació del sistema mitjançant la simulació a nivell de mostra sense restriccions de memòria. El proper pas seria la implementació en plaques DSP, peró això s'escapa del objectiu d'aquest projecte. És per això que s'ha dissenyat un sistema que pugi processar les dades amb pocs recursos mitjançant Matlab, tots marcats per una serie de variables. El transmissor es composa de diversos mòduls invariants que son el codificador, modulador, spreader, zero padder, pols conformador i el up converter que estan encadenats per generar la senyal a transmetre per cada un dels diversos usuaris. Totes aquestes senyals passen per un canal d'esvaniment lent amb soroll Gaussià blanc que modelitza un medi de comunicacions mòbil. Finalment el receptor rep totes les senyals y les processa en una serie de mòduls independents formats per un filtre pas baix, downconverter, filtre adaptat, sincronitzador, downsampler, equalitzador, despreader, demodulador y decodificador. En aquest treball es pot observar en la secció de Resultats les captures de la senyal a cada una de les diverses fases seguides d'una breu explicació. Finalment es tracten les conclusions i les properes vies d'investigació.Castellano: En este proyecto se ha analizado e implementado un sistema basado en DSSS-CDMA con un receptor común y varios transmisores sobre una plataforma modular en Matlab, siendo ésta una herramienta de validación teórica. Se ha primado esta sobre una implementación en DSP por el coste económico de las placas DSP. Así que se ha decidido hacer una implementación en Matlab con las constricciones propias de una placa DSP. El objetivo principal del proyecto es la validación del sistema mediante la simulación a nivel de muestra sin restricciones de memoria. El siguiente paso sería la implementación en placas DSP pero esto se escapa del objetivo de este proyecto. Para ello se ha diseñado un sistema que pueda procesar los datos con pocos recursos en Matlab, marcados por una serie de variables. El transmisor se compone de varios módulos invariantes que son el codificador, modulador, spreader, zero padder, pulse shaper y el up converter que encadenados generan la señal a transmitir de cada uno de los distintos usuarios. Todas estas señales pasan por un canal con desvanecimientos lentos y ruido aditivo gaussiano que modeliza un medio de comunicaciones móvil. Finalmente el receptor recibe todas las señales y las procesa en una serie de módulos independientes formados por un filtro paso bajo, downconverter, filtro adaptado, sincronizador, downsampler, equalizador, despreader, demodulador y decodificador. En este trabajo se puede observar en la sección Resultados las capturas de la señal en cada una de las distintas fases seguida de una breve explicación. Para finalmente llegar a la sección de Conclusiones y Futuras líneas de investigación.English: This project has analyzed and implemented a system based on DS-CDMA with a common receiver and multiple transmitters on a modular platform in Matlab, which is used for theoretical validation tool. This platform has been chosen over a DSP implementation due to the economic cost of DSP boards. So, it was decided to implement it using Matlab considering the inherent constraints in a DSP board. Project's main objective is to validate this system by having a simulation at a sample level which has no memory constraints. The next step would be to implement this in DSP boards; however this is beyond the scope of this project. A system has been designed that can process data with few resources in Matlab environment. The system developed is highly configurable using some input parameters. The transmitter consists of several modules that are invariant which are encoder, modulator, spreader, zero padder, pulse shaper and converter. These chained modules generate each user transmitted signal. Once these transmittersâ signals have been generated, they pass through a slowly fading channel with additive Gaussian noise which models a means of mobile communications. Ultimately the receiver gets all signals and processes them in a series of independent modules consisting of a low pass filter, downconverter, matched filter, synchronizer, downsampler, equalizer, despreader, demodulator and decoder. This work can be seen in the â Resultsâ section where there are screens of the signal in each of the phases followed by a brief justification

Narrowband Interference Suppression in Wireless OFDM Systems

Signal distortions in communication systems occur between the transmitter and the receiver; these distortions normally cause bit errors at the receiver. In addition interference by other signals may add to the deterioration in performance of the communication link. In order to achieve reliable communication, the effects of the communication channel distortion and interfering signals must be reduced using different techniques. The aim of this paper is to introduce the fundamentals of Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA), to review and examine the effects of interference in a digital data communication link and to explore methods for mitigating or compensating for these effects

Dispensing with Channel Estimation…

In this article, we investigate the feasibility of noncoherent detection schemes in wireless communication systems as a low-complexity alternative to the family of coherent schemes. The noncoherent schemes require no channel knowledge at the receiver for the detection of the received signal, while the coherent schemes require channel inherently complex estimation, which implies that pilot symbols have to be transmitted resulting in a wastage of the available bandwidth as well as the transmission power