Interference-free spectrum sharing in cognitive radio based on space time coding

Next generation wireless communications and mobile systems demand higher data rates with maximum bandwidth utilization. Over the past years, some techniques in the literature are provided in various ways where wireless systems should support high data rates. However, this requirement is faced with spectrum scarcity accommodation problem as in spectrum allocation chart where majority of the spectrum is under license. Nevertheless, spectrum measurements show that most of the spectrum is not continuously used based on the frequency, geographical location and time. As a result, this requirement necessitates the efficient use of the available resources by managing spectrum distribution to provide acceptable service quality. Therefore, several approaches in wireless systems are introduced in literature for sharing the available spectrum to overcome the spectrum scarcity problem. Cognitive Radio (CR) is considered as a potential solution to boost the spectrum utilization and solve the spectrum scarcity in which two different wireless systems could operate in the same frequency band with respect to their priority. CR is an innovative technique and developing area of research for the next wireless generation systems. In CR, to access the licensed bands interweave, underlay and overlay patterns can be used by the Secondary Users (SUs). In the interweave protocol, SUs sense the existence or nonexistence of Primary Users (PUs) to use spectrum holes without causing interference to PUs. In the underlay protocol, SUs are certified to use the licensed spectrum if the interference caused at PUs is below a predetermined interference level. In the overlay protocol, unlicensed users generally pay a price to share a licensed spectrum by cooperating for the benefit of spectrum owners. Different kinds of spectrum sharing model such as hybrid and superposition coding techniques are proposed in the literature. In superposition coding SU combines the PU's and its own signal and then broadcast to both Primary Receiver (PR) and Secondary Receiver (SR). However, this technique causes severe interference in both receivers. Two CR patterns can be combined to approach the hybrid mode where in this case the system has flexibility to operate in different protocols to obtain benefits from the spectrum and cooperate with PU. One diversity technique that could mitigate the fading channels effect on the transmitted signal is Signal Space Diversity (SSD) also known as modulation diversity. SSD provides performance improvement over fading channels without using extra bandwidth or power. In SSD, constellation points of the modulation are rotated by an angle before the transmission which maximize the minimum product distance of the rotated constellation, and after applying Coordinate Interleaved Orthogonal Design (CIOD) on the rotated constellation an irregular modulation points are acquired. Normally, in cooperative communications overall system spectral efficiency and rate are decreased due to the fact that data takes more than one time slot to be transmitted to the receiver. However, using SSD and CIOD technique in the system configuration, one can easily enhance the overall spectral efficiency and rate in cooperative systems without adding any complexity to the system. In this thesis, two different spectrum sharing protocols for cognitive radio operating in overlay mode are proposed. In the first protocol, the primary and secondary system is comprised of PT, PR, ST and SR equipped all with one antenna. SU is allowed to use the shared spectrum band in accordance to cooperate with primary system. We take advantages of the SSD and CIOD concepts in this three-phase overlay protocol to enhance the spectral efficiency, rate and diversity as well as to provide single symbol decoding. In the first transmission time slot, PT broadcasts its signal to PR and ST where ST tries to decode the transmitted signal from PT. If ST correctly decodes the PU's signal, ST forwards the coordinate interleaved signal pair to PR in the second time slot and PR tries to estimate the symbol by received signals from PT and ST. However, in the case that ST could not correctly decode the signal received from PT, PR decodes the symbol only from PT. Finally, SU's signal is transmitted to SR by ST at the third time slot. Due to the fact that a specific time slot is dedicated to SU data transmission, this protocol avoids interference. A critical distance between PT and ST is obtained such that as long as ST is located within that distance the outage probability of PU will be equal or lower than the case that of without spectrum sharing and ST will benefit from the band. The outage probabilities for PU and SU over Rayleigh fading channels are derived in a closed-form expressions and depicted for different target rates. Note that, the theoretic outcomes match perfectly with simulation results. It is shown that a significant performance improvement in the proposed scheme is notable both for PU and SU in comparison to the reference protocol where superposition coding is used at ST. Furthermore, an upper bound on the Bit Error Probability (BEP) of primary system is obtained and supported via simulation results. The simulation results agree well with the theoretical upper bound in the high Signal to Noise Ratio (SNR), respectively. Finally, for the secondary system, BEP performance is the same as classical direct transmission. The BEP performance for 16-QAM and 4-QPSK modulation for different values of the distance between ST and SR are depicted where in both cases simulation. The results confirm the efficiency of the proposed spectrum sharing compared to the reference protocol. In the second protocol, PU consists of a pair of PT and PR with one antenna and for SU, ST is equipped with two antennas to use the benefits of Maximum Ratio Combining (MRC) and Alamouti techniques and SR has only one antenna. A CR protocol which is configured on overlay mode to share the spectrum of the primary system is presented. SSD, CIOD and Alamouti concepts are used together to benefit from the single symbol decoding, diversity gain and to increase the overall system rate and spectral efficiency. The protocol comprises of three time slots to transmit PU's and SU's data. In the first time slot, PT transmits the primary signal to PR and to both antennas of ST. Then, ST uses the MRC technique to decode the transmitted signal. If ST correctly decodes the transmitted signal from PT, it applies Alamouti coding to transmit the primary and its own symbols to PR and SR in the second and third time slots. Nevertheless, if ST could not correctly decode the signal pair received from PT, PR will estimate the symbol from the direct link PT$\to$PR. Meanwhile, secondary signal is transmitted in the second and third time slots which provides diversity which enhances SU system performance. This protocol guarantees an interference-free communication for both users by considering the fact that PR and SR could extract their own signal based on the single symbol decoding technique provided by Alamouti coding. An upper bound for the BEP of the primary system is derived and it is shown that computer simulation results are in perfectly match with the theoretical upper bound in high SNR which validates the theoretical derivations. Results for different values of the distance between PT, ST and PR are obtained where in all cases, they validate the significant improvement in the Bit Error Rate (BER) performance for the proposed protocol. For the secondary system, BEP performance is the same as classical Alamouti code and is depicted for various values of the distance between ST and SR for classical 16-QAM and is compared with the straightforward transmission scheme and with the reference scheme where SU uses nearly all of its power to transmit the PU's signal and SU is in outage in most cases and operates a relay.