In this paper, we present a multi-user resource allocation framework using fragmented-spectrum synchronous OFDM-CDMA modulation over a frequency-selective fading channel. In particular, given pre-existing communications in the spectrum where the system is operating, a channel sensing and estimation method is used to obtain information of subcarrier availability. Given this information, some real-valued multi-level orthogonal codes, which are orthogonal codes with values of @math , are provided for emerging new users, i.e., cognitive radio users. Additionally, we have obtained a closed form expression for bit error rate of cognitive radio receivers in terms of detection probability of primary users, CR users' sensing time and CR users' signal to noise ratio. Moreover, simulation results obtained in this paper indicate the precision with which the analytical results have been obtained in modeling the aforementioned system.
In @cite_0 , an interesting dynamic spectrum access method based on non-contiguous carrier interferometry MC-CDMA has been introduced and its performance is evaluated. Particularly, CR users need to deactivate some of their subcarriers to avoid interference to primary users; and orthogonal Carrier Interferometry codes are used to eliminate the loss of orthogonality among spreading codes caused by deactivating subcarriers. However, compared with simple binary Hadamard-Walsh signature codes with values @math , orthogonal Carrier Interferometry codes, with length @math , take complex values @math for @math chip of the @math signature code that result in complexity of the system, from implementation point of view; also, this scheme does not consider the probabilities of false alarm and detection and @cite_3 of primary users in computing error probability. In a similar study, the authors in @cite_7 , @cite_11 , introduced a partially matched filter to eliminate narrow band interference (NBI), which can be interpreted as interference from primary users. However, detection of NBI was assumed to be ideal, which is far from reality.
@cite_0 @cite_7 @cite_3 @cite_11
2102191992 1967582724 2071324566 2123528608
In this paper, we present a quantitative performance evaluation of non-contiguous multi-carrier code vision multiple access (NC-MC-CDMA) for cognitive radio in a dynamic spectrum access (DSA) network. In a DSA network, multi-carrier based cognitive radio transceivers need to deactivate some of its subcarriers to avoid interference to primary users. However, by deactivating subcarriers, orthogonality among different spreading codes are lost, leading to poor BER performance. The performance of the NC-MC-CDMA can be improved by adaptive spreading code adjustment to compensate for the loss of orthogonality. However, since Hadamard-Walsh codes only exist for certain code length, loss of orthogonality can only be minimized instead of eliminated for many cases. On the other hand, orthogonal Carrier Interferometry codes exist for code length of any integer. Hence, by applying non-contiguous CI MC-CDMA into DSA, the loss of orthogonality among spreading codes caused by deactivating subcarriers can be eliminated. As a direct result, adaptive NC-CI MC-CDMA significantly outperforms adaptive NC-MC-CDMA using Hadamard-Walsh codes. In this paper, we present and discuss the physical implementation and testing of a Spectrally-Encoded Spread-Time Code Devision Multiple Access (SE ST-CDMA) transceiver. The proposed design is suitable for multiple-access and anti jamming wireless communication systems. The design and implementation takes into account the ability to automatically detect and mitigate tone jammers by altering the corresponding Gold sequence codes assigned uniquely to each user and setting the corresponding code's chip to zero. A simple real-time anti jamming mechanism capable of dealing with frequency hopping jammers is also proposed. The simulation results are provided in terms of Bit Error Rate (BER) versus Signal to Noise Ratio (SNR) in one case and the number of multi tone jammers in another, both conditioned on the number of users. Finally, the experimental and simulation results are compared and we observe that they are in a very good agreement. In this paper, we study and analyze the performance of matched and partially-matched receivers for a typical ultra wide-band Spectrally-Encoded Spread-Time (SE ST) CDMA in the presence of multiple tone jammers. We investigate a scenario where there is no information about the jammer frequencies in the transmitter side while it is desirable to cancel it in the receiver side. We will demonstrate the performance of the SE ST system would be improved through partially matched filter at the receiver. In addition, we approximate the two-dimensional RAKE receiver by a simple one-dimensional RAKE for the scenario where there are K asynchronous users. Furthermore, closed form formula for the characteristic function of the decision variable will be found and used to evaluate the probability of error. Our numerical results show that the partially-matched receiver outperforms the conventional matched filter in case of high jammer to signal power ratio.