Dissertation
Dissertation > Industrial Technology > Radio electronics, telecommunications technology > Communicate > Data communication > Data transmission technology

Spectrum Sensing Based Interference Avoiding Mechanism for MB-OFDM UWB Systems

Author LiMan
Tutor WangDeQiang
School Shandong University
Course Communication and Information System
Keywords UWB MB-OFDM Spectrum sensing Interference avoidance
CLC TN919.3
Type Master's thesis
Year 2011
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Nowadays, portable devices such as computing devices, communication terminals, data storage, and multi-media processing equipments are getting more and more popular. A reliable, immediate and convenient wireless technology is desirable to provide links between these devices. Ultra-wideband (UWB) technology is one of the hot topics in wireless communication research area, which supports high data rates with low power consumption and low cost. Due to the unlicensed nature of this spectrum, the spectrum of the UWB system will overlap with the existing narrowband systems and the UWB devices will coexist with incumbent users operating in the 5-6GHz bands, e.g., IEEE802.11a systems. As we know, the power spectrum density (PSD) of UWB is limited to -41.3dBm/MHz which causes negligible interference to the incumbent users. However, UWB systems are unavoidably disturbed by the incumbent systems with much higher PSD. Therefore, anti-interference measures should be taken in UWB systems so as to boost their performance. Cognitive radio technology is a kind of new intelligent wireless communication, and the spectrum sensing is the key technology of cognitive radio. To improve the performance of MB-OFDM UWB, we proposed an interference avoidance mechanism supported by spectrum sensing. This thesis focuses on the frequency hopping scheme of MB-OFDM UWB system and explores the combination of cognitive radio technology with ultra-wideband networks.The main contributions of this thesis can be summarized as follows:1. Combined with the CR spectrum sensing, the thesis proposes the interference pattern arithmetic based on energy detection. A cognitive radio (CR) module is deployed for the purpose of spectrum sensing over all available sub-bands and sub-carriers to detect whether the IEEE 802.11a narrow band is existing or not. Take statistic of the binary decision di,q, a binary sequence of length 128,ηi(k), is obtained, which represents the interference pattern within the i-th sub-band at stage k. Further define the interference pattern of the specific band group at stage k,ξ(k)=[D1, D2, D3],a binary sequence of length 3.2. Based on the interference pattern detected by the CR module, a CR-based FH scheme is proposed for MB-OFDM UWB systems to avoid co-channel interferences adaptively. For extreme scenarios, where interferences are detected over all available sub-bands, a refined interference avoidance mechanism, besides CR-based FH, is enabled by turning off disturbed sub-carriers to further enhance the performance of MB-OFDM UWB systems. Simulations have been carried out to verify the effectiveness of the proposed schemes in typical environment.The performance of the proposed interference avoidance mechaine in typical UWB channels is analyzed and compared with that of transitional frequency hopping seheme by simulations. The simulation results suggest that:1) The performance of spectrum sensing FH seheme is much better than traditional FH for a given value of y, It implies that CR-FH is an effective scheme for MB-OFDM UWB to avoid co-channel interferences. Espectial, when y=0, In comparison with the traditional FH, a SNR gain of 3dB can be achieved by the proposed FH when the bit error rate is 10-3. However, it also can be observed that the BER performance of CR-FH decreases monotonically when y increases from 0 to 0.9. It suggests that CR-FH is not robust enough if all three sub-bands are disturbed at the same time frequently.2) For the case ofγ≠0, we verify the performance of refined interference avoidance mechanism (RIAM). The BER performance of RIAM is much better than that of tradional FH and similar to that of with no interference for any given value of y, especially when y takes big values. When y=0.3, a SNR gain of 4dB can be achieved by the proposed FH when the bit error rate is 10-3.and when y=0.9, the corresponsing SNR gain is7dB. So far, it suggests that RIAM is a feasible way to improve the performance of MB-OFDM UWB systems.3) The proposed interference avoidance mechanism is robust no matter at the typical UWB channel CM1 or CM3. The tendency of the performance of MB-OFDM UWB that implemented the proposed mechanism is approximately the same.

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