Thesis or Dissertation Studies on Reliable Public Safety Wireless Communication Systems Employing Software De ned Radio and Cognitive Radio

森山, 雅文  ,  モリヤマ, マサフミ  ,  Moriyama, Masafumi

pp.1 - 114 , 2015-03-25 , The University of Electro-Communications
Public safety mobile wireless communication systems (PMCSs) are widely used by public safety personnel, such as firefighters and police, as well as local governments. PMCSs are crucial to protect safety and security of communities. Conventional PMCSs effectively cover underpopulated areas as well as urban areas by employing long-zone scheme. Since the PMCSs can cover areas that are not covered by commercial cellule systems, they play the important role as the only communication tool. Moreover, the conventional PMCSs have enhanced robustness and reliability. The conventional PMCSs can keep their services even if backbone lines are cut off. In contrast, short-zone scheme systems cannot offer stable and wide service area without backbone line connection. For example, the Great East Japan Earthquake in Japan, police mobile communication systems had kept their functions while cellular phones became disabled. PMCSs are required to be quite high robustness and reliability in order to save human life. Recently, conventional PMCSs are required to realize further expansion of service areas and high speed transmission although they have stably provided users with wide service areas so far. Nowadays, in order to solve complicated public affair quickly, more stable service areas and broadband communication are required. Compared with conventional PMCSs in urban areas, commercial wireless mobile communication systems (CWMCSs) such as cellular systems supply stable service areas and broadband communication in times of peace. In accordance with development of wireless technology, PMCSs need to keep pace with CWMCSs. However, conventional PMCSs can hardly realize further stable service areas and high speed transmission because of large-zone scheme. In terms of realization of further stable service areas, no-service areas cannot be eliminated easily. This is because no-service areas are mostly attributed to shadowing; in large-zone scheme, a no-service area that must essentially be covered by a certain base station is seldom covered by other neighboring base stations. Although new allocation of base stations is fundamental answer to solve no-service area problem in PMCSs, building new base stations of PMCSs that are not used for a commercial purpose is restricted by national and local budget. Realization of high speed transmission of PMCSs is also difficult because of large-zone scheme. To realize high speed transmission, increase of transmit power or shrinking of service area coverage is required to compensate Signal to Noise Power Ratio (SNR) deterioration caused by expanding bandwidth. Increase of transmission power of mobile station used in large-zone scheme systems is almost impossible because transmission power of mobile station is originally high. Thus, shrinking of service areas is necessary for high speed communication. Currently, to realize high speed transmission, next generation broadband PMCSs (BPMCSs) employing short-or middle-zone scheme are being developed. In the 3GPP, it is considered that the Long Term Evolution (LTE) is used for communication of public safety. In Japan, National Institute of Information and Communications Technology (NICT) has researched and developed Public Broadband Wireless Communication System (PBWCS), which employs 200MHz as carrier frequency. The PBWCS has already been equipped in national police agency in Japan. However, we consider that the conventional narrowband PMCSs (NPM-CSs) are not replaced with the BPMCSs completely. This is because the BPMCSs cannot cover all the areas that the conventional NPM-CSs have covered. Moreover, there are problems of robustness and reliability when accidents happen. Hence, users of PMCSs will utilize both of NPMCSs and BPMCSs in accordance with the situation. In this case, users equipping several terminals feel inconvenient and also radio resources are not used effectively. The best solution to realize optimal PMCSs is employing heterogeneous cognitive radio (HCR) for PMCSs. By applying the HCR to PMCSs, service areas expansion and high speed transmission in PM-CSs will be realized effectively. We propose an integrated system combining NPMCSs with CWMCSs and BPMCSs to make communication quality of the PMCSs improve. The proposed HCR recognizes communication conditions of several systems and then provides PMCS's users with optimal communication quality. Although software defined radio techniques (SDR) are ideal to operate cognitive radio more flexibly, we deal with HCR mainly to realize combined systems in this thesis. We study advantages, problems, and their solution to realize the HCR for PMCSs. Firstly, we research service area expansion of NPMCSs using HCR. The proposed HCR is utilized for stabilization of NPMCS's service area. If communication quality of a NPMCS deteriorates owing to shadowing, the proposed HCR terminal obtains a part of NPMCS's data called subsidiary information (SI) from CWMCSs or BPMCSs. The proposed HCR terminal can improve PMCS's bit error rate (BER) performance by combining the SI with received signals of the NPMCS and then decoding the combined signals using forward error correction (FEC). Since convolutional codes are often used in FEC of NPMCSs, we consider BER improvement methods of the convolutional code. We derive modified Viterbi algorithm from maximum likelihood sequence estimation (MLSE) of the combined signals. Moreover, we introduce the distance spectrum to evaluate characteristics of the convolutional codes. The distance spectrum is used for estimating improvement of BER performances. Next, we consider synchronization methods to realize the proposed HCR. In the HCR, there are two types of synchronization method; one is the self-synchronization method to synchronize each system itself. The other is the co-synchronization method to combine different systems. In this thesis, we consider self-synchronization methods of NPMCSs mainly. This is because the HCR aims to improve communication quality of NPMCSs equipping conventional self-synchronization methods that are not probably available in low SNR environments. In this environment, since NPMCSs can hardly obtain their self-synchronization alone, powerful self-synchronization methods using HCR techniques are required. We propose two synchronization methods that are utilization of global portioning system (GPS) signals and utilization of the SI, respectively. The synchronization methods utilizing GPS signals can acquire timing synchronization. To obtain timing synchronization, the proposed HCR acquires accurate time and own location using the GPS signals. The HCR also gets the location of base stations and the frame timing by making the SI convey their information. Since the HCR can know accurate time and distance between the base station and the HCR, synchronization timing can be calculated. However, in GPS based method, preciseness of timing synchronization may be deteriorated by measurement error of GPS signals, diffraction caused by mountains, and propagation delay caused by reection. For this reason, we consider a mitigation method of the timing error and then evaluate BER performances using computer simulation. Moreover, we propose a SI based synchronization method that can obtain timing synchronization without GPS signals. The proposed method is employed when a NPMCS uses differential coded π/4 shift QPSK as the modulation scheme. The notable feature of the proposed method is to convey the phase rotation of the π/4 shift QPSK as the SI. The HCR can forecast PMCS's envelopes from the obtained SI and then obtain the timing synchronization by correlating the forecasted envelopes with real received envelopes. Since the proposed method can also be used for co-synchronization and BER improvement, CWMCS's resource consumption to convey the SI is suppressed. Finally, we consider HCRs combining several PMCSs. In this thesis, the combination of NPMCSs and the combination of a NPMCS and a BPMCS are researched. In the combination of NPMCSs, we consider that several PMCSs are integrated by SDR. In the combination of a NPMCS and a BPMCS, we propose site diversity based on HCR to improve uplink communication quality of the BPMCS. In this diversity, since uplink interference must be avoided, we employ combination of the adaptive array and HCR techniques. Moreover, we propose information compression methods for narrow band backbone lines so that received data can be conveyed to head office with little BER deterioration. PMCSs will have played an important role to ensure social safety. In the thesis, we consider the one of the next generation PMCSs employing SDR and HCR. Using this research, we can obtain a direction of optimal PMCSs. The next step that we need to perform is to apply our proposed method to actual radio systems. We must continue this research so that high reliable and compact PMCSs can be realized.

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