Continuous Monitoring Techniques for a Cognitive Radio Based GSM BTS

GSM is a cellular wireless network which uses a fixed spectrum assignment policy. The allocated GSM spectrum is densely used in some places but sparingly used in others like in rural areas and inside a building. This provides an opportunity to improve the efficiency in the GSM spectrum usage. The cognitive radio based GSM base station (cognitive GSM BTS) can use the unused GSM spectrum, thereby increasing the spectral efficiency. Spectrum sensing and continuous monitoring are the key cognitive radio techniques which are used to identify and use the unused spectrum. These techniques are studied for an upcoming wireless standard the IEEE 802.22, but these schemes cannot be directly used for the cognitive GSM BTS. In this paper two modified continuous monitoring schemes are proposed for the cognitive GSM BTS. This paper also presents novel ways to implement these schemes for the BTS, without modifying the GSM standard. Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used intermittently. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new paradigm is referred to as cognitive radio. The cognitive radios (unlicensed users) share spectrum with the licensed users, thereby increasing the spectrum usage efficiency. GSM is a cellular wireless network which uses a fixed spectrum assignment policy. The allocated GSM spectrum is densely used in some places but sparingly used in others like in rural areas and inside a building. This provides an opportunity to improve the efficiency in the GSM spectrum usage. The cognitive GSM BTS can automatically identify and use the part of the GSM spectrum which is not used by the licensed users, thereby, increasing the overall spectral efficiency of the GSM network. The cognitive GSM BTS is the secondary user (cognitive radio) of the GSM band. Spectrum sensing and continuous monitoring are the key cognitive radio techniques which are used to identify and use the unused spectrum, without causing interference to the licensed users. Spectrum sensing helps cognitive radios to identify the vacant channels in the primary band. The spectrum sensing techniques for a cognitive GSM BTS are studied in (1). Once the vacant channels are identified the secondary users can use these channels without causing harmful interference to the primary user. To do so, the cognitive radio must continuously sense the spectrum it is using in order to detect re-appearance of the primary user. Once the primary user is detected, the cognitive radio should withdraw from the spectrum instantly so as to minimize the interference it may possibly cause. This is referred as continuous monitoring in this paper. Continuous monitoring offers different challenges, some of them are discussed below. • For continuous monitoring, cognitive radios must detect weak primary user's signal in the same frequency band as used by the cognitive radio for transmission. Detecting weak primary signal in strong interference is a very difficult task, and the continuous monitoring schemes should take care of that. • The cognitive radio receiver should be able to receive it's own signal and the primary user's signal. To explain this requirement, consider the cognitive GSM BTS, like any other GSM BTS it transmits (downlink) and receives (uplink) on two different frequencies. For the cognitive GSM BTS the primary user is another GSM BTS, so the cognitive GSM BTS receiver has to receive transmitted signal from the other GSM BTS (primary signal) and transmitted signal from its mobile users (own users signal). Which means that it has to receive signals at two different frequencies. Consequently to receive two frequencies the cognitive receiver should have two receive chains.