Continuous Monitoring Techniques for a Cognitive Radio Based GSM BTS

Transcription

1 NCC 2009, January 6-8, IIT Guwahati 204 Continuous Monitoring Techniques for a Cognitive Radio Based GSM BTS Baiju Alexander, R. David Koilpillai Department of Electrical Engineering Indian Institute of Technology Madras {baiju,koilpillai}@iitm.ac.in Abstract 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 , 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. I. INTRODUCTION 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 []. 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. For continuously monitoring the primary user s signal, detection has to be done periodically, so detection algorithm complexity should be low. If during continuous monitoring the cognitive radio finds the presence of the primary user in its frequency band of operation then the cognitive radio must stop transmitting on this band and switch to a new band. For switching cognitive radio should have at least one backup frequency and the cognitive radio must inform its users to change to the new frequency. Continuous monitoring is important to avoid interference to the licensed users. In this paper, two monitoring schemes are studied for a single carrier cognitive GSM BTS. Single carrier BTS is a BTS that uses only one carrier or one operating frequency. These schemes are adopted from the upcoming wireless air interface (i.e., MAC and PHY standard based on cognitive radios: the IEEE working group [2]. The two methods are single carrier or non-frequency hopping method

2 NCC 2009, January 6-8, IIT Guwahati 205 Quiet Control Data Transmission Quiet Control Data Transmission Downlink TimeSlot Fig.. Time Non-hopping monitoring scheme proposed for systems Uplink TimeSlot [3] and frequency hopping method [4]. These methods are explained in this paper. However, these methods can not be used directly for the cognitive GSM BTS. This paper studies the modifications needed to implement these methods for the cognitive GSM BTS. II. SINGLE CARRIER OR NON-FREQUENCY HOPPING METHOD This section gives an introduction of the non-frequency hopping method of systems and then steps to adopt it for the cognitive GSM BTS is presented. The non-hopping mode is the basic mode of systems [3]. In this scheme the operating time is divided into sensing time and data transmission time which are mutually exclusive, i.e. during the channel sensing interval, the BTS does not transmit any data. Such periodic interruption in data transmission could impair the QoS (Quality of Service of the system. Fig. shows the non-hopping monitoring scheme proposed for systems [3]. It can be seen that there is periodic slot allocated for sensing and during this time, data transmission is stopped. The advantage of this scheme is that there is no selfinterference while sensing the primary user. The main disadvantage is that the data transmission is interrupted periodically and hence, the system throughput is lowered. This method is also called single carrier scheme because sensing and data transmission can be done on the same carrier frequency. A. Non-hopping mode for cognitive GSM BTS The non-hopping monitoring scheme can be adopted for the cognitive GSM BTS, to facilitate this, fixed time should be allocated for sensing. GSM uses time division multiple access (TDMA [5] and each frequency channel of 200 KHz bandwidth is divided into 8 time slots. GSM time slot structure is shown in Fig. 2. If data transmission is stopped in one of the time slots and sensing is done during this time, it can be seen from the time slot structure that this would mean that two time slots can not be used for data transmission. For example, data transmission can be stopped on downlink time slot 4 and sensing can be done on uplink time slot (refer Fig. 2. But, in GSM same numbered downlink and uplink time slots form a pair for transmission and reception. Which means that downlink time slot can not be used for data transmission. So both time slots 4 and cannot be used for data transmission. So if non-hopping monitoring scheme is used, one time slot is wasted, i.e., on this time slot neither sensing is done nor data is transmitted. Fig. 2. GSM time slot structure The other problem in directly adopting the non-hopping monitoring scheme proposed for systems is that the GSM standard does not allow to stop transmission on broadcast channel carrier [6]. Since the cognitive GSM BTS under study is a single carrier GSM BTS, the GSM standard does not allow stopping of transmission on this carrier. To overcome this a new scheme is proposed wherein a known waveform like single tone is transmitted during the unused time slot and interference caused by this is canceled out in the receiver. It is known that all one bits modulated by GMSK (Gaussian minimum shift keying modulator manifests as a complex sine wave [6], of frequency R b /4, where, R b is the bit rate, R b = 2733 Kbps. So sine wave of frequency R b /4 can be sent in the unused time slot. Fig. 3 shows the proposed method to cancel interference caused due to transmission of sine wave in unused time slot. Steps for canceling interference are given below: Channel estimation: By doing channel estimation the power of interference in the received signal is found. The received signal can be represented as: y(n = x(n + h i(n + w(n ( where x is primary user s signal, i is interfering signal and it is known to receiver, h is the fading coefficient, it is modeled as independent complex Gaussian random variable and w is the additive white Gaussian noise. By doing channel estimation, ĥ which is the estimate of h is found. Since it is known that i is sine wave, estimate of the channel can be found by correlating received signal y with i at zero lag. ĥ = N N y(ni (n (2 n= where N is the number of samples. 2 After finding ĥ. Multiply it with the known signal i and subtract it from the received signal y to get y. 3 Pass the subtracted signal y to a signal detector. After canceling the interference caused due to sine wave transmission, the signal is passed to a detector. For detection, the cyclostationary detector [7] is used. The simulated detection performance of this detector after self interference cancellation is shown in Fig. 4. It can be seen that their is only a small performance loss between interference cancellation method and one with no interference. For simulation study, the channel was assumed to be quasi-static Rayleigh fading, Typical Urban fading profile of GSM was used. Sampling rate was taken to be two times the symbol rate, i.e. 2R b,

3 NCC 2009, January 6-8, IIT Guwahati 206 Known Signal (i Without Interference With interference an cancellation Input (y Channel Estimation (h (h + To Signal (y Detector Probability of detection (P d Fig. 3. Block diagram of the self-interference cancellation method SNR (db 5 Fig. 6. Detection performance of cross-correlation based detector with interference cancellation scheme for Rayleigh fading. Probability of detection (P d Without interference With interference and cancellation SNR (db Probability of false alarm (P f Fig. 4. Detection performance of cyclostationary detector with interference cancellation scheme for Rayleigh fading. average interference power was taken as 0 dbm and noise variance of the detector was 6 dbm. Fig. 5 shows the false alarm rate for interference cancellation method with improved cyclostationary detector. Performance was also evaluated for cross-correlation-based detector [] after interference cancellation. Simulation was done for detecting normal burst with 26-bit training sequence in fading. Simulated detection curve and false alarm curve are Probability of false alarm (P f SNR (db Fig. 5. Probability of false alarm for cyclostationary detector with interference cancellation method SNR (db Fig. 7. Probability of false alarm for cross-correlation based detector with interference cancellation method shown in Fig. 6 and Fig. 7. Sampling rate was taken to be the same as symbol rate, i.e. R b. So any of the two detectors can be used with the interference cancellation method. One disadvantage of interference cancellation method is that it requires a large dynamic range analog to digital converter (ADC in the receiver. Dynamic range of ADC depends on transmission power of cognitive BTS and sensitivity requirement for receiver. A simple calculation is given below. Dynamic range (db = 6.02n n db, where n is number of bits in ADC. Let interfering signal power be I p = 0 dbm and noise power in dbm = 6 dbm. It was found through simulation that cyclostationary detector gives good performance with minimum 6 bit quantizer, which is equal to dynamic range of D r = 6 6 db. Minimum signal level which can be detected with the cyclostationary detector min lev = 26 dbm So minimum dynamic range required is I p min lev +D r, on substituting the values it comes to be 52 db. Number of bits for ADC is (dynamic range/6 = 52/6 28 bits ADC required.

4 NCC 2009, January 6-8, IIT Guwahati 207 Validation time Validation time Start Initial Free channels = F, F, F... Bk 2 Initial Transmitting on on Transmitting on on Time Choose Two Frequencies F = F, F = F Opr Sense 2 Fig. 8. Dynamic Frequency Hopping mode of systems Start Operation on F Opr Start on F Sense Initialize Frame Counter It can be seen that as transmit power increases, interfering power I p increases, so the dynamic range also increases. No Primary Signal Detected Frame counter is 56 No III. FREQUENCY HOPPING METHOD In the non-hopping mode sensing and data transmission can not happen at the same time, hence, data transmission is periodically interrupted. This critical issue can be addressed by an alternative operation mode proposed in called dynamic frequency hopping (DFH, where cognitive radio data transmission is performed in parallel with spectrum sensing without interruptions. In the DFH mode, a cognitive BTS hops over a set of channels. During operation on a working channel, sensing is performed in parallel on the intended next working channels. After a fixed time, a channel switch takes place, one of the intended next working channels becomes the new working channel and the channel previously used is vacated. Hence, no interruption is required for sensing any longer. To explain the working of DFH scheme an example of DFH applied two frequency channel, channel A and channel B, is studied. DFH scheme is shown in Fig. 8. The DFH mode works as follows: During initial sensing, channel to be used in the next operation period is validated. So channel A is validated first (refer Fig.8. After initial sensing, the validated channel is used for operation for a fixed duration, during this period, channel to be used in the next operation period is validated. In this example when channel A is used, channel B is validated for use in the next operation period. So data transmission is not stopped during sensing. A disadvantage of DFH scheme is that it requires atleast two frequency channels for operation. A. Frequency hopping mode for the cognitive GSM BTS Hopping scheme cannot be directly adopted for cognitive GSM BTS, which is a single carrier BTS. GSM standard does not allow hopping on broadcast channel carrier. To solve this problem a scheme is developed by which the active mobile users are informed about the change of frequency so that they can change to new frequencies and calls are not dropped. The forced synchronized handover [8] of GSM is proposed to be used for this purpose. A handover is defined as the change of the currently used radio channel to another radio channel during an existing and active connection between mobile station and BTS. GSM has Yes Fsense = F Bk Yes Send Handover Command between frame 56 to 520 Change frequency in Timeslot to 7 of frame 560 Complete handover process Interchange F and F Opr Sense Fig. 9. Flow diagram of proposed frequency hopping algorithm for cognitive GSM BTS defined several handover types [8] that can be activated by the network operator and executed when necessary. Synchronized intra-bts handover is used to implement DFH. In intra-bts handover, a new channel in the same BTS is assigned to the mobile station. It is worth pointing out that an intra-bts handover is always synchronized, since all TRXs (Transmitters and receivers of a BTS have to use the same clock. Algorithm proposed to change the frequency with the help of handover is shown in Fig. 9. Initially all the free channels in GSM band are identified and marked as F, F 2, F BK,, F BKN. It is assumed that BTS hops between 2 frequencies: operation frequency F opr = F and sensing frequency F sense = F 2. It uses each frequency alternatively for data transmission and sensing for a fixed duration of 56 frames, 8 time slots make a frame. The duration was fixed at 56 frames as it corresponds to approximately 2.4 seconds, in this duration is 2 seconds [4]. Another reason for choosing 56 frames is that frame number 520 is an idle frame, this is explained later in this section. A frame counter is started. When the counter reaches 56 frames BTS sends handover command on all traffic channel time slots to initiate a synchronized intra-bts handover. Most

5 NCC 2009, January 6-8, IIT Guwahati 208 handover-based frequency hopping algorithm. Implementation of these algorithms in a cognitive GSM BTS, helps it to continuously monitor the presence of the primary user and thereby avoiding interference to the primary user. Fig. 0. Intra BTS handover scenario [0] important content of the message is on which time slot and on what frequency is the new channel. Synchronized intra-bts handover scenario is shown in Fig. 0. After sending handover command on all traffic time slots between frame number 56 and 520 (it takes 4 frames to send handover command, BTS changes its frequency of operation in time slot to 7 of frame 520. Time slots to 7 of frame 520 are idle time slots so frequency change can take place without any data loss. Then the mobile station sends up to four handover acknowledge (HND ACC messages to the BTS on the new frequency (refer Fig. 0. After this all-further handover procedure is carried out in the new frequency. Once the handover procedure is complete, frequencies F opr and F sens are interchanged and frame counter is re-initialized. If during sensing on frequency F sens primary user is detected, then this frequency is replaced by a backup frequency, F BK. So this algorithm enables mobile and BTS to switch to new frequency without dropping call. In DFH scheme, sensing and data transmission takes place at the same time so minimum two receiver chains are required to implement it. One receiver chain to receive primary signal and other to receive own user s signal. So implementing DFH scheme is more costly than non-hopping scheme. REFERENCES [] Baiju Alexander, R. David Koilpillai, Cognitive Radio Techniques for GSM Band, NCC [2] IEEE WG, IEEE Project homepage, [3] IEEE P802.22/D0., Draft Std for Wireless Regional Area Networks Part 22: Cognitive Wireless RAN Medium Access Control (MAC and Physical Layer (PHY Specifications: Policies and Procedures for Operation in the TV Bands. [4] Hu, W. and Willkomm, D. and Abusubaih, M. and Gross, J. and Vlantis, G. and Gerla, M. and Wolisz, A., Dynamic Frequency Hopping Communities for Efficient IEEE Operation, IEEE Communications Magazine, Special Issue: Cognitive Radios for Dynamic Spectrum Access, vol. 45, number 5, pp 80-87, [5] GSM 05.02, Multiplexing and multiple access on the radio path, ETSI, 999. [6] Halonen T., Melero J. GSM, GPRS and EDGE Performance: Evolution Towards 3G/UMTS, Second Edition, John Wiley and Sons Ltd., [7] Menguc Oner, Fredrich Jondral, On the Extraction of llocation Information in Spectrum Pooling System, IEEE Journal on selected areas in communications, Vol. 25, No. 3, April 2007, pp [8] GSM 03.09, Handover procedures, ETSI, 998. [9] GSM 03.22, Functions related to Mobile Station (MS in idle mode and group receive mode, ETSI, 999. [0] Gunnar Heine, GSM Networks: Protocol, Terminology and Implementation, Artech House, 998. IV. SUMMARY In this paper two continuous monitoring schemes were studied for the cognitive GSM BTS. These schemes were adopted from the and modified to suite the GSM standard. The first technique was self interference cancellation based non-hopping scheme. Performance of this scheme was presented in this paper. The second scheme was the novel