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An Efficient Medium Access Control (MAC) Layer Protocol for a UMTS High Altitude Platform Station

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An Efficient Medium Access Control (MAC) Layer Protocol for a UMTS High Altitude Platform Station An Efficient Medium Access Control (MAC) Layer Protocol for a UMTS High Altitude Platform Station Nikolaos Batsios(1) , Fotini-Niovi Pavlidou(2) (1)Intracom S.A 14 Km Thessaloniki-N.Moudania 57001 Thermi Greece Email: [email protected] (2)Aristotle University of Thessaloniki, School of Engineering, Department of Electrical & Computer Engineering, Telecommunications Division 54006 Thessaloniki, P.O.Box: 1641, Greece Tel/Fax: +30 31 996285 Email: [email protected] INTRODUCTION The employment of High Altitude Platforms (HAPs) has been recently proposed as an alternative to traditional terrestrial and satellite-based infrastructure for the design of future communication systems [1, 2]. The cost-effective coverage of rural low population density areas, the capability to upgrade their telecommunication payload periodically in order to fulfill future demands, are some advantages of HAP based systems. The International Telecommunication Union (ITU) has granted spectrum for HAPs in IMT-2000 frequency bands [3, 4]. Under the above considerations in this paper the performance of a MAC protocol based on the combination of the well- known Packet Reservation Multiple Access (PRMA) scheme with CDMA technologies is studied for a UMTS HAP station operating at 2GHz frequency range in an altitude of 22Km. The PRMA scheme is based on TDMA and combines random access with slot reservation. Combining it with the characteristics of CDMA more efficient radio packet communication systems can be achieved since each slot can support more than one transmitted packets limited by multiple access interference (MAI) [5-8]. Protocol performance is investigated in a single and in a cellular HAP constellation. Furthermore the impact of acknowledgement delay has been examined through computer simulations, along with the selection of suitable channel access functions to control the access of mobile users. SYSTEM DEFINITION It is assumed that the High Altitude Platform Station uses a phased array antenna to project hundreds of spot beams to provide telecommunication service, in a pattern similar to that created by a traditional cellular system. In our case a HAPBS carrying a CDMA payload is considered, at an altitude of 22Km above the service area. Stability issues have been ignored meaning that the HAP station is kept stationary to the ground surface. The effect of earth’s curvature is neglected due to low altitude and the service area is further consisted of equal sized circular cells of radius r. As it was shown in [9] for radius greater than 100Km the elevation angle falls below 10o, therefore it is difficult for a HAP station to provide high quality coverage beyond this radius. Furthermore, in such cells serving large areas, the propagation delay plays a significant role. A mobile user, which has sent a packet in contention mode in a particular time slots will not be allowed to contend again in the next s slots, regardless of whether there are time slots for contention available in this period. For instance as it was shown in [2] the maximum propagation delay for elevation angles less than 10o is about 3ms means that a contending user re-attempts for transmission after at least 3 ms. The impact of acknowledgement is studied through computer simulation in this paper. Mobile users are uniformly distributed in each cell and perfect power control is employed, ensuring that signals from all mobile users in a given cell arrive at the HAP station with the same power. Two different scenarios have been studied. A stand alone configuration where a HAP station can serve a single macrocell, and cellular configuration as shown in Fig. 1 with more than one cell rings. Fig. 1. System configuration Single Cell HAP Constellation For DS/CDMA environment the Standard Gaussian Approximation has been widely used to determine the BER of the channel. Assuming that the dominant interference contribution is Multiple Access Interference (MAI), and AWGN is ignored, the average BER or the probability of bit error Pe, can be calculated using (1) Pe " Q(SNR) (1) where Q(x) is equal to 2 1 $ #u Q(x) = % e 2 du ! 2" x (2) and ! 3G SNR = p 2(K "1) (3) Perfect power control has been assumed and QPSK modulation. When packets of length L are transmitted over such a radio channel the probability of packet success Qpe is obtained from e ! " L% i L(i Qpe = $ '( 1( Qe ) (Qe ) ) i i= 0 # & (4) where Qe = 1 - Pe is the average probability of data bit success. For a spreading factor Gp=15 a packet length L=255 and a (255, 107, 22) BCH code, the packet success probability is depicted in Fig. 2. 1.2 ! 1 0.8 0.6 0.4 0.2 Burst Success Probability Qpe[K] 0 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Voice Users K per time slot Fig. 2. Qpe[K] for Gp=15 and a (255, 107, 22) BCH code Multiple Cell HAP Constellation In the case of a cellular environment, users located in neighboring cells, also generate interference, yielding in an increased BER. The average signal to noise ratio with perfect power control and QPSK modulation is denoted by 3G SNR = p 2(K "1) + 2(K f ) aver (5) In (5) Kaver is the average number of users per time slot. In the case where each cell is equally loaded with M mobile voice users Kaver is equal to ! M • au Kaver = N (6) In (6) N is the number of slots per frame and au is the voice activity factor. It is further assumed that Kaver denotes the number of users per time slot also in the test cell. Another important parameter in (5) is the factor f, which denotes the other cell interference factor. As it was shown in [10] taking f = 0.155 for a cellular HAP en!vi ronment and 4 tiers of interfering cells could be a good approximation. From (4) the packet success probability can be derived. Since the average SNR in a cellular environment is strongly related to the number of users M, increasing the load someone will expect a reduction in the number of simultaneous transmitted packets in a time slot. This effect is depicted in Fig. 3 From Fig. 3 when M=100 taking Kmax=9 we can achieve a packet success probability equal to 0.9937 whilst for K=10 the average packet error probability falls below 5%. Increasing the number of users M we can obtain that for M=240 no more than 8 packets can be correctly received at the aerial station VOICE TRAFFIC MODEL Each voice terminal uses a slow speech activity detector so the speech source can be modeled as a two state Markov chain. The two statuses are talkspurts and silentgaps. The lengths of both are assumed to be exponentially distributed. The Markov chain is assumed to be of discrete time with transition on slot boundaries. Voice packets are generated only during talkspurts, at the rate one per frame. The mean duration of talkspurts is Dtalkspurt = 1sec and for silentgaps Dsilentgap = 1.35 sec, yielding to a voice activity factor of 0.426. A finite number of voice terminals M is considered, which are all involved in a conversation and this number remains fix per simulation run. PROTOCOL DESCRIPTION CDMA/PRMA protocol is an extension of the conventional PRMA in order to operate in a DS/CDMA environment [6, 7]. The time axis is divided into time slots where a fixed number of them consist a TDMA frame. The TDMA frame rate is equal to the basic source frame rate. In our case only voice traffic is considered and each voice source generates one packet per TDMA frame. 1.05 1 0.95 0.9 M=100 0.85 M=180 0.8 M=200 0.75 0.7 M=260 0.65 0.6 0.55 0.5 0.45 0.4 0.35 0.3 0.25 0.2 Burst Success Probability Qpe[K] 0.15 0.1 0.05 0 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Users K per time slot Fig. 3. Qpe[K] for Gp=15 and a (255, 107, 22) BCH code and different values of users M In an uplink information channel, each time slot can support more than one simultaneous transmitted packet. The maximum number of correctly received packets at the HAPBS is limited by the MAI. These time slots are either available for contention or reserved for the information transfer of a particular mobile user. The HAP station informs all mobile users in the downlink about the status of each time slot (reserved or available). When a packet arrives at a mobile user it will switch from idle mode to contention mode, trying to perform a transmission at the first contending slot (Sc). The mobile user has first to perform a Bernoulli experiment with some permission probability Pv in order to gain access to the channel. In the case of positive outcome (the output of the experiment is less than Pv) the user transmits the first packet of the talkspurt. If this packet is received correctly by the aerial station, it will send an acknowledgment, which implies a reservation of the same slot (which is now characterized as a reserved slot, Sr) in subsequent frames for the rest of the talkspurt. In the case of a negative outcome or corruption of the packet due to excessive MAI the contention procedure is repeated. Packets are dropped when exceeding a delay threshold value Dmax and contention is repeated with the next packet of the talkspurt.
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