channel simultaneously, the transmitted packets are lost in collision. The piconets .... slave in several piconets, but only as a master in a single piconet. Usually ...
Adaptive Mitigation of Self-Interference in Bluetooth Scatternets Petar Popovski, Hiroyuki Yomo, Sergio Guarracino, Ramjee Prasad Center for TeleInFrastructure (CTIF), Aalborg University Niels Jernes Vej 12, DK-9220 Aalborg, Denmark Email: {petarp, yomo, sergiog, prasad}@kom.auc.dk
Abstract— Bluetooth technology supports operation of short-range wireless ad hoc networks. The basic networking entity is piconet and several piconets can be interconnected into a scatternet. Since Bluetooth operates in the unlicensed ISM band, each piconet is uses pseudorandom frequency hopping. If collocated piconets use the same channel simultaneously, the transmitted packets are lost in collision. The piconets that are networked into scatternet exhibit spatial overlapping and naturally produce multi-piconet interference, termed selfinterference. The collisions cause retransmissions and thereby energy wasting. In this paper we propose a distributed algorithm for adaptive self-interference avoidance (A-SIA). As opposed to the earlier selfinterference avoidance (SIA) proposal which is oblivious with respect to the physical scatternet topology, the A-SIA algorithm adapts to the actual interference pattern, determined by the piconet deployment and activity. The results show that A-SIA algorithm maintains high energy efficiency, while not introducing the goodput degradation which is featured in the operation of SIA algorithm for sparse scatternets. Keywords— Bluetooth, scatternet, interference avoidance, energyefficiency
I. I NTRODUCTION The recent developments in the wireless technology have been largely focused on supporting disposable wireless links among proximate devices and users [1]. The short-range wireless networks are usually operating according to the ad hoc networking paradigm, where the network is established and maintained without relying on a preexisting infrastructure. The applications of short-range wireless networks encompass Wireless Personal Area Network (WPAN) [2], collaborative computing, interconnection of sensors and actuators etc. There are two important features of the shortrange wireless networking technologies. First, regarding the target ubiquitous usage scenarios with local interactions, a short-range wireless technology should be available worldwide, without imposing charges to the user for the utilization of the wireless medium, which implies usage of the unlicensed spectrum. Second, the devices involved in these networks are usually battery powered and thereby energy limited. Therefore, the energy-efficient operation should be considered in the design of all elements of the protocol stack. Bluetooth [3] is a prominent representative of the short-range wireless technologies. Bluetooth technology operates in the 2.4 GHz unlicensed ISM band. A basic networking entity in Bluetooth is a piconet, which is a star topology with a central coordinating unit-master and up to seven active slaves. Two or more piconets can be interconnected into a scatternet, which can be considered as a instance of extendable multi-hop ad hoc networking structure. The piconet is using slow frequency hopping (FH), such that for each packet, the piconet picks a frequency channel according to a pseudorandom pattern. Two different piconets are not coordinated in their hopping patterns, such that if the piconets are in close proximity there is a nonzero probability that the packets in the collocated piconets will simultaneously use the same frequency hop and lose the packets in collision. This inter-piconet interference is termed self-interference. When a piconet is participating in a scatternet, it does not change its hopping pattern compared to the case when the piconet is independent. Since the spatial overlapping
of the piconets is unavoidable for the case of scatternet, the self-interference becomes a more acute problem. The collisions cause packet errors and packet retransmissions, which increases the energy spent per transmission of a data portion. This can be a serious drawback for scenarios where densely deployed scatternets should operate for a long time and thus the scarce energy should be spent in an economic manner. The problem of self-interference has received significant research attention [4] [5] [6] [7] [8]. When the collocated piconets are not interconnected, then to avoid self-interference they should apply non-collaborative coexistence mechanisms. Such mechanisms have been proposed in [9] and [10]. If the piconets are networked into a scatternet, then they can exchange information to aid the avoidance of the self-interference. In [11] the authors propose collaborative approach for interconnected piconets, but the self-interference is not completely eliminated. In [12] we have proposed a distributed Self-Interference Avoidance (SIA) algorithm, which relies on the information exchange among the piconets in scatternet. SIA algorithm results in overall energy-efficient operation of the scatternet. However, the basic SIA algorithm proposed in [12] is oblivious with respect to the physical topology: A piconet A assumes that a transmission in any other piconet of the scatternet has enough power to produce interference to the piconet A. This is certainly not true for sparse scatternets, where the propagation effects are mitigating the self-interference. The other assumption adopted by a piconet that runs the basic SIA algorithm is that the other piconets are always active. These two pessimistic assumption does not deteriorate the energy gain brought by the SIA algorithm, but they can dramatically reduce the goodput in the piconets and introduce unnecessarily large delays. To cope with these deficiencies of the SIA algorithm, in this paper we propose the adaptive SIA (A-SIA) algorithm. The ASIA algorithm equips a piconet with a mechanism to probe which piconets in the scatternet can cause actual interference to that piconet. The simulation results show that the A-SIA algorithm largely retains the energy gain brought by the SIA algorithm, while it avoids the goodput degradation featured in the SIA operation. By learning about the actual interference environment, the ASIA algorithm is adaptive with respect to spatial density of the interconnected piconets. The paper is organized as follows. After the introduction, we give a system background about the Bluetooth system and introduce the terms and notations used further in the paper. Section III contains a detailed description of the S-SIA algorithm. The simulation model and performance evaluation are presented in Section IV. Finally, Section V concludes the paper. II. S YSTEM DESCRIPTION Bluetooth piconet is which is a star topology with a central coordinating unit-master and up to seven active slaves. Two piconets can communicate with each other if there is at least one bridge device—unit that is a member of both piconets in a time-division multiplex basis. The bridge devices enable creation of scatternet
of arbitrary size. The FH communication channel in a piconet is slotted with a nominal slot value 625 µs. The slaves are time- and hop- synchronized to the master. All transmissions occur between master and slaves only. The full-duplex communication is realized by the exchange of packets in a Time-Division Duplex (TDD) manner. The start of a packet occurs at the beginning of the slot and on the frequency that is determined by the unique Bluetooth device address (BD_ADDR) of the master and the value of the master clock in that slot. The nominal hopping rate is 1600 hops/s. Between a master and a slave there can be two link types: Synchronous Connection-Oriented (SCO) and Asynchronous Connection-Less (ACL). SCO is symmetric point-to-point connection defined to carry voice stream and is maintained by using reserved slots at regular intervals. Non-SCO slots can be used to provide ACL connections between a master and all active slaves. The packets used in the ACL links occupy 1, 3, or 5 time slots. The 3- and 5-slot packets retain the initial transmission frequency during all packet slots, making the actual hopping rate lower than the nominal. Different ACL packet types are denoted by DHx and DM x, where x ∈ {1, 3, 5} denotes the packet length, H denotes high rate packet and M denotes medium rate. Due to the applied ARQ, ACL packets are retransmitted in case of loss until a positive acknowledgement is received at the packet source. The slots of the piconet channel are uniquely numbered by the clock of the master. The master starts transmission of master-toslave (MS) packet in even-numbered slots. A slave transmits only after being polled by the master. If the master has data for the slave, it polls the slave implicitly by addressing the MS-packet to that slave. Otherwise, the master sends explicit POLL packet to the slave. All slaves are listening the channel at the even slots and the slave which detects that it has been addressed within the header of MS-packet continues to receive the packet payload. The addressed slave must respond with a slave-to-master (SM) packet in the odd-numbered slot that immediately follows the reception of the MS-packet. If the slave does not have any data, it responds with a NULL packet. However, if the addressed slave fails to detect the header of MS-packet, it will not respond with a SM-packet. After receiving the header of the MS-packet, the non-addressed slaves are going to sleep mode until the next even slot. By listening the channel at the even slots the slaves are re-synchronizing to the master clock. The hopping mechanism is robust in a sense that master and slaves remain synchronized even if no transmissions occur in the channel for hundreds of milliseconds [13]. The FH pattern and the time reference in the piconet do not change when the piconet is a member of a scatternet, i.e. the piconets continue to hop independently. The clocks of the different masters should not be synchronized. A Bluetooth unit can act as a slave in several piconets, but only as a master in a single piconet. Usually, the bridge device is a slave in both piconets. To participate on the proper channel in a piconet, the bridge unit should use the associated master device address and proper clock offset to obtain the correct phase. We will use πk to denote the k−th piconet and µk to denote the master of πk . Define a transaction τk in piconet πk to be a sequence of MS-packet and the SM-packet from the addressed slave. Then, the goal of the proposed algorithm is to provide each piconet in the scatternet with a two-way data pipe that consists of interference-free transactions. A transaction is called active if the master and the addressed slave transmit the packets during the transaction. Here we assume that, whenever the master transmits MS-packet, the addressed slave receives the header (but not necessarily the payload) and sends SM-packet. The A-SIA algorithm relies on two essential assumptions: (1) A piconet operates solely with ACL traffic and (2) Always 1slot packet is transmitted. The first assumption is needed to have
piconet πk
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Fig. 1.
← transaction τ (k, i) → fM (k, i) fS (k, i)
← transaction τ (l, j) → fM (l, j) fS (l, j)
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Overlapping between the transactions of two piconets k and l
flexibility in the traffic, in a sense that a packet can be rescheduled to postpone its transmission, which is not possible with the SCO traffic. The second assumptions renders the information about the clock and address of a master sufficient to know the frequency used in the piconet of that master at any instant. III. A LGORITHM FOR A DAPTIVE S ELF -I NTERFERENCE AVOIDANCE (A-SIA) We start by description of the basic SIA algorithm, introduced in [12]. Let us consider two piconets πk and πl , connected into a scatternet. The interconnection of the piconets enables the master µk of πk to obtain the clock and the address of the master µl , and vice versa. Since we have assumed that all the piconets in in the scatternet are using solely single-slot packets, then by knowing the clock/address, µk knows which frequency is used in µl at each instant. The same can be claimed for µl with respect to µk . Since the piconets are not synchronized, each slot of piconet k overlaps with two slots in piconet l. In addition, from Fig. 1, the transaction τ (k, i) in πk overlaps with two transactions in piconet l, τ (l, j) and τ (l, j + 1). The same is true for each transaction in πl . We define the transaction τk in πk to be concurrent with the transaction τl in πl if a packet from τk should simultaneously use the same frequency with a packet from τl . For the example on Fig. 1, τ (k, i) is concurrent with τ (l, j) if at least one of the following is true: fM (k, i) = fM (l, j) or fM (k, i) = fS (l, j) or fS (k, i) = fS (l, j). If the concurrent transactions are both executed, then a collision between πk and πl occurs. The SIA algorithm guarantees a distributed consensus, such that when there are concurrent transactions, at most one transaction is executed. The master µk starts the transaction τ (k, i) by transmitting MS-packet only if τ (k, i) is assessed to be usable. For defining usable transaction, first we define that the transaction τ (k, i) is usable with respect to τ (l, j) if either C1 or C2 is true: C1 C2
τ (k, i) and τ (l, j) are not concurrent; τ (k, i) and τ (l, j) are concurrent, while it holds fM (k, i) + fS (k, i) > fM (l, j) + fS (l, j).
If τ (k, i) is usable with respect to both overlapping transactions τ (l, j) and τ (l, j + 1), then τ (k, i) is usable with respect to πl . If there are in total N piconets in the scatternet, µk assesses τ (k, i) usable if and only if it is usable with respect to each of the (N −1) other piconets. For some realistic scenarios regarding the scatternet operation, the basic SIA algorithm results in unnecessary performance degradation. In essence, it can be said that a master that applies SIA makes pessimistic assumptions about the potential collisions with the other piconets. Let τ (k, i) be concurrent with τ (l, j). Let the devices of πk be sufficiently distant from the devices of πl , such that, due to the propagation, a transmission in πl does not reach a receiver in πk with power that is sufficient to cause packet error. In such case both τ (k, i) and τ (l, j) can be executed, without interfering with each other. However, by receiving the clock/address information about some µl , the master µk does not get any information about the physical distance from the devices in πl , such that in SIA a pessimistic assumption is made that all collisions are destructive. Another such assumption is that µk considers that µl has traffic to start transmission during
Receive CA-packet from ml
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Fig. 2. The algorithm that the master µk runs within the A-SIA algorithm with respect to the piconet πl .
any transaction. Clearly, this assumption also yields suboptimal operation when the piconets exhibit pattern of variable activity. In order to cope with such suboptimalities, we introduce the adaptive version of the SIA algorithm, the A-SIA algorithm. Note first that the SIA run by µk can be seen as a union of SIA-instances with respect to all other piconets in the scatternet. The target of the A-SIA is to allow execution of concurrent transactions that can not cause collision and packet error to each other. For describing the actual A-SIA algorithm, we start from the instant at which the µk receives for the first time a packet (called CA-packet) that carries information about the clock/address of the master µl . After receiving the CA-packet, µk starts to run an instance of the SIA algorithm with respect to πl . Let us now observe the first transaction τ (k, i1 ) which µk claims unusable with respect to πl . If the basic SIA is applied, then µk should not start τ (k, i1 ). In the A-SIA algorithm, µk starts τ (k, i1 ). If there is not an error during τ (k, i1 ), the next transaction τ (k, i2 ) which is claimed unusable with respect to πl is also executed. An error during the transaction is detected by the master both explicitly and implicitly: • •
•
explicitly, by receiving erroneous SM-packet; explicitly, by receiving NAK in the SM-packet, which implies an error in the previous MS-packet; implicitly, by not receiving an SM-packet from the addressed slave.
If there is an error during the transaction, then µk sets the value of the backoff counter Wl to Wmax , where Wmax is referred to as backoff window. The next Wmax transactions which µk claims to be unusable with respect to πl are not started and πk stays silent during those transactions. In other words, µk starts to run the SIA algorithm with respect to µl . After Wmax such transactions, the next transaction that is claimed unusable with respect to πl is started by µk . If there is an error, µk again backs off for Wmax transactions. The diagram of the A-SIA algorithm as run by µk with respect to πl is shown on Fig. 2. Each master µk in the scatternet runs an instance of the algorithm shown on Fig. 2 with respect to each other piconet πl and the complete A-SIA can be regarded as a union of these instances. In essence, after applying the SIA algorithm during Wmax unusable transactions, the master µk probes whether the other piconet is
“active” by starting an unusable transaction. The overall effect is that µk adaptively turns on/off the SIA algorithm as the activity of the interfering piconets within the scatternet changes. Note that πk keeps separate backoff counter Wl for each piconet πl from which a CA-packet is received. It may happen that the transaction τ (k, i) is claimed not usable with respect to two different piconets, πl1 and πl2 . In that case, the transaction is started if both Wl1 = 0 and Wl2 = 0. The backoff window Wmax is subject to optimization, for which we need a knowledge about the traffic pattern in the piconets. In general, µk can adapt individual values of Wmax with respect to each piconet from which it receives a CA-packet. Since our goal here is only design of the interference avoidance at the MAC layer, we are not dealing with the optimization of A-SIA for specific traffic patterns generated by the higher layers. Instead, we consider two extreme cases with respect to the required behavior of the ASIA algorithm: • Multihop scatternet and all piconets with activity G = 1. The master µk should set Wmax (l1 ) = ∞ for each piconet πl1 that is in range of πk and set Wmax (l2 ) = 0 for each piconet πl2 that is physically distant, such that it does not interfere with πk . • All piconets are in range, but each piconet has only an intermittent traffic activity. In this case, the traffic in the piconets can be represented by a bursty Markov ON/OFF source rather than a single activity factor G. The parameter Wmax should be adapted to the parameters of the ON/OFF source and, in general, Wmax should vary over time. IV. P ERFORMANCE E VALUATION The A-SIA algorithm has been evaluated by the BlueHoc [14] extension of the network simulator ns-2 [15]. The BlueHoc simulator has been significantly modified in order to obtain scenarios with realistic modelling of the self-interference among the piconets. The detailed modifications can be found in [?]. The scenario simulated is represented by square area of L × L m2 in which N piconets are deployed randomly. Both L are N were varied. Each piconet consists of a master and a single slave. The position of each master is chosen uniformly within the square area, while the slave is put at distance 2m in a random direction from the master. The transmit power of each device is set to 0 dBm. The propagation model has been taken as in [6], but the noise has been neglected. At the start of the simulation it is assumed that each master has received CA-packets from all other masters. The traffic in each piconet is generated according to the exponential ON/OFF model. The traffic parameters are chosen such that a piconet is kept at high load: the average ON time is 900ms, average OFF time is 100ms and data at each device is generated with 200 kbps during the ON period. Each device always transmits data by using the DH1 packet with 27 bytes payload. The POLL and NULL packets are transmitted when needed, but the master does usability assessment according to the A-SIA algorithm also before transmitting a POLL packet. The backoff window for the A-SIA algorithm is taken to be Wmax = 5. Two performance measures have been considered: goodput and energy efficiency. Goodput is measured as average amount of useful data exchanged within a piconet during the simulation time (30s). The results are obtained as averages over 10 runs. The POLL and NULL packets, as well as the data in the packet header were not calculated in the useful data. Due to the exclusive usage of DH1 packet, the maximal possible goodput per piconet is 345.6 kbps. The energy efficiency is measured through the average energy spent in a piconet per useful data byte. The amount of energy spent for transmitting and receiving a byte is ET X and ERX ,
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Fig. 3. Average energy spent per useful data byte in a piconet as a function of the side of the square area for three cases: no SIA applied (SIA-OFF), ordinary SIA (SIA-ON) and A-SIA. The values are normalized with respect to the energy spent for SIA-ON when L = 10m. The number of piconets is N = 30.
respectively. When the master does not send MS-packet to start a transaction, the slave is still considered to spent the amount of energy needed to receive the packet header. Following [16], we have taken ERX to be one energy unit and ET X = 2ERX . Figures 3 and 4 show the energy efficiency and goodput, respectively, as functions of the spatial density for the piconets into the observed area. The number of piconets in the scatternet is N = 30 and spatial deployment density is varied via the change of the square side L. The graphs compare the performance when no SIA is applied (SIA-OFF) to the cases when basic SIA (SIAON) or adaptive SIA (A-SIA ON) is applied. The energy values are normalized with the value of the energy used with SIA-ON algorithm for square side L = 10m. The energy and the goodput of SIA-ON case do not change with L, since SIA is oblivious with respect to the physical topology. From Fig. 3 it can be seen that the gain in energy offered by the SIA algorithm with respect to the SIA-OFF case is only slightly deteriorated in case A-SIA is used. This is because some self-interference can still occur in the A-SIA algorithm, while it is completely eliminated by the usage of the SIA algorithm. The energy gain brought by the usage of SIA or A-SIA decreases as the scatternet becomes sparse. This is because the destructive collisions are reduced in the sparse scatternet and this brings reduction in the number of retransmitted packets in SIA-OFF scatternet. Fig. 4 clearly shows the suboptimal operation introduced by the pessimistic assumptions adopted by SIA. As L increases, the goodput of SIA-OFF and A-SIA increases, while the goodput offered by the basic SIA remains constant. This implies that for sparse scatternets, SIA introduces unnecessary large delays in the piconet traffic. The A-SIA algorithm successfully repairs the goodput degradation featured in the operation of the basic SIA variant. It can even it be noted that there are values for L for which the goodput of A-SIA is higher than the goodput of SIA-OFF.
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Fig. 4. Average goodput of the individual piconets as a function of the side of the square area. Number of piconets is 30.
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Fig. 5. Average energy spent per useful data byte in a piconet as a function of the number of piconets in the scatternet for three cases: no SIA applied (SIA-OFF), ordinary SIA (SIA-ON) and A-SIA. The values are normalized with respect to the energy spent for SIA-ON when N = 10. The square side is L = 15m.
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Figures 5 and 6 depict the performance comparison between SIA-OFF operation, SIA-ON and A-SIA operation for variable number of piconets N , while keeping the square area unchanged with side L = 15m. The energy benefit from the usage of SIA or A-SIA is more prominent for larger scatternets. Again, the energy gain brought by the SIA algorithm is largely preserved in the ASIA variant. From Fig. 6 it can be seen that, for the chosen values of L and Wmax , the A-SIA algorithm yields largest goodput.
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Fig. 6. Average goodput of the individual piconets as a function of the number of piconets in the scatternet. The square side is L = 15m.
V. C ONCLUSION In this text we have addressed the problem of self-interference in Bluetooth scatternets. We introduce the adaptive self-interference avoidance (A-SIA) algorithm, which is run by each piconet in a distributed manner the piconets and reduces the packet errors and retransmissions. In effect, that decreases the energy spent per useful data portion and results in overall energy efficient operation of the scatternet. The A-SIA algorithm uses the basic mechanism of the previously proposed SIA algorithm [12], but it improves the SIA algorithm in an important way. The SIA algorithm is oblivious with respect to the physical scatternet topology as well as to the variable activity pattern of the piconets. This yields significant throughput degradation when the number of piconets is large, while the spatial density of the piconets is not so high. As opposed ti SIA, the A-SIA algorithm contains probing mechanism by which a piconet infers which other piconets are able to interference. Hence, the A-SIA algorithm adapts to the actual interference pattern and a piconet applies interference avoidance only with respect to those piconets. We have evaluated SIA via extensive simulations. The results show that A-SIA avoids the goodput degradation which SIA introduces in sparse scatternets. A-SIA does not deteriorate the goodput in the piconets since it attempts not to suppress transmissions which are highly probable to be successful. On the other hand, A-SIA maintains high energy-efficiency as compared to the scatternet where no self-interference avoidance is used. As a future work, the A-SIA algorithm will be generalized to allow usage of packets of various lengths, instead of restricting all piconets to constantly use identical packet type. R EFERENCES [1] D. G. Leeper, “A long-term view of short-range wireless,” IEEE Computer, vol. 34, no. 6, pp. 39 –44, June 2001. [2] IEEE 802.15 Working Group for WPAN . [Online]. Available: http://grouper.ieee.org/groups/802/15/ [3] Specification of the Bluetooth system, Std., Rev. 1.2, Nov. 2003. [4] A. El-Hoiydi, “Interference between Bluetooth networks - upper bound on the packet error rate,” IEEE Commun. Lett., vol. 5, no. 6, pp. 245–247, June 2001. [5] A. Zanella, A. M. Tonello, and S. Pupolin, “On the impact of fading and inter-piconet interference on Bluetooth performance,” in Proc. IEEE WPMC ’02, vol. 1, Honolulu, Hawaii, Oct. 2002, pp. 218–222. [6] F. Mazzenga, D. Cassioli, P. Loreti, and F. Vatalaro, “Evaluation of packet loss probability in Bluetooth networks,” in Proc. IEEE Intl. Conf. Commun (ICC’02), vol. 1, New York, Apr. 2002, pp. 313 –317. [7] T.-Y. Lin and Y.-C. Tseng, “Collision analysis for a multi-Bluetooth picocells environment,” IEEE Commun. Lett., vol. 7, no. 10, pp. 475– 477, Oct. 2003. [8] G. Pasolini, “Analytical investigation on the coexistence of Bluetooth piconets,” IEEE Commun. Lett., vol. 8, no. 3, pp. 144–146, Mar. 2004. [9] H. Yomo, P. Popovski, and R. Prasad, “Adaptive radio resource sharing for collocated Wireless Personal Area Networks,” in Proc. IEEE WPMC ’03, Yokosuka, Japan, Oct. 2003. [10] P. Popovski, H. Yomo, and R. Prasad, “Frequency rolling: A cooperative frequency hopping for mutually interfering WPANs,” in ACM Mobihoc, Tokyo, Japan, May 2004, pp. 199–209. [11] M. Sun, S. Wang, C. Chang, T. Lai, H. Sawatari, and H. Okada, “Interference-aware MAC scheduling and SAR policies for Bluetooth scatternets,” in Proc. IEEE GLOBECOM ’02, Taipei, Dec. 2002. [12] P. Popovski, L. Gavrilovska, T. Renier, H. Fathi, and R. Prasad, “Energy-efficient interference avoidance for interconnected Bluetooth Personal Area Networks,” in Proc. IEEE VTC-Spring 2003, vol. 2, Jeju, Korea, Apr. 2003, pp. 1484 –1488. [13] J.C.Haartsen and S.Matisson, “Bluetooth—a new low-power radio interface providing short-range connectivity,” Proc. IEEE, vol. 88, no. 10, pp. 1651 –1661, Oct. 2000. [14] BlueHoc: Open-source Bluetooth simulator. [Online]. Available: http://oss.software.ibm.com/developerworks/opensource/bluehoc/ [15] Network Simulator 2 (ns-2). [Online]. Available: http://www.isi.edu/nsnam/ [16] S. Garg, M. Kalia, and R. Shorey, “MAC scheduling policies for power optimization in Bluetooth: A master driven TDD wireless system,” in Proc. IEEE Veh. Technol. Conf. (VTC 2000), Tokyo, Japan, May 2000.
