IEEE COMMUNICATIONS LETTERS, VOL. 17, NO. 11, NOVEMBER 2013. Beacon-Based Slotted ALOHA for. Wireless Networks with Large Propagation Delay.
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IEEE COMMUNICATIONS LETTERS, VOL. 17, NO. 11, NOVEMBER 2013
Beacon-Based Slotted ALOHA for Wireless Networks with Large Propagation Delay Hoki Baek, Student Member, IEEE, Jaesung Lim, and Sangyoon Oh, Member, IEEE
Abstract—In a wireless network with large propagation delay S-ALOHA(Slotted ALOHA) requires large guard time which causes lower normalized throughput. To reduce the large guard time in the ISA-ALOHA [4], a time alignment mechanism was proposed under the assumption of propagation delay estimation. In this letter, we propose a framed structure which is able to estimate propagation delay by employing coordinator beaconing. The framed structure consists of a time period for beaconing and a group of multiple time slots for random access. The proposed Beacon-based S-ALOHA(BS-ALOHA) can make packets generated during the time of beaconing evenly distributed over the random access period. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time and show that BS-ALOHA provides higher normalized throughput than both ALOHA and S-ALOHA employing the large guard time. Index Terms—Propagation delay, framed structure, slotted ALOHA.
I. I NTRODUCTION
U
NLIKE terrestrial networks, the propagation delay in airborne and underwater networks is not negligible and is dynamically changed owing to mobility of nodes. In common, these networks have a coordinator to which all nodes send packets and a well-known characteristic, space-time uncertainty, which results from large propagation delay [1]. In terrestrial networks, S-ALOHA uses a time slot whose length is the almost same as transmission time of a packet. It then provides higher normalized throughput than ALOHA. However, if the propagation delay is a significant factor, the performance can be different. In [2], simulation results show that S-ALOHA yields the equal normalized throughput to that of ALOHA when the propagation delay becomes large and the space-time uncertainty problem cannot be resolved. According to [3], when the guard time length is longer than transmission time of a packet in a slot, S-ALOHA even provides lower normalized throughput than ALOHA. ISA-ALOHA(Improved Synchronized Arrival slotted ALOHA) [4] was proposed to reduce the large guard time by using the time alignment mechanism which helps each node adjust the start time of transmission, i.e., a packet arrives Manuscript received August 8, 2013. The associate editor coordinating the review of this letter and approving it for publication was P. Chatzimisios. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A1A01016423). This research was supported by the MSIP (Ministry of Science, ICT & Future Planning), Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency (NIPA-2013-H0301-13-2003)). The authors are with the Graduate School of Information and Communications, Ajou University, Suwon, Republic of Korea (e-mail: {neloyou, jaslim, syoh}@ajou.ac.kr). Digital Object Identifier 10.1109/LCOMM.2013.101413.131802
at the start time of a slot at the coordinator. For the time alignment, Propagation Delay to Coordinator(PDC) should be estimated. ISA-ALOHA assumed that each node can estimate the propagation delay by exchanging packets with coordinator. On the other hand, in a single-channel wireless network with large propagation delay, any specific mechanism for exchanging packets is required to estimate the propagation delay. In this letter, we adapt a framed structure which is able to estimate propagation delay by employing coordinator beaconing. All nodes are assumed to be synchronized by Global Positioning System(GPS). The PDC should be obtained for time alignment even in the perfect time synchronized network. The framed structure has some advantages. Firstly, the framed structure is adequate to support coordinator beaconing by allocating a coordinator time slot which includes guard time enough to resolve space-time uncertainty. The beaconing can reduce the number of packet exchanges required for PDC estimation. Beacon message may include time information used for PDC estimation and acknowledgment for successful reception of packets. Secondly, the framed structure can provide a periodic PDC estimation which can help dynamically determine the error bound of PDC estimation, especially in high mobility environments. In the proposed scheme, the framed structure consists of a time period for beaconing and a random access period which is a group of multiple time slots. Each node estimates PDC during the beacon time period and executes the time alignment mechanism proposed in [4] during the random access period. The proposed Beacon-based S-ALOHA(BSALOHA) can make packets generated during the time of beaconing evenly distributed over the random access period. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time and show that BS-ALOHA provides higher normalized throughput than both ALOHA and S-ALOHA employing the large guard time. II. B EACON -BASED S LOTTED ALOHA We consider a single-channel single-hop wireless network with large propagation delay. Our network consists of a coordinator and multiple nodes. We assume that all nodes are synchronized perfectly by GPS and that they transmit packets of equal size to the coordinator. If all nodes are synchronized, BS-ALOHA can be applied to any networks and provides more effective performance in the wireless networks with large propagation delay, e.g., the airborne and underwater networks. We define a number of variables for the framed structure of BS-ALOHA as shown in Fig. 1. Let tstart , Tf rame , TB , TMAX RT T , and ts,j be the start time of a frame,
c 2013 IEEE 1089-7798/13$31.00
BAEK et al.: BEACON-BASED SLOTTED ALOHA FOR WIRELESS NETWORKS WITH LARGE PROPAGATION DELAY
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is given by k=
Fig. 1.
PDC estimation and time alignment in a frame of BS-ALOHA.
frame duration, beacon transmission time, maximum roundtrip time of the maximum communication range R, and start time of the j-th slot, respectively. TMAX RT T is given by TMAX RT T = 2R/c, where c is the signal propagation speed. The framed structure consists of a coordinator time slot and a group of multiple time slots. The coordinator time slot whose guard time length is TMAX RT T is for beaconing. After the coordinator time slot of length TB + TMAX RT T , M slots of length TS are available for random access. Including an initial delay of length TMAX RT T enables the farthest node from the coordinator to access the first slot. We define ti to be the time at which a packet at the i-th node is generated, and ttx,i to be the time at which the i-th node starts to transmit the packet. We also define tv,i as the virtual packet arrival time, which is the time a packet generated by the i-th node would arrive if the packet was transmitted as soon as it was generated. Thus, tv,i is obtained by tv,i = ti + τi , where τi is the PDC of the i-th node. All nodes can estimate PDC in every frame by using coordinator beaconing. As shown in Fig. 1, the coordinator broadcasts a beacon at tstart , the start of a frame, and the ith node receives it at tB,i , the beacon arrival time. The PDC of the i-th node then is obtained by τi = tB,i − tstart . By using GPS and framed structure, periodic PDC estimation is possible. Next, we explain random access period in which each node executes the time alignment mechanism proposed in [4] to reduce guard time. In addition, we present distribution method of packets generated during the time of beaconing. BS-ALOHA evenly distributes these packets over the random access period. When the i-th node generates a packet, it determines the start time of transmission, ttx,i , based on tv,i and τi . Firstly, it checks whether tv,i happens to fall within the M slots available for transmission. For simplicity, let γ be tstart + TB + TMAX RT T . If tv,i falls within the first M − 1 slots, i.e., γ ≤ tv,i ≤ γ + (M − 1) × TS , the node selects the next slot to begin after tv,i . If tv,i falls within slot M , i.e., γ + (M − 1) × TS ≤ tv,i ≤ γ + M × TS , the packet will be transmitted in the first slot of the next frame, as there are no more slots available in the current frame. Therefore, if γ ≤ tv,i ≤ γ + M × TS , then the index k of the selected slot
(t
− γ) mod M + 1. TS
v,i
(1)
On the other hand, if tv,i < γ, i.e., the packet is generated during the time of beaconing, the node postpones transmission and selects a slot based on tv,i . For this, we uniformly divide γ into M intervals (I1 , · · · , IM ) and define TI as a length of interval, which is obtained by TI = (TB + TMAX RT T )/M . If tv,i is within Ik , the packet will be transmitted in slot k which is obtained by (t − t v,i start ) . (2) k= TI In this way, BS-ALOHA can evenly distribute packets generated during the time of beaconing over the random access period. Consequently, BS-ALOHA can equalize collision probability of every slot and provide a fair packet delay by letting k be proportional to tv,i . After selecting the slot, the node performs time alignment to reduce guard time while resolving space-time uncertainty by adjusting the start time of transmission so that the packet will arrive at the beginning of a slot. If the slot k is selected, ttx,i is obtained by ttx,i = ts,k − τi . For example, in Fig. 1, tv,2 and tv,3 are within slot 2 and M , respectively. Thus, node 2 selects slot 3. However, in case of node 3, there are no more slots after slot M in the frame. Thus, node 3 selects the slot 1 of the next frame. Unlike node 2 and 3, node 1 selects the slot j because tv,1 is within Ij . After selecting the slots for packet transmission, each node adjusts the start time of transmission so that the packet will arrive at its beginning. Thus, ttx,1 , ttx,2 , and ttx,3 can be obtained by ts,j − τ1 , ts,3 − τ2 , and ts,1 − τ3 , respectively. If all nodes are synchronized, BS-ALOHA can be applied to both single-hop airborne networks and single-hop underwater networks by determining parameters: R, c, vMAX , and TP . In the airborne networks [5] [6], R is 300 nautical miles(nmi) and c is 3 × 108 m/s because Radio Frequency(RF) signal is used. On the other hand, in the underwater networks [4], R is 1500 m and c is 1500 m/s because acoustic signal is used. We can set the values of vMAX and TP according to the given system parameters. Moreover, BS-ALOHA can support acknowledgment by using a beacon message because it may include acknowledgment for successful reception of packets as well as time information used for PDC estimation. III. P ERFORMANCE A NALYSIS We assume that infinite nodes generate packets according to a Poisson process at a rate of λ packets/s and that TB is negligible. We also assume that all the packet losses are caused by collisions. We do not consider any packet retransmission. We consider an additional guard time, TG , in a slot to compensate for the drift of the estimated PDC due to the high mobility. Thus, TS is TP + TG , where TP is transmission time of a packet. Even if the GPS provides perfect time synchronization, the estimated PDC at the start of a frame can be changed as all nodes move rapidly during a frame. The amount of the drift can be greatest when the two nodes
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IEEE COMMUNICATIONS LETTERS, VOL. 17, NO. 11, NOVEMBER 2013
move in opposite directions with maximum velocity during Tf rame . Thus, we set TG as Tf rame (3) TG = 2vMAX × c where vMAX is the maximum velocity of a node. We replace Tf rame in (3) with TMAX RT T + M (TP + TG ) as Tf rame contains TG recursively. After that, we can obtain TG by 2vMAX (TMAX RT T + M TP ) . (4) TG = c − 2M vMAX We define normalized throughput, X, as the ratio of frame time used for successful packet transmission to frame duration. The normalized throughput of BS-ALOHA can be expressed as E[S] × TP (5) X= Tf rame where E[S] is the average number of successful slots in a frame and is given by E[S] =
M
sP (S = s)
Fig. 2.
Normalized throughput according to G (TP = 3.0 ms).
Fig. 3.
Normalized throughput according to G (TP = 0.1 ms).
(6)
s=0
where S is a random variable describing the number of successful slots and P (S = s) is the probability that the number of successful slots is s. We can obtain P (S = s) by P (S = s) =
M
P (S = s|B = b) × P (B = b)
(7)
b=s
where B is a random variable describing the number of busy slots, P (S = s|B = b) is the conditional probability that the number of successful slots is s given that the number of busy slots is b, and P (B = b) is the probability that the number of busy slots is b. Since not all the busy slots are successful owing to collisions, we consider those b whose values are equal to or larger than s. In (7), P (S = s|B = b) is expressed as
b s (8) P (S = s|B = b) = ρ (1 − ρ)b−s s where ρ is the conditional probability that the number of arrival packets in a slot is one given that the slot is busy as in [7]. Thus, ρ is expressed as λTP GT e−λTP GT (9) pB where TP GT is the amount of time, which is used for generating packets arriving at a slot, and pB is the probability that a slot is busy. In S-ALOHA, the packets arriving at a certain slot were generated during the length of its previous slot, TS . Thus, TP GT of S-ALOHA is expressed as TP GT = TS . On the other hand, in BS-ALOHA, the packets arriving at a certain slot were generated during its corresponding TI as well as during the length of its previous slot, TS . Thus, TP GT of BSALOHA is given by TP GT = TS + TI . The probability pB is obtained by (10) pB = 1 − e−λTP GT . ρ=
In (7), P (B = b) is expressed as
M b P (B = b) = pB (1 − pB )M−b . b
(11)
IV. N UMERICAL R ESULTS In this section, we compare BS-ALOHA with ALOHA, SALOHA employing large guard time, and ISA-ALOHA which uses the assumptions that all nodes know their PDCs. ISAALOHA can provide higher normalized throughput than BSALOHA because it does not require overhead duration of TB + TMAX RT T for coordinator beaconing time. Thus, we use ISA-ALOHA as an ideal scheme to show the effect of the overhead. In order to show the effect of guard time, we use S-ALOHA employing a guard time whose length is the same as the maximum propagation delay given by TMAX RT T /2, i.e., R/c. The normalized throughput of ALOHA is Ge−2G and that of SALOHA is Ge−(1+α)G [3], where G is the offered load given by λTP , and α is the ratio of the maximum propagation delay to TP . When α is larger than 1, S-ALOHA actually provides a lower normalized throughput than ALOHA. In ISA-ALOHA, normalized throughput is Ge−(1+β)G , where β is the ratio of the guard time to TP . We assume that the guard time length of ISA-ALOHA is the same as TG of BS-ALOHA when M is 100.
BAEK et al.: BEACON-BASED SLOTTED ALOHA FOR WIRELESS NETWORKS WITH LARGE PROPAGATION DELAY
We determine several parameters based on Link-16 [5] and Tactical Targeting Network Technology (TTNT) [6]. These are representative military airborne communication systems. All nodes can communicate directly because their networks are single-hop. In these systems, the maximum communication range is 300 nmi, where 1 nmi is 1.852 km and RF signal is used. Thus, the corresponding maximum propagation delay is 1.852 ms. The F-16 Fighting Falcon equipped with Link-16 has the maximum speed of 680 m/s (Mach 2.0). Therefore, we set R, c, and vMAX as 300 nmi, 3 × 108 m/s, and 680 m/s, respectively. We also consider two types of TP s: one whose transmission time is longer than the maximum propagation delay (3.0 ms) and another whose transmission is shorter (0.1 ms). Fig. 2 shows X according to G when TP is 3.0 ms, longer than the maximum propagation delay. BS-ALOHA provides higher X than S-ALOHA and ALOHA regardless of M as the guard time is reduced by time alignment. Moreover, BSALOHA and the ideal ISA-ALOHA provides almost same X even if BS-ALOHA only considers the overhead due to the coordinator beaconing time. We can also see that M does not significantly affect X of BS-ALOHA. Fig. 3 shows X according to G when TP is 0.1 ms, smaller than the maximum propagation delay. BS-ALOHA provides higher X than S-ALOHA and ALOHA regardless of M . SALOHA provides a much lower X than ALOHA, as α is much larger than 1. When M is 1000, BS-ALOHA provides the highest X except ISA-ALOHA. This means that X of BSALOHA is affected by Tf rame . The longer Tf rame provides the higher X when TP is small. The difference of Xs between BS-ALOHA and ISA-ALOHA is not zero. However, it is still very small when M is large. This means that BS-ALOHA with large M and ISA-ALOHA provide almost same X even if BS-ALOHA only considers overhead due to the coordinator beaconing time.
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V. C ONCLUSIONS In this letter, we adapt a framed structure which can estimate PDC periodically by employing coordinator beaconing and reduce guard time by executing the existing time alignment mechanism based on the estimated PDC, not assumption. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time. Numerical results show that BS-ALOHA always provides higher normalized throughput than both S-ALOHA and ALOHA. Moreover, BSALOHA with large M and ISA-ALOHA provide almost same normalized throughput even if BS-ALOHA only considers the overhead due to the coordinator beaconing time. R EFERENCES [1] A. Syed, W. Ye, B. Krishnamachari, and J. Heidemann, “Understanding spatio-temporal uncertainty in medium access with ALOHA protocols,” in Proc. 2007 ACM WUWNet, pp. 41–48. [2] L. F. M. Vieira, J. Kong, U. Lee, and M. Gerla, “Analysis of Aloha protocols for underwater acoustic sensor networks,” 2006 ACM WUWNet. [3] D. Makrakis and K. M. S. Murthy, “Spread slotted ALOHA techniques for mobile and personal satellite communication systems,” IEEE J. Sel. Areas Commun., vol. 10, no. 6, pp. 985–1002, Aug 1992. [4] Y. Zhou, K. Chen, J. He, and H. Guan, “Enhanced Slotted Aloha protocols for underwater sensor networks with large propagation delay,” in Proc. 2011 IEEE Vehicular Technology Conference – Spring, pp. 1–5. [5] Northrop Grumman Corporation Information Technology Communication & Information Systems Division, Understanding Link-16: A Guidebook for New Users, NCTSI, San Diego, CA, pp. 5.47–5.49, Sep. 2001. [6] P. T. Highnam, “Tactical targeting network technologies (TTNT),” 2002 DARPATech Symposium. Available: http://archive.darpa.mil/DARPATech2002/presentations/ixo pdf/slides/Hi ghnamIXO v4.pdf. [7] R. Rom and M. Sidi, Multiple Access Protocols: Performance and Analysis. Springer-Verlag Inc., 1990, pp. 49–53.
