Slotted Random Access Protocol with Dynamic Transmission Probability Control in CDMA System

Transcription

1 Slotted Random Access Potocol with Dynamic Tansmission Pobability Contol in CDMA System Intaek Lim 1 1 Depatment of Embedded Softwae, Busan Univesity of Foeign Studies, itlim@bufs.ac.k Abstact In packet adio netwoks, simultaneously tansmitted packets act as multiple access intefeence. In ode to maximize the system pefomance, the numbe of simultaneously tansmitted packets should be kept at a pope level. This pape poposes a slotted andom access potocol with dynamic tansmission pobability contol scheme in the spead spectum system. In the poposed scheme, the base station calculates the packet tansmission pobability in the next slot accoding to the offeed loads and then boadcasts it to all the mobile stations. Mobile stations, which have a packet to tansmit, tansmit packet with the eceived pobability. Simulation esults show that the poposed scheme can offe bette system thoughput and aveage delay pefomance than the conventional one. Keywods Tansmission pobability contol, S-Aloha potocol, CDMA system I. INTRODUCTION Spead spectum code division multiple access (CDMA) technique has been widely used in militay communication systems. CDMA technique povides the following advantages: a multiple access capability, a potential fo high spectum efficiency, extenal intefeence ejection capability, and inheent fequency divesity [1][2]. Slotted ALOHA (S-ALOHA) access potocol has been widely ecognized fo packet adio applications because of its simplicity in managing packet tansmissions. In S-ALOHA potocol, it is assumed that wheneve moe than one packet is tansmitted at the same time slot, the infomation in the tansmitted packets will be lost. Moeove, if a high taffic load is offeed to the system, then the system will become unstable [3]. This would not be the case if S- ALOHA was used with CDMA technique. Application of the conventional S-ALOHA potocol to CDMA technique, namely CDMA S-ALOHA system, offes elatively high system capacity [4][5]. In CDMA S-ALOHA system with a tansmitte-based code method, mobile stations that have a packet tansmit the packet with a given tansmission pobability[6]. Mobile stations that fail to tansmit a packet etansmit it with a given etansmission pobability. If the numbe of simultaneously tansmitting mobile stations, i.e. the level of multiple access intefeences, inceases above a cetain theshold, almost all the packets eceived by the base station can be eoneous. Hence, unsuccessful packet tansmissions ae caused entiely by the numbe of simultaneously tansmitting mobile stations. If the level of multiple access intefeences can be emained close to the level that the system can suppot, it is expected to achieve the best system pefomance. Theefoe, the numbe of simultaneously tansmitting mobile stations needs to be limited with the use of tansmission pobability contol scheme. Most pevious eseaches have been based on a fixed tansmission pobability without the tansmission pobability contol algoithms, and have been done without distinguishing between new packet and etansmission packet. In the case of a high tansmission pobability, packet eos will fequently occu due to the inceased level of multiple access intefeences as becoming the offeed load high. On the othe hand, if the tansmission pobability is low, the system thoughput will decease because of the excessive estiction of packet tansmissions. Thee ae some eseaches to contol the tansmission pobability aiming at impoving the system thoughput [7][8]. In these eseaches, mobile stations that fail to tansmit a packet etansmit it with a deceased tansmission pobability. Continuously deceasing the tansmission pobability of mobile stations that fail All Rights Reseved 124

2 tansmit, the tansmission pobability of a specific mobile station becomes excessively deceased. As a esult, the system thoughput pefomance can be deceased, the tansmission delay of failed mobile station can be inceased, and moeove, fainess between mobile stations cannot be guaanteed. This pape is intended to impove the thoughput and delay pefomance and guaantee the fainess between mobile stations. Fo these puposes, this pape poposes a tansmission pobability contol scheme fo CDMA S-ALOHA system with a tansmitte-based code method. In the poposed scheme, the base station dynamically contols the tansmission pobability of new and unsuccessful packets based on the taffic loads. This pape is oganized as follows. Section II descibes the system model of CDMA S-ALOHA. The poposed tansmission pobability contol scheme is explained in Section III, and simulation esults ae pesented in Section IV. Concluding emaks ae pesented in Section V. II. SYSYEM MODEL In this pape, the bit eo pobability Pe(m) of CDMA system is assumed as follows [6]: 1 m 1 N 2 P e ( m) Q (1) 3N Eb Hee, N is the pocessing gain, m is the numbe of simultaneously tansmitted packets, Eb/N is the atio of enegy-pe-bit to noise powe spectal density, and Q(x) is given by Q( x) 1 2 x 2 u / 2 e du (2) Fom (1), the numbe of simultaneously tansmitted packets as well as the pocessing gain has a stong influence on the bit eo pobability of CDMA system. Accodingly, the system thoughput, which is defined as the numbe of successful packets, can be affected by the bit eo pobability. It is assumed that a packet is successfully eceived at the base station when all the bits of a packet ae eo-fee. When m packets ae tansmitted simultaneously, the pobability Ps(m) that a packet is successfully eceived and the numbe of successful packets S(m) can be expessed as follows, espectively: s 1 Pe ( m L (3) m 1 P L e ( m (4) P ( m) ) S ( m) ) whee L is the length of a packet in bits. Figue 1 shows the achievable thoughput of CDMA system vesus the numbe of simultaneously tansmitted packets, whee the packet length is 432 bits, the pocessing gain is 64, and Eb/N is 15dB. As shown in Figue 1, when the numbe of simultaneously tansmitted packets is ove 12, the thoughput deceases as a esult of packet eos due to the excessive multiple access intefeences. Theefoe, in ode to achieve the maximum thoughput in CDMA system, the numbe of simultaneously tansmitted packets should be contolled at a pope All Rights Reseved 125

3 . Intenational Jounal of Recent Tends in Engineeing & Reseach (IJRTER) 1 8 S(m) N=64, E b /N =15dB L=432 bits m Figue 1. The achievable thoughput of CDMA system vesus numbe of simultaneously tansmitted packets. III. TRANSMISSION PROBABILITY CONTROL SCHEME The system model of tansmission pobability contol scheme, which is named as the popotional backoff (PB) scheme, is pesented in Figue 2. In the poposed scheme, the base station contols the tansmission pobability of mobile stations in the centalized manne. The mobile stations in the contention state and etansmission state attempt to tansmit packets with the tansmission pobability Pn and etansmission pobability P, espectively. The base station calculates these pobabilities based on the estimated taffic loads and boadcasts ove an eo-fee downlink contol channel. MS 1. P n o P MS 2. CDMA S-ALOHA channel BS MS K. MS: Mobile Station BS: Base Station Figue 2. System model of tansmission pobability contol scheme. Evey mobile station geneates a packet in each slot with aival ate. When the mobile station geneates a packet, it is stoed at the buffe. Stoed packets ae seved on a fist-in-fist-out discipline. Figue 3 shows the opeation mode of each mobile station. As depicted in Figue 3, all of the mobile stations may be in one of thee diffeent opeation states: idle state, contention (CON) state, and etansmission (RETX) All Rights Reseved 126

4 IDLE Aival of a packet CON Packet tansmission Success? no RETX yes Buffe empty? yes no IDLE Figue 3. Opeation mode of mobile station. The mobile station, which does not have any packet in the buffe, is said to be in the idle state. When the mobile station in the idle state geneates a packet, it entes into the contention state and tansmits a packet at the next slot with a given tansmission pobability Pn. The mobile stations ae infomed as to whethe o not the tansmitted packets ae successfully eceived by the base station in the fom of acknowledgement though an eo-fee downlink channel. The mobile station that fails to tansmit a packet o does not pemitted to tansmit entes into the etansmission state, and etansmits the packet at the next slot with a given etansmission pobability P. Afte successful tansmission, the packet is emoved fom the buffe and the mobile station seves the next packet if exists. The etansmission pocess is epeated until the packet is successfully eceived. The mobile station that fails to tansmit its packet at slot t will etansmit with P(t+1) at slot (t+1), while the mobile station that entes into the contention state at slot t will tansmit with Pn(t+1) at subsequent slot. The Pn(t+1) and P(t+1) ae calculated as follows: 1, if N ( t 1) THm P n( t 1), othewis e (5) 1, if N ( t 1) THm P ( t 1) THm, othewis e N ( t 1) (6) whee TH m is the numbe of simultaneously tansmitted packets at which the system thoughput can be maximized, and N(t+1) is the total numbe of mobile stations in the etansmission state at slot (t+1). Let Nf(t) be the numbe of failed mobile stations at slot t, and Nb(t) the numbe of mobile stations not being pemitted to tansmit at slot t. Then N(t+1) can be deived as follows: N ( t 1) N ( t) N ( t) (7) f b The theshold value THm can be detemined by (4), and Nb(t) is given All Rights Reseved 127

5 N b ( t) N ( t 1){1 P ( t)} N ( t){1 P ( t)} (8) n n Nn(t) in (8) is the numbe of mobile stations that ente into the contention state at slot t, and can be deived by Nn ( t) { K N ( t) N ( t 1)} (9) n In (9), K is the total numbe of mobile stations in the system, and is the pobability that each mobile station geneates a packet in each slot. The base station cannot exactly know how many packets ae geneated in one slot. Theefoe, is computed using a moving time aveage of the numbe of new packets that ae successfully eceived. In the poposed scheme, if the numbe of mobile stations in the etansmission state is less than THm, all of the mobile stations in both the contention state and the etansmission state should be allowed to tansmit a packet. If the numbe of mobile stations in the etansmission state becomes moe than THm, the base station sets Pn into to suppess the tansmission of new packets. Also, in this case, the base station sets P as the values at which the total numbe of simultaneously etansmitted packets becomes THm, in ode to minimize the tansmission delay. IV. SIMULATION RESULTS This section pesents simulation esults fo the poposed tansmission pobability contol scheme. In this pape, it is assumed that the packet length is 432 bits equal to slot duation, the pocessing gain is 64, Eb/N is 15 db, the total numbe of mobile stations is 1, and each mobile station geneates packets accoding to the Poisson pocess. With these assumptions, it can be seen that THm is equal to 12 fom Figue 1. Also, it is assumed that the total length of window used fo computing the moving time aveage of packet aival ate is set to 1, slots. The pefomance measues of inteest ae the system thoughput, aveage delay. The system thoughput is defined as the numbe of successfully tansmitted packets duing one slot time. The aveage delay is defined as the aveage time between the aival of packet and its eception at the base station. In this pape, we analyze the pefomance of the poposed PB scheme in compaison to the conventional hamonic backoff (HB) scheme [7]. In the HB scheme, the mobile station that fails in the tansmission of packet deceases its tansmission pobability independently with the taffic load. Fo the fist attempt of packet tansmission, the tansmission pobability P1 is set to 1. If the tansmission becomes unsuccessful, then Pi+1 fo (i+1)th attempt is deceased accoding to Pi Pi 1, i 1 P 1 (1) i P1 1 The numbe of simultaneously tansmitted packets, thoughput, and aveage delay vesus the offeed load ae shown in Figue 4, 5, and 6, espectively. As shown in Eq. (1), in the HB scheme, the mobile station that fails to tansmit a packet deceases its tansmission pobability independently with the numbe of mobile station in the etansmission state. As the taffic load inceases, so the bit eo ate of each packet inceases because of the unconditional tansmission of new packets. Because the mobile station in the etansmission state continuously deceases the tansmission pobability due to the packet eo, the tansmission pobability of a specific mobile station becomes excessively deceased. As a esult, as shown in Figue 4, the numbe of simultaneously tansmitted packets in the HB scheme is fa less than TH m. On the othe hand, in the poposed PB scheme, the base station contols the tansmission pobability of mobile stations based on the taffic load and the theshold THm. Theefoe, the PB scheme maintains the numbe of simultaneously All Rights Reseved 128

6 packets the theshold THm in spite of the inceased offeed load. Fom Figue 5 and 6, the PB scheme gives bette pefomance than the HB scheme. The easons ae as follows: i) the PB scheme can contol the numbe of simultaneously tansmitted packets moe pecisely that the HB scheme; ii) in the PB scheme, the base station does not pemit the tansmission of new packets in the heavy taffic load. no. of tansmitted packets PB HB L=432 bits, K=1 N=64, E b /N =15dB Offeed load (packets/slot) Figue 4. Numbe of tansmitted packets vesus offeed load. Thoughput (packets/slot) PB HB L=432 bits, K=1 N=64, E b /N =15dB Offeed load (packets/slot) Figue 5. Thoughput vesus offeed load. Aveage Delay (slots) PB HB L=432 bits, K=1 N=64, E b /N =15dB Offeed load (packets/slot) Figue 6. Aveage delay vesus offeed load. V. CONCLUSIONS This pape has poposed a slotted andom access potocol with a dynamic tansmission pobability contol scheme. The design objective of the poposed scheme is to impove the system thoughput and delay pefomance. In the poposed scheme, the base station dynamically detemines the All Rights Reseved 129

7 tansmission pobability of mobile stations accoding to the offeed load and then boadcasts it to all the mobile stations. Simulation esults have showed that the poposed scheme maintains the numbe of simultaneously tansmitted packets at the optimum theshold, which can achieve the maximum thoughput of spead spectum system. The thoughput and delay pefomance of the poposed scheme have outpefomed by fa those obtained with the conventional scheme. REFERENCES 1. A. F. Al-Junaid, and F. S. Al-kamali, Efficient wieless tansmission scheme based on the ecent DST-MC-CDMA, Wieless Netwoks. Vol.22, no.3, pp , Ma Shu-Ming Tseng, Li-Hsin Chiang, and Yung-Chung Wang, Thoughput of Coded DS CDMA/Unslotted ALOHA Netwoks with Vaiable Length Data Taffic and Two Use Classes in Rayleigh Fading FSMC Model, KSII Tans. on Intenet and Infomation Systems, vol.8, no.12, pp.4324~4342, Dec Zhu-wei Wang, Da-Cheng Yang, and L. B. Milstein, Multi-use-Resouse Allocation fo a Distibuted Multi-caie DS-CDMA Netwok, IEEE Tans. On Wieless Communications, vol.6, no.1. pp , Jan Shu-Ming Tseng, Hung-Pin Lin, Chih-Hao Chen, and Yung-Chung Wang, Thoughput Analysis of DS CDMA/Unslotted ALOHA Wieless Netwoks with Fixed Packet Length in Rayleigh Fading Finite-State Makov Channel Model, Wieless Pesonal Communications, vol.71, Issue 4, pp.391~314, August Hoga Saagih, and Fendy Santoso, Thoughput Analysis of Adaptive Slotted-ALOHA CDMA ove a Multipath Fading Channel with Captue Effects, Intenational Jounal of Wieless Infomation Netwoks, vol.16, no.4, pp , Dec A. W. Lam, and F. M. Ozlutuk, Pefomance bound fo DS/SSMA communications with complex signatue sequences, IEEE Tans. Commun., vol.4, no.1, pp , Oct Ch. V. Veikoukis, and J. J. Olimos, Up-link pefomance of the DQRUMA MAC potocol in a ealistic indoo envionment fo W-ATM netwoks, in Poc. VTC2, pp , Sept J. F. Figon, and V. C. M. Leung, A pseudo-bayesian ALOHA algoithm with mixed pioities, ACM Wieless Netwoks, vol.7, Issue 1, pp.55-63, Jan. All Rights Reseved 13