Adaptive Reverse Link Rate Control Scheme for cdma2000 1xEV-DO Systems

Transcription

1 Adaptive Reverse Link Rate Control Scheme for cdma2000 1xEV-DO Systems HyeJeong Lee, Woon-Young Yeo and Dong-Ho Cho Korea Advanced Institute of Science and Technology Abstract The cdma2000 1xEV-DO standard which is designed to support an increasing demand for high-speed wireless data service has a distributed rate control scheme on e reverse link. In is paper, we propose an adaptive rate control scheme based on e control of RateLimit, which is e maximum possible data rate in e reverse link, and analyze e proposed scheme rough a discrete Markov process modeling. It is shown at our proposed scheme outperforms e 1xEV-DO rate control scheme in view of e overload probability and e effective reverse link roughput. Moreover, we show at e reverse link capacity can be increased by using e proposed scheme. R1 R2 R3 R4 R5 9.6 kbps 19.2 kbps 38.4 kbps 76.8 kbps kbps Fig. 1. p 1 RAB=0 p 2 RAB=0 p 3 RAB=0 p 4 RAB=0 q 2 RAB=1 q 3 RAB=1 q 4 RAB=1 q 5 RAB=1 Rate control scheme for 1xEV-DO (Self-loops are not shown) I. INTRODUCTION The cdma2000 1xEV-DO (EVolution-Data Only) standard [1], also known as IS-856, is designed to accommodate packet data services wi asymmetric data rates and provides bandwid-efficient solution to support high-speed wireless data services. In 1xEV-DO, a Forward Traffic Channel is shared by all mobiles in a cell or a sector, and supports data rate from 38.4 kbps to 2.4 Mbps. On e oer hand, each mobile has a Reverse Traffic Channel on which data rates from 9.6 kbps to kbps can be supported. In e forward link, a base station can control e traffic flow to all mobiles in a cell based on a packet transmission scheduling such as e proportional fair scheduling. In contrast, since e explicit scheduling in e reverse link increases signaling traffic and it is hard to predict e data amount of mobiles, e 1xEV-DO adopts a simple and distributed rate control mechanism. The reverse link capacity of 1xEV-DO has been discussed in [2][3][4] and e analytical model of e reverse link rate control for 1xEV-DO has been provided in [5]. In addition, much research on e enhanced rate control algorim for 1xEV-DO reverse link has been performed in [6]-[9]. In is paper, we propose an adaptive reverse link rate control meod at adjusts e value of RateLimit according to e traffic load and guarantees a stable operation even in e heavy traffic load. This paper is organized as follows. In Section II, we provide a brief description of e rate control scheme adopted in e 1xEV-DO standard. We explain e proposed adaptive rate control scheme for e reverse link traffic channel of e 1xEV-DO in Section III and analyze e proposed scheme by using Markov chain modeling in Section IV. In Section V, we provide numerical results to show e performance comparison between e 1xEV-DO rate control and e proposed rate control schemes. In Section VI, we present our conclusions. II. REVERSE LINK RATE CONTROL OF THE 1XEV-DO In e reverse traffic channel, ere are five available levels of data rates, such as R i =9.6 2 i 1 kbps (i =1,, 5) as shown in Fig. 1. Each mobile has an initial data rate of R 1 and cannot have e data rate above e RateLimit, which is given by a base station rough e Control Channel. A base station broadcasts e Reverse Activity Bit (RAB) over e Reverse Activity (RA) channel, in order to indicate wheer or not e reverse traffic load is above a certain reshold. A typical example of e traffic load is e noise rise over ermal noise (η), which represents e level of interference at e base station. If e traffic load at e base station exceeds e reshold (η ), e RAB is set to 1, and oerwise it is set to 0. Before e beginning of a new frame, each mobile determines its data rate according to a distributed rate control, which is based on e latest RAB value and persistence probabilities (p i and q i ). If a mobile wi a data rate of R i receives an RAB of 0 (or 1), it increases (or reduces) its data rate to R i+1 (or R i 1 ) wi probability p i (or q i ), and stays at e current rate wi probability 1 p i (or 1 q i ). Fig. 2 shows e 1xEV-DO reverse channel structure. In 1xEV-DO, reverse link power control is applied only to e Pilot Channel. During e transmission of e Reverse Traffic Channel, e output powers of e Data Channel, e Data Rate Control (DRC) Channel, and e Acknowledgement (ACK) Channel are adjusted by a fixed gain relative to at of e Pilot Channel. The Data Channel gain relative to e Pilot Channel power has a different value, depending on e data rate. The nominal values of Data Channel gain are represented in Table I based on [1]. Note at e Data Channel gain is /05/$20.00 (c)2005 IEEE

2 Access Reverse Pilot Data Pilot Fig. 2. Reverse Rate Indicator Medium Access Contol Traffic Data Rate Control Ack Channel structure of e 1xEV-DO Reverse link TABLE I NOMINAL DATA CHANNEL GAINS Symbol Data rate (kbps) Data channel gain (db) R R R R R Data not entirely proportional to e data rate. While every doubling e data rate between R 1 and R 3 increases e corresponding Data Channel gain by 3dB, e Data Channel gains for R 4 and R 5 are 3.5 db and 5.25 db higher an ose of e R 3 and R 4, respectively. Because e Data Channels wi high data rates may significantly increase interference level for e heavy traffic load, it is desirable to restrict e available maximum data rate by controlling e RateLimit according to e traffic load. Even ough performance evaluation and improvement algorims for e 1xEV-DO reverse link have been provided in [6]-[9], ere is no research on e effect of e RateLimit and e reliable means to control e value of e RateLimit. Thus, e 1xEV-DO typically sets e RateLimit to be kbps, regardless of e traffic load. III. PROPOSED SCHEME We now consider e problem to find e adequate value of RateLimit, given e traffic load. The traffic load is defined as e number of mobiles which wish to transmit data to e base station. In 1xEV-DO reverse link rate control, two performance metrics are defined: 1) Reverse link roughput, which indicates e sum of e data rates of all mobiles on e reverse link, and 2) Overload probability, which is e probability at e reverse link traffic load is above e maximum allowable value. We assume at e overload situation occurs at η>η max. In e proposed scheme, we assign e value of RateLimit at minimizes e traffic overload wiout e loss of e reverse link roughput. Let Γ be e number of mobiles in a cell and r j be e data rate of mobile j, where r j {R i i = 1,, 5} and j =1,, Γ. We first find e maximum roughput for e given Γ and RateLimit. Since ere is a trade-off between e reverse link roughput and e overload probability [5], e roughput maximization problem is constrained by e requirement at e noise rise is restricted to be below a reshold (η ). Thus, e maximum reverse link roughput obtainable at e RateLimit=R i, T max (R i ), is given as follows. T max (R i )=max Γ r j (1) j=1 subject to R 1 r j R i η η In (1), noise rise η is given by Γ c(r j ) η= 1 (c(r j )+1/τ(r j )) j=1 1, (2) where c(r j ) is e ratio of e received power of mobile j at e base station to e received pilot channel power of mobile j, and τ(r j ) is e target (E c /N 0 ) of mobile j. More detailed derivation of η can be found in [5]. For e different values of RateLimit in (1), T max (R 5 ) provides e upper bound for e maximum roughput. However, according as Γ increases, e maximum roughput T max (R i ) for all i converges to e same upper bound because of e noise rise constraint. If e values of T max (R i ) for different RateLimit values are e same, it would be desirable at RateLimit is set to e lowest one in order to minimize e overload probability. Therefore, for a given Γ and η,weset e RateLimit as follows. RateLimit = min R i (3) such at {T max (R i 1 ) <T max (R i )=T max (R 5 )}, where i =2,, 5. When e number of mobiles in a cell or e noise rise constraint is changed, e base station newly determines a value of RateLimit and informs mobiles of e RateLimit by BroadcastReverseRateLimit message [1]. Because e base station knows e number of mobiles in its own cell and e noise rise constraint is given as e system parameter, it is very simple for e base station to determine e optimum value of RateLimit by adopting e proposed scheme. IV. PERFORMANCE ANALYSIS To analyze e proposed scheme, we herein consider a single cell environment. We assume at e reverse link frame boundaries of e mobiles are synchronized, so at all mobiles change eir data rates simultaneously at e beginning of a new frame. The RAB update period is short enough for mobiles to use e latest RAB value. All mobiles always have data to transmit and ey have an RAB decoding error probability z wi which each mobile decode its received RAB as an opposite value. The value of RateLimit is determined by e proposed scheme.

3 Let Λ denote e number of available data rates on e reverse traffic channel. (e.g., if RateLimit=R 3, Λ is 3.) Let S i (t) be e number of mobiles wi a data rate of R i at e t frame. Since S Λ (t) can be deduced from Γ= Λ S i(t), we can define a state for a data rate distribution as a vector S(t) = (S 1 (t),,s Λ 1 (t)) [5]. Note at e number of elements of S(t) is limited by e RateLimit. Because e RAB depends only on e current state, and not on e previous states, is analytic model is considered as a first-order discrete Markov process. Let U i and D i be e number of mobiles at increase and reduce eir data rates from e current data rate of R i, respectively, at e beginning of e (t +1) frame. Denoting e samples of S(t) and S(t +1)by x = (x 1,,x Λ 1 ) and y = (y 1,,y Λ 1 ), respectively, e state transition probability p xy Pr(S(t) = y S(t 1) = x) can be equivalently expressed as p xy = Pr(D U = e S(t 1) = x), (4) where D = (D 2,,D Λ ), U = (U 1,,U Λ 1 ), e = (e 1,,e Λ 1 ), and e i i k=1 (y k x k ) [5]. Let N i denote e number of mobiles at decode e RAB as 0 at e data rate of R i. Then, by conditioning on N i and U i, p xy can be written as p xy = Pr(D=u+e u,n,x) Pr(U=u n,x) Pr(N=n x), (5) n u where N = (N 1,,N Λ ), {n 0 n i x i, i = 1,,Λ} and {u max(0, e i ) u i min(n i, x( i+1 ) n i+1 e i ),i = 1,, Λ 1}. Defining f γ (α, β) = αβ γ β (1 γ) α β, each probability in (5) is expressed as follows. Λ 1 Pr(D = u+e u,n,x) = f qi+1(x i+1 n i+1,u i +e i ) (6) Λ 1 Pr(U=u n,x) = f pi (n i,u i ) (7) Λ f 1 z (x i,n i ), x {x u } Pr(N=n x) = Λ f z (x i,n i ), x {x o } In (8), {x u } and {x o } are set of x at satisfies η(x) <η and η(x) η, respectively. Because e Markov model is finite, irreducible and aperiodic, we can get a unique steady-state probability π x for a state x. Accordingly, we can obtain {π x } by solving a set of linear equations π y = x p xyπ x and x π x =1. If we define {x out } as a set of x at causes e overload situation (i.e., η> η max ), e overload probability is given by x {xout} π x.in addition, e effective reverse link roughput, which is defined as e average traffic volume transmitted wiout occurring overload situation, is obtained as x/ {xout} ( Λ x ir i )π x. (8) Fig. 3. Data rate distribution of e 1xEV-DO rate control scheme (η =4.8 db, z =5%) Fig. 4. Data rate distribution of e proposed scheme (η = 4.8 db, z =5%) V. NUMERICAL RESULTS We set (p 1,p 2,p 3,p 4 )=(0.2, 0.1, 0.05, 0.025) and (q 2,q 3, q 4,q 5 ) = (0.1, 0.3, 0.6, 0.9) for all mobiles. z =5%, τ( ) = 22 db and η max =9dB are assumed. Figs. 3 and 4 show e data rate distribution of e 1xEV- DO rate control scheme and e proposed rate control scheme when η =4.8 db, respectively. As e number of mobiles increases for bo schemes, e proportion having a high data rate such as 76.8 or kbps decreases and e proportion having a low data rate such as 9.6 or 19.2 kbps increases. Because data rates of e mobiles in e 1xEV-DO are controlled by probabilistic way, mobiles having a high data rate are remained when e number of mobiles is large. Meanwhile, as shown in Fig. 4, e proposed rate control scheme limits e maximum possible data rate according to e number of mobiles.

4 Overload probability 1E-4 1E-6 1E-8 1xEV-DO ( =4.8dB) Proposed scheme ( =4.8dB) 1xEV-DO ( =6.0dB) Proposed scheme ( =6.0dB) Effective reverse link roughput (kbps) xEV-DO ( =4.8dB) Proposed scheme ( =4.8dB) 1xEV-DO ( =6.0dB) Proposed scheme ( =6.0dB) 50 Fig. 5. Overload probability (η max =9dB, z =5%) Fig. 6. Effective reverse link roughput (η max =9dB, z =5%) Figs. 5 and 6 show e overload probability and e reverse link roughput for bo e proposed and 1xEV-DO rate control schemes as a function of Γ. For bo schemes, setting e η to a higher value causes not only higher roughput but also higher overload probability. As Γ increases, e proposed scheme significantly reduces e traffic overload compared to e 1xEV-DO. This is because at e proposed scheme reduces e traffic overload by efficiently restricting e high data rates wi high Data Channel gain. While e overload probability of e proposed scheme is much lower an at of e 1xEV-DO, e reverse link roughput of e proposed scheme is slightly higher an at of e 1xEV-DO. Fig. 7 shows e reverse link capacity, which is e peak traffic volume transmitted wiin an overload criterion at e ROT tail above 9dB is less an 1% of e time. We can see at our proposed scheme can achieve significant capacity gains compared to e 1xEV-DO rate control. Therefore, we can conclude at e proposed scheme suppresses traffic overload effectively by controlling e RateLimit and improves e reverse link roughput. VI. CONCLUSIONS In is paper, we have investigated e effect of e Rate- Limit on e performance of e reverse link rate control in 1xEV-DO systems and provided a reliable meod to adjust e value of RateLimit to improve e reverse link capacity. From e numerical results, out proposed rate control scheme was compared wi e conventional 1xEV-DO rate control scheme. The numerical results showed at e proposed rate control scheme effectively reduces e overload probability according to e increasing number of mobiles, us significantly increases e reverse link capacity compared to e 1xEV-DO rate control scheme. Moreover, our rate control scheme can be easily adopted in e practical 1xEV- DO systems wiout any modification. Reverse link capacity (kbps) Fig. 7. 1xEV-DO Proposed scheme Reverse link capacity (η max =9dB, 1% ROT tail=9db) REFERENCES [1] 3rd Generation Partnership Project 2 (3GPP2), cdma2000 High Rate Packet Data Air Interface Specification, 3GPP2 C.S20024 v.4.0, Oct [2] E. Esteves, On e reverse link capacity of cdma2000 High Rate Packet Data systems, in Proc. of IEEE International Conference on Communications, vol. 3, pp , [3] S. Chakravarty, R. Pankaj, and E. Esteves, An algorim for reverse traffic channel rate control for cdma2000 High Rate Packet Data systems, in Prof. of IEEE Global Telecommunications Conference, vol. 6, pp , [4] S. J. Oh and V. Vanghi, HDR (1xEV-DO) Reverse link roughput wi fast rate control, in Proc. of IEEE Wireless Communications and Networking Conference, vol. 1, pp , [5] W. Y. Yeo and D. H. Cho, A Markovian approach for modeling IS-856 reverse link rate control, in Proc. of IEEE International Conference on Communications, vol. 6, pp , [6] P. Tinnakornsrisuphap and C. Lott, On e fairness and stability of e reverse-link MAC layer in cdma2000 1xEV-DO, in Proc. of IEEE International Conference on Communications, vol. 1, pp , [7] W. Y. Yeo and D. H. Cho, Enhanced rate control scheme for 1xEV-DO

5 reverse traffic channels, IEE Electronics Letters, vol. 39, no. 23, pp , Nov [8] M. Yavuz and D. W. Paranchych, Adaptive rate control in high data rate wireless networks, in Proc. of IEEE Wireless Communications and Networking Conference, vol. 2, pp , [9] D. G. Jeong and W. S. Jeon, Congestion control schemes for reverse link data transmission in multimedia CDMA systems, IEEE Trans. Veh. Technol., vol. 52, no. 6, pp , Nov