Nhiều giao thức truy cập đối với truyền thông di động P7

Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond Alex Brand, Hamid Aghvami Copyright  2002 John Wiley & Sons Ltd ISBNs: 0-471-49877-7 (Hardback); 0-470-84622-4 (Electronic) 7 MD PRMA WITH LOAD-BASED ACCESS CONTROL Chapter 5 provided descriptions of channel and traffic models used for investigations on the MD PRMA protocol, which was defined in detail in Chapter 6. Starting with this chapter, and continuing in Chapters 8 and 9, the outcomes of our research efforts on MD PRMA will be discussed. In this chapter, the focus is on load-based access control (for MD PRMA), a technique adopted to protect reservation-mode users from multiple access interference generated by contending users. Only voice traffic is considered. We are investigating an interference-limited scenario, where code-slots are not distinguished, and random coding is assumed instead, such that ‘classical’ code-collisions cannot occur. However, users may still suffer ‘collisions’ due to excessive MAI, which will cause packet erasure. With access control, the overall packet-loss probability Ploss is composed of the packet-dropping ratio Pdrop and the packet-erasure rate Ppe. Load-based access control is applied to trade off packet dropping against packet erasure in a manner which minimises Ploss. To assess the benefits of load-based access control through so-called channel access functions (CAFs), several benchmarks are introduced. After a section defining the system considered, both analytical and simulation results for the first benchmark, a random access protocol, are presented. The analysis is expanded to underpin some of the comments made in the introductory chapter of this book on multiplexing efficiency. Other bench-marks include one used as a reference to assess multiplexing efficiency, and an ideal backlog-based access control scheme. Following considerations on channel access func-tions used for load-based access control, performances of the different schemes considered are compared for various scenarios. This includes a study of the impact of power control errors and the choice of the spreading factor. 7.1 System Definition and Choice of Design Parameters 7.1.1 System Definition and Simulation Approach MD PRMA for frequency division duplexing as defined in Section 6.2 is considered, assuming immediate acknowledgements and using load-based access control as described in Section 6.4. Physical layer performance is accounted for assuming random coding and applying the standard Gaussian approximation to assess the error performance, as outlined in Section 5.2. When intercell interference is considered, all cells are assumed to be 280 7 MD PRMA WITH LOAD-BASED ACCESS CONTROL equally loaded, that is, K in Equation (5.16) is the average load per time-slot experienced in the test cell, as defined below, and Iintercell is taken to be 0.37 and 0.75, for values of the pathloss coefficient γpl of 4 and 3 respectively. Code-slots are not considered. Therefore, in Subsection 6.2.3, the specific considerations provided for the random-coding case apply. In other words, we are considering a purely interference-limited system, where, however, instantaneous interference levels are only considered for interference generated within the test cell, while intercell interference is assumed to be constant at its average level. To assess the benefit of load-based access control, MD PRMA performance is compared with that of various benchmarks, which are defined below. The only traffic considered in this chapter is packet-voice traffic, using the two-state voice model specified in Section 5.5 with parameters Dspurt = 1 s and Dgap = 1.35 s, which results in a voice activity factor αv = 0.426. The number of conversations M supported simultaneously determines the system load. Ploss performance as a function of M is of interest here, and particularly, M0.01 and M0.001, the number of conversations which can be supported at tolerated maximum Ploss values, (Ploss)max, of 1% and 0.1%, respectively. A static scenario is considered, where Ploss is established as a function of M, and M remains fixed over the relevant period of observation. Therefore, the average number of users per time-slot K can be obtained through K = M · αv . (7.1) Simulations were performed using a commercial, event-driven and object-oriented tool for network simulations. Each simulation-run with fixed M covered 1000 s conversation time. Where required, several simulation-runs were performed for the same value of M, in which case the Ploss reported is the averaged result over these simulation-runs. 7.1.2 Choice of Design Parameters The starting point for the choice of design parameters is to be found in Reference [146]. In this reference, a voice source rate Rs of 8 kb/s and a frame length Dtf of 20 ms are considered, yielding 160 information bits per packet1, to which 64 header bits are added. With a PRMA channel rate Rp of 224 kbit/s, neglecting guard periods, a slot duration Dslot of 1 ms is required to accommodate a packet. A frame is therefore composed of N = 20 time-slots. The dropping delay threshold Dmax is set to 20 ms, which is half the value considered in Reference [146]. This is to keep the total transfer delay low, to which also other sources of delay contribute, such as framing delay and processing delay. It remains to specify the FEC code-rate rc and the spreading factor X. In Section 5.2, the optimum value for rc was established for packets with 224 message bits, applying the Gilbert–Varshamov bound. It was found that, irrespective of X, the bandwidth-normalised throughput was maximised when rc was between 0.4 and 0.6. A suitable BCH code with a code-rate in this range of values is the (511, 229, 38) BCH code. It supports five more message bits than required (they will be attributed to the header), and has a code-rate rc of 0.45. With this choice, Rp increases to 229 kbit/s before error coding, while the channel-rate after error-coding Rec is 511 kbit/s. Interleaving is not applied, every packet 1 In other Goodman publications, such as References [8] and [142], the voice source rate assumed was 32 kb/s, which is rather high for a basic voice service in cellular systems. 7.2 THE RANDOM ACCESS PROTOCOL AS A BENCHMARK 281 Table 7.1 Parameters relevant for the physical layer, protocol operation and traffic models Description Symbol TDMA Frame Duration Dtf Time-Slots per Frame N Message bits per Packet B Channel-Rate before Error-Coding Rp Channel-Rate after Error-Coding Rec Chip-Rate Rc Dropping Delay Threshold Dmax Voice Terminal Source-Rate Rs Mean Talk Gap Duration Dgap Mean Talk Spurt Duration Dspurt Parameter Value 20 ms 20 160 information bits + 69 header bits 229 kbit/s 511 kbit/s 3.577 Mchip/s 20 ms 8 kbit/s 1.35 s 1 s is separately error-coded, and contention and reservation-mode packets have the same packet format. This also implies that contention packets contain the same amount of user data as those sent on reserved resources. No dedicated request bursts are generated. In order to limit computer resource requirements for simulations, a rather low spreading factor of X = 7 was chosen when we started our investigations on PRMA-based protocols back in 1994. The resulting chip-rate Rc of 3.577 Mchip/s is surprisingly close to the one having been chosen for UTRA. Most of the results presented in the following are for X = 7. If larger spreading factors are considered, this is explicitly mentioned. The complete set of parameters used is listed in Table 7.1. 7.2 The Random Access Protocol as a Benchmark 7.2.1 Description of the Random Access Protocol In References [28–31] we established the benefits of load-based access control through a performance comparison with what was referred to there as random access CDMA. Strictly speaking, the name ‘random access CDMA’ is somewhat misleading, since the same hybrid CDMA/TDMA channel structure as in MD PRMA is used. A more generic name will therefore be used for this protocol; it will be referred to here as random access protocol (RAP). In RAP, every user may access the channel at will. In other words, the access permission probability p is always set to one. For a voice user, this simply means that the next time-slot after the arrival of a talk spurt will be accessed. In Reference [31], it was assumed that a voice terminal needed to retransmit the first packet of a spurt until it was successfully received and acknowledged by the base station. This can be viewed as MD PRMA with p = 1. Here, in order to have completely unconstrained channel access, packets are never retransmitted, and the time-slot number used for all packets in a spurt depends only on the arrival instance of the first packet in that spurt. Therefore, Pdrop is always zero, and the Ploss performance is entirely determined by Ppe. 282 7 MD PRMA WITH LOAD-BASED ACCESS CONTROL 7.2.2 Analysis of the Random Access Protocol According to Section 5.5, the steady-state distribution for the number of simultaneously active terminals v given M voice sources is µ ¶ Pr{V = v} = PV (v) = v · αv · (1 − αv)M−v (7.2) with mean V = M · αv. These v simultaneously active users will be distributed in some fashion over the N available time-slots. With completely unconstrained access as discussed above, there is no reason to expect that some time-slots are more likely to be chosen by any one of the users than others are. Furthermore, with exponentially distributed spurt and gap duration, any particular user will choose time-slots for successive spurts independently of each other. It can therefore be assumed that each slot in a TDMA frame is chosen with equal likelihood, i.e. Pslot = 1/N. The probability of k users accessing a slot conditioned on v active users is then µ ¶ Pr{K = k|V = v} = PK|V (k|v) = k · Pkot · (1 − Pslot)v−k, (7.3) and the unconditional probability can be calculated through M Pr{K = k} = PK(k) = PV (v) · PK|V (k|v). (7.4) v=k Note that the summation starts from k, since in order that k users access a certain time-slot, there must be at least k users active in total. Finally, Ploss can easily be calculated according to M Ploss = k · PK(k) · Ppe[k], (7.5) k=0 with K from Equation (7.1). To establish the packet erasure probability Ppe[k], depending on the circumstances considered, Equations (5.7) and (5.3) together with either Equa-tion (5.6) or Equation (5.16) are used2. Alternatively, Equation (5.20) may be used. The steady-state distribution (Equation (7.2)) is a binomial distribution with parame-ters M and αv. The Poisson distribution with mean V = M · αv is a good approximation of the binomial distribution, provided that αv ¿ 1 and M large (e.g. αv < 0.05 and M > 10). The first condition must hold for the variance of the binomial distribution, M · αv · (1 − αv), to match roughly that of the Poisson distribution, V. Here, αv is signif-icantly larger than 0.05, and thus, the variances of the two distributions cannot match, irrespective of M. Assume for now all the same, that Equation (7.2) can be approximated by a Poisson distribution with mean V. In this case, since every slot is selected indepen-dently with probability Pslot, the probability distribution per slot is again Poisson with 2 The attentive reader will have noticed that upper case ‘K’ was used for the number of users per time-slot in Chapter 5, while ‘k’ was used as an index for a particular user out of these K users. For consistency of notation in this chapter, ‘k’ is here the number of users per time-slot, and ‘K’ the respective random variable. Instead of 8K, we can write PK(k) for the probability distribution of this random variable. 7.2 THE RANDOM ACCESS PROTOCOL AS A BENCHMARK 283 mean V/N = K owing to the ‘splitting property’ of the Poisson process discussed in Section 6.5. Therefore, k −K PK(k) = k! . (7.6) This approximation is useful for the discussion on multiplexing efficiency provided in Subsection 7.2.4. Its accuracy is assessed below. 7.2.3 Analysis vs Simulation Results In Figure 7.1, Ploss values resulting with the random access protocol are reported as a function of M for two cases, namely an isolated test cell and a test cell in a cellular environment, in both cases assuming perfect power control. In the single-cell case, there is no intercell interference, and Equation (5.6) is used for the average SNR which determines Ppe[k]. For the results shown for the cellular environment, a pathloss coefficient γpl of 4 is assumed, and average intercell interference is accounted for by using Equation (5.16), with Iintercell = 0.37, instead of Equation (5.6). The curves with markers represent simulation results, whereas the solid and the dashed curves refer to analytical results with the binomial steady-state distribution according to Equation (7.2) and the Poisson approximation for the steady-state distribution respectively. Two conclusions can be drawn from this figure. Firstly, judging from the Ploss values reported, the Poisson approximation models the Ploss performance obtained with the binomial distribution quite well for large values of M, regardless of the variance mismatch. With decreasing M, however, the gap between the Ploss values calculated widens. Secondly, in general a very good agreement between analysis and simulation results can be observed. Normally, even simulation results obtained from individual 1000 s simulation-runs closely match the analytical results, although most points shown in the 0.1 Perfect power control spreading factor X = 7 0.01 Cellular environment, gpl = 4 Single cell 0.001 Simulation Binomial Poisson 0.0001 90 110 130 150 170 190 210 230 250 Simultaneous conversations M Figure 7.1 Performance of the random access protocol with perfect power control ... - tailieumienphi.vn