ICUPC’97, S AN D IEGO , CA, O CT. 12-16, 1997
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Throughput comparison between the CDPA packet system and FDMA/TDMA system in the cellular environment Flaminio Borgonovoy , Luigi Frattay , Michele Zorziz y Dipartimento di Elettronica e Informazione, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 MILANO - ITALY – fax: +39-2-2399-3413 e-mail: fborgonov,
[email protected] – http://www.elet.polimi.it
z Center for Wireless Communications, University of California San Diego
9500 Gilman Drive – La Jolla, CA 92093-0407, USA – fax: +1-619-534-2486 e-mail:
[email protected] – http://www-cwc.ucsd.edu/ zorzi
~
Abstract— In this paper we provide a performance com-
parisons between the Capture Division Packet Access (CDPA), a novel cellular packet-access architecture recently proposed, and more classic systems based on the FDMA/TDMA scheme. CDPA is a single-frequency interference-limited system in which the packet access scheme is integrated with the interference protection scheme, being the protection attained through packet retransmission and time diversity. The comparison is obtained by simulating the operation of the two schemes in a real cellular environment in which co-channel interference, propagation and receiver structure are taken into account. The FDMA/TDMA scheme assumes that a suitable frequency reuse plan and forward error correction with perfect interleaving are used. Results show that CDPA provides considerably higher throughput than FDMA/TDMA in all propagation conditions. It is also shown that the CDPA throughput can be delivered within the delay and the packet-dropping probability constraints usually requested for real-time applications. I. I NTRODUCTION
The packet approach has been widely recognized as the most suitable for traffic integration and variable bandwidth services and has received support even in the cellular environment, as testified by the many studies of packet access schemes appeared in the recent literature, e.g., [1]–[7]. Some of these proposals originate from, or are inspired by, multi-access protocols which were conceived for different environments. All of them assume a radio channel that is shared by the users within each cell, and no interference from adjacent cells is considered. This, in fact, can be avoided by adopting some channel reuse techniques, namely FDMA frequency reuse plan or spread spectrum modulation, whose design is usually accomplished to fulfill circuit-oriented traffic requirements. This practice results in a complete separation between the physical and the MAC layer operation that turns out to limit the system capacity. A study of slotted ALOHA in a cellular environment where the same channel is used in all cells has been presented in [8]. The recently introduced Capture Division Packet Access
This work was partially supported by MURST 1995-1996. The work of M. Zorzi was supported by the Center for Wireless Communications, UCSD.
40%
(CDPA) [9, 10] implements a cellular access architecture in which multiple access and channel reuse are designed in an integrated way to achieve the best efficiency while using a single frequency in all cells. CDPA uses a hybrid reservation/polling mechanism (HRP) at the base station (BS) to solicit packet transmission from the mobile terminals (MT). Parallel transmissions from different cells exploit capture. If capture fails, the MT is solicited again to retransmit the collided packet. This mechanism has been shown to be particularly effective because it can easily adapt to rapidly changing co-channel interference and propagation conditions. Finally, despite the delay introduced by packet retransmissions, it has been shown that this technique can cope with delay sensitive traffic such as voice [10, 11]. In previous works, the CDPA performance has been evaluated under different environment assumptions, both analytically and by simulations. From those results emerged that the capacity of CDPA is generally higher than the maximum capacity allowed in FDMA/TDMA systems with frequency reuse and cluster size K , an usual figure for those systems [12]. In this paper we explicitly attempt a comparison between the performance of CDPA and FDMA/TDMA schemes. To this purpose, we consider the same cellular environment for both systems and consider the effect of interference by simulating the activity in the surrounding cells that use the same channel. This evaluation procedure differs from those produced in the literature for FDMA/TDMA systems, which usually are based on the reduced description given by the long term signal-to-interference ratio (SIR). This description is inaccurate because in the environment we are considering the SIR changes dynamically as transmitting terminals and propagation conditions change. Moreover, given the value of the SIR, the error rate on the received signals can greatly differ and depends on the nature and composition of the interfering signal (the Gaussian approximation is not accurate in general). To overcome these limitations we have adopted for both system a more realistic detection model that decodes each bit of the transmitted packet in the presence of interference. This model allows a correct evaluation of the throughput of a TDMA/FDMA system, and also provides a capacity evaluation for CDPA that is more accurate than the one obtained with the receiver thresholdmodel considered in [9, 10]. The throughput of the FDMA/TDMA system is evaluated as-
=7
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suming a given channel reuse factor K , forward error correction (FEC) and perfect interleaving, so that the errors caused by interference are independent and identically distributed. The FEC code is chosen among the convolutional and BCH codes of a given complexity that provide the best coding rate in guaranteeing ?3 residual error probability. For our comparison we are interested in spectral efficiency and, thus, we are not considering the effect of multiple access protocols superimposed on the FDMA/TDMA scheme. Therefore, in the sequel, we only refer to the capacity that can be attained by constant rate sources. The rest of the paper is organized as follows. Section II presents the adopted evaluation model, and the CDPA performance obtained in different environments is discussed in Section III. The comparison with FDMA/TDMA is presented in Section IV. Comments on the CDPA delay performance are given in Section V.
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II. CDPA EVALUATION MODEL The core of the operation of CDPA is a Hybrid Reservation/Polling (HRP) scheme, whereby the base station of each cell maintains a list of active users and assigns slots to them according to their traffic and quality of service needs. The slot assignment is performed by an appropriate scheduling algorithm which generates polling commands that are sent to the users and trigger the transmission of packets. A detailed description of the scheme, not reported here for brevity, can be found in [10, 13]. For the purpose of this study, we assume that in every slot only one user per cell may be allowed to transmit, and therefore contentions among MT in the same cell are avoided. In the cellular environment taken into account, the BSs are evenly spaced on the plane, at the center of ideal hexagonal cells, and operate with omni-directional antennas. Each MT is assumed to communicate with the nearest base station according to the HRP mechanism. Packet transmissions in different cells are assumed synchronized on a common slotted time basis, so that transmissions in different cells overlap completely. Similarly, each BS transmits packets to the MTs within its own cell. The propagation model takes into account fading due to multipath, log-normal shadowing, due to terrain irregularities and obstructions, and an -th power loss law [10]. In our model, we assume that fading is a short term effect and can be independently drawn at each transmission by the same MT, whereas lognormal shadowing and the MT position are long term effects, and affect all transmissions bythe same MT (at least on the time scale of a packet’s lifetime). The receiver demodulates bit by bit the received signal, which is the sum of the intended signal and interfering signals, perfectly overlapped, BPSK modulated with the same frequency but with random phases. We also assume that all packets are preceded by a synchronization preamble and that the receiver locks to the phase of the superposition of the preambles. This results in a phase detection error that adds to the amplitude error caused by the interference. A packet is considered captured (no retransmission is required) if no errors occur. The throughput evaluation presented here does not take into account any overhead factor, such as packet or commands overhead and, according to the above uplink model, the throughput density s }; at location } with shadowing is evaluated as the fraction
( )
of packets correctly received, i.e.,
s(}; ) = Ps (G; }; )g(}; );
(
)
(1)
where Ps G; }; is the capture probability of a packet transmitted from } with shadowing , g }; is the traffic density at location } with shadowing , and
( )
G=
Z
Z
cell
g(}; )dAd
(2)
is the average offered load per cell. In steady-state conditions, and under equal throughput requirements, the HRP mechanism guarantees uniform throughput, because it keeps polling the same packet until it is successfully received. In these conditions, Equation (1) shows that, in order to have a uniform throughput s }; , the traffic density g }; is, in general, non-uniform, because the capture probability Ps G; }; depends on the signal power at the receiver, which, in turn, depends on } and . This has been taken into account in the model for the intended user, whereas it has been ignored for the interferers, which are assumed to generate a uniformly distributed traffic for analytical convenience. Our investigations have shown that the impact of this approximation is negligible [14]. With uniform throughput requirement, s }; s, following [10] we have
( )
( ) (
)
( )
?1 S (G) = G E P (G;1 }; ) : s
(3)
Although a simplified analytical approach can be used [15], in this study we have directly evaluated the throughput (3) by simulating the transmission and retransmission process within the cell, the interfering transmissions and the receiver capture according to the following scheme. Each MT, uniformly chosen in the considered cell, is assumed to generate only one packet at a time, which is retransmitted from the same location and with the same shadowing attenuation factor until it is captured by the BS. The interfering transmissions, composed by transmissions and retransmissions in the adjacent cells are represented by an independent process of intensity G packet/slot/cell. The interfering MTs, no more than one per cell, are chosen in each slot with probability G and uniformly located in each of the 36 cells surrounding the considered cell. We also considered power control, i.e., a mechanism that compensates for the near-far effect and the log-normal shadowing and assures a constant long-term power at the receiver. The use of perfect power control drastically simplifies the analysis shown, since the channel traffic density becomes independent of } and . Finally, the model exposed above can be easily adapted to a system with partial frequency reuse, which subdivides the total bandwidth into K sub-bands and assigns each cell a sub-band, accord; and 7 will be considered. Note ing to a regular pattern. K corresponds to the complete frequency reuse of CDPA. that K
1
=1
=1 3
III. CDPA PERFORMANCE
WITH FULL FREQUENCY REUSE
In Figure 1 the throughput S of the uplink channel of CDPA with full frequency-reuse is plotted versus the offered traffic G, with and without power control and in the absence of shadowing, with
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0.45 0.4 0.35
1
CDPA - Uplink - no FEC 511-bit payload - BPSK η=4 η=3 σ=0
PC
G=1, L=511
0.8
eta=4 0.7
no PC
0.3 PC
0.25
eta=3
0.6 0.5 0.4
0.2 0.3
no PC
0.15
0.2
0.1
0.1
0.05 0 0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Channel traffic G
0.35
150
200
250
300
N occur-
300
CDPA uplink channel with PC
CDPA - Uplink - no FEC 511-bit payload - BPSK η=4 η=3 σ=6
250
0.3
PC
0.25 0.2
no PC
0.15
L=511
200
eta=4 150
eta=3 100
50
PC
0.1
no PC 0
0.05 0 0
100
Fig. 3. Complementary cumulative distribution of the number of errors ring in an erroneous packet.
Average # of errors
0.4
50
n
0.5 0.45
0
1
Fig. 1. Throughput of the CDPA uplink channel, with and without power control, in the absence of shadowing.
Normalized Throughput S
CDPA uplink channel with PC
0.9
P(N>n)
Normalized Throughput S
0.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Channel traffic channel traffic G 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Channel traffic G Fig. 2. Throughput of the CDPA uplink channel, with and without power control, with 6 dB shadowing.
511-bit packets. Both S and G are expressed in packets per slot. Two values of the path loss exponent, , are considered, namely = 4 and = 3, typical of macrocell and microcell environ-
ments, respectively. The results show that CDPA offers good performance, even without power control, especially if one considers that the throughput efficiency of TDMA cellular systems, where the frequency reuse is based on clusters of seven or more cells, cannot exceed : . Figure 2 shows similar curves in the presence shadowing with parameter 6 dB (i.e., the shadowing attenuation in dB is a Gaussian random variable with standard deviation ). As we can see, shadowing noticeably degrades the throughput if no power control is used. This happens because it significantly reduces the power received from some users, which, in order to get through, must retransmit many times, thus wasting a considerable amount of bandwidth. Power control limits only in part the throughput loss, because of the increased interference from other cells. However, the situation analyzed is very pessimistic since, in practice, long term
0 143
=6
Fig. 4. Average number of errors in an erroneous packet versus the channel traffic G.
shadowing determines the choice of the BS and it is likely that the users that cause the dramatic throughput loss should be considered attached to other BS. This issue greatly complicates the analysis since the observed shadowing variables become correlated. The interested reader is referred to [15, 16]. We have also studied the error statistic in erroneous packets. Figure 3 reports the complement of the cumulative distribution of the number of errors N occurring in an erroneous packet, on the and ; , in the presence of power uplink channel for G control and for no shadowing. Even though the probability of hav? S=G, is very different in the two ing at least one error, Pe : for and Pe : for , the cases, namely Pe error distribution is almost the same. Also, the number of errors is on the average and larger than 100 in likely to be large, and is of the cases. In Figure 4 we report the average number of errors per erroneous packet as a function of the channel traffic G for the two values of already considered, when power control is used. We observe that, even though the value of Pe obviously decreases as G decreases,
50%
=1 =3 4 =1 = 0 84 =3 125
= 0 54
=4
ICUPC’97, S AN D IEGO , CA, O CT. 12-16, 1997
4
(511 )
1 0.9 hard
0.8
soft BCH
0.7
code rate
the average number of errors per erroneous packet increases. In fact, as the traffic decreases, the number of interfering packets also decreases, and the interference “noise” on erroneous packets becomes more correlated. As an extreme example, consider the case in which the interference is given by only one packet. In this case and with our modulation model, either no errors are present, if the level of the interferer is below that of the intended signal, or half of the bits in the packet are wrong on the average. Very similar results have also been observed for other values of the shadowing parameter and reuse factor. All these results suggest that no substantial advantage can be obtained in CDPA by en; k BCH codes [13]. In fact, when coding each packet with the interference is strong enough to cause errors, the number of such errors is quite often larger than the error correcting capability of the codes usually adopted, whereas more powerful codes waste too much bandwidth. The above argument strongly suggests that TDMA/FDMA cellular system, that completely relies on the correcting capability of FECs, may be outperformed by CDPA, which is based on the immediate retransmission of erroneous packets.
conv L=5
0.6 0.5 0.4 0.3 0.2 0.1 0 10 -4
10 -3
10 -2
10 -1
10 0
input error rate Fig. 5. Rate reduction provided by BCH codes and convolutional codes with hard and soft decoding in order to provide a residual error rate of ?3 , plotted as a S=G ne = . function of the input error rate "
= (1 ?
) 511
10
IV. FDMA/TDMA SYSTEM In this section we derive the performance of FDMA/TDMA systems in which partial frequency reuse and FEC with perfect interleaving are used. In order to use the same interference results for both systems, we assume TDMA packets of 511 bits. Moreover, we assume that packet bits are obtained by a perfect interleaving mechanism of bits among blocks and then coded by a suitable code. Perfect interleaving is such that the distribution of errors after de-interleaving can be assumed as a purely random process, the best condition to operate a FEC coding. Given our assumptions, a unique simulation measure can provide the performance of both systems, for both full and partial frequency reuse, provided that we assume power control. In this case, in fact, the channel traffic density g }; within the cell does not depend on }; , as expected in FDMA/TDMA systems where no retransmission is allowed. The ratio S=G provides the capture probability for CDPA and represents the probability that no errors are present in FDMA/TDMA packets. If ne is the number of erroneous bits in an erroneous packet, then after de-interleaving the ? S=G ne = , which is the parameter error rate becomes " of the binomial distribution of the number of errors in a block Ne after de-interleaving. The throughput of the FDMA/TDMA system is defined as S 0 =K rG=K , where K is the frequency reuse factor, and r the rate of the code used to guarantee that the resulting bit error rate after decoding is not greater than ?3 . We consider the code ; k BCH codes and that provides the highest rate r among the . the convolutional codes with constrained length L The residual bit error rate for the BCH codes is evaluated as
( )
= (1
) 511
=
10
BER =
P511
(511 )
= +1 kP (Ne = k)
k t
511
=5
10?3
(4)
where t is the error correcting capability of the code. For the convolutional codes, the residual error rate is provided by the union bound [17].
Figure 5 shows the maximum code rate that provides a residual error rate of ?3 versus the input error rate ". The three curves represent the rate reduction needed by the cited BCH codes and convolutional codes with hard and soft decoding. The figures used to evaluate " have been obtained by the CDPA simulator and are reported in Table 1. This Table shows the average number of errors ne and the throughput S=K of the CDPA for K ; ; and for uplink with power control and ; . Because of the power control, the capture probability S=G is independent of the position of the MT. In Table 2 we show the results concerning the TDMA/FDMA system with optimal FEC and perfect interleaving. Both the optimum code rate and the maximum throughput have been evaluated in correspondence to the cases shown in Table 1. The code rates in italic refer to convolutional codes, while the others to the BCH codes. The throughputs in bold-face, in both Tables 1 and 2 are the maximum values observed for each technique and propagation factor . The results of Tables 1 and 2 are summarized in Figure 6 where the maximum throughputs of CDPA and FDMA/TDMA for differ, are coment reuse factors and propagation conditions, with ; . pared. CDPA is more efficient than FDMA/TDMA for K This shows that, in all conditions considered, the packet repetition both mechanism is superior to FEC and interleaving. For K techniques provide almost the same performance, as expected, since in this case the throughput is constrained by the reuse factor rather than by the interference noise. Similar results are shown dB. We observe that the in Figure 7 for a shadowing factor advantage of CDPA over TDMA is even more significant. As far as CDPA is concerned, the results show that a reuse factor equal to one is practically the best, (in fact, the improvement for is very small), showing that the offered by K single-frequency CDPA approach is a fortunate case that conjugates simplicity with maximum performance.
10
=0
=34
=137
=0
=6
=3
=3
=1 3 =7
ICUPC’97, S AN D IEGO , CA, O CT. 12-16, 1997
G=0.1 G=0.2 G=0.3 G=0.4 G=0.5 G=0.6 G=0.7 G=0.8 G=0.9 G=1.0
5
=3; K=1 =3; K=3 =3; K=7 =4; K=1 =4; K=3 =4; K=7 181=0:091 210=0:033 239=0:014 207=0:096 195=0:033 213=0:014 146=0:159 173=0:065 210=0:028 174=0:180 192=0:066 228=0:028 128=0:201 158=0:095 184=0:042 156=0:254 163=0:099 229=0:043 121=0:223 137=0:124 176=0:056 143=0:314 151=0:131 154=0:057 117=0:229 123=0:151 168=0:070 136=0:361 138=0:163 188=0:071 119=0:228 115=0:176 151=0:084 133=0:400 124=0:194 165=0:085 119=0:215 107=0:198 138=0:097 132=0:427 113=0:224 126=0:100 120=0:198 100=0:219 132=0:111 128=0:448 110=0:254 145=0:114 122=0:181 99=0:238 129=0:124 128=0:459 109=0:284 120=0:128 125=0:160 94=0:254 115=0:136 126=0:460 99=0:315 123=0:142
Table 1. CDPA: Number of errors ne of non captured packets and throughput .
=0
G=0.1 G=0.2 G=0.3 G=0.4 G=0.5 G=0.6 G=0.7 G=0.8 G=0.9 G=1.0
=3; K=1 0:595=0:059 0 :42 =0:084 0 :37 =0:111 0 :32 =0:128 0 :278 =0:139 0 :245 =0:147 0 :215 =0:150 0 :195 =0:156 0 :178 =0:160 0 :165 =0:165
=3; K=3 0:911=0:030 0:859=0:057 0:788=0:079 0:735=0:098 0:682=0:114 0:647=0:129 0:594=0:138 0:559=0:149 0:489=0:146 0:471=0:157
S=K for partial frequency reuse factor K , different values of the channel traffic G and
=3; K=7 =4; K=1 0:982=0:014 0.736/0.074 0:964=0:027 0.577/0.115 0:947=0:040 0.472/0.141 0:929=0:053 0.415/0.166 0:911=0:065 0.37 /0.185 0:894=0:076 0.35 /0.21 0:876=0:087 0.32 /0.224 0:876=0:100 0.301/0.241 0:859=0:110 0.281/0.253 0:841=0:120 0.265/0.265
Table 2. TDMA with optimal FEC and interleaving: optimum code rate r and the corresponding throughput . the channel traffic G and
=0
V. CDPA
DELAY PERFORMANCE FOR CONSTANT RATE SOURCES
The previous performance analysis has proved the validity of the CDPA architecture when channel utilization is considered. However, some concerns may arise on its ability to support real time traffic, because the CDPA retransmission mechanism can not strictly guarantee bounded delay. In FDMA/TDMA systems, where no retransmission is allowed, the delay is deterministic. Note, however, that such delay is not zero, due to the presence of the coding/interleaving scheme. In particular, a strict delay constraint may make perfect interleaving impossible to use, so that the results obtained in this papers for TDMA/FDMA may be considered optimistic. In FDMA/TDMA, packets are lost due to channel errors, since erroneous packets cannot be retransmitted. Packet loss can also occur in CDPA, where a delay constraint will result in dropping the packets which are not delivered in time. Since in CDPA the delay is strictly related to the system load, the question becomes how much throughput can be delivered with constraints on delay and packet dropping probability. In [10, 11] we have shown the feasibility of a traffic control procedure, that prevents the CDPA system from operating beyond the maximum throughput point (congestion region). This mechanism
C=c 4 4 4 3 3 3
156 31 31 156 156 31
=4; K=3 0:982=0:033 0:964=0:064 0:929=0:093 0:911=0:121 0:894=0:149 0:876=0:175 0:859=0:200 0:841=0:224 0:823=0:247 0:806=0:268
=4; K=7 1=0:014 1=0:028 1=0:043 1=0:057 1=0:071 1=0:085 1=0:100 0:982=0:112 0:982=0:126 0:964=0:137
S 0 for partial frequency reuse factor K , different values of
S
G
0.372 0.355 0.323 0.192 0.181 0.162
0.6 0.54 0.44 0.3 0.27 0.215
0.9 0.9 0.8 0.49 0.49 0.49
u E [A] 58 11 10 30 28 5
104 18.6 15.1 95.5 86 13.6
[ ] P (A > C=c) 5 x 10?9 ?4
VAR A 82.38 12.83 7.78 208.3 175.7 23.24
3 x 10 7 x 10?9 1.5 x 10?5 6.5 x 10?8 1.6 x 10?4
Table 3. Examples of parameters related to the distribution of the number A of slots needed to successfully transmit all the packets generated in a packet arrival period.
has been successfully used in the CDPA simulations showing that a throughput close to the maximum can be achieved without any packet loss. More recently [13], we have obtained a bound on the packet dropping probability for the case of u constant-rate sources, assuming that they generate traffic at rate c and transmit over a channel of rate C . The maximum allowed delay before dropping a packet is equal to the inter-packet generation time. This bound is shown in Table 3 as P A > C=c , where A is the random variable representing the total number of slots used to transmit the packets generated in a packet inter- arrival time. The parameter repre-
(
)
ICUPC’97, S AN D IEGO , CA, O CT. 12-16, 1997
6 signal-to-interference ratio (SIR). The results show that CDPA is inherently more efficient than FDMA/TDMA. It has also been shown that this gain can be provided even for delay constrained traffic, such as voice. This results, together with the inherent flexibility of CDPA and other attractive features, such as single frequency operations and protocol simplicity, makes it an attractive solution for future cellular systems.
0.5
Uplink with power control L=511 σ=0 dB
Max. Throughput (pkt/slot)
0.45
CDPA
0.4
η=4
0.35
η=3
0.3
TDMA
0.25
CDPA
0.2 0.15
ACKNOWLEDGMENTS The authors would like to thank P. Carmine, R. Gianini, M. Quinzio for their help in obtaining the numerical results.
TDMA
0.1
R EFERENCES
0.05 0
1
2
3
4
5
6
7
Channel reuse factor K
[2] N.M. Mitrou, T. D. Orinos, E. N. Protonotarios, “A reservation multiple access protocol for microcellular mobile-communication system”, IEEE Trans. Veh. Tech., vol. VT-39, pp. 340-351, Nov. 1990.
Fig. 6. Maximum throughputs of CDPA and FDMA/TDMA for different reuse fac. tors, propagation conditions and
=0
0.5
Uplink with power control L=511 σ=6 dB
Max. Throughput (pkt/slot)
0.45 0.4
η=3
0.3
[5] N. Amitay, L.J. Greenstein, “Resource auction multiple access (RAMA) in the cellular environment”, IEEE Trans. on Vehic. Technol., vol. 43, pp. 110111, Nov. 1994.
CDPA
0.25
[6] A. Urie, M. Streeton, C. Mourot, “An advanced TDMA mobile access system for UMTS”, IEEE Personal Communications, vol. 2, no. 1, pp. 38-47, Feb. 1995.
0.2
CDPA
0.15
0
[7] S. Nanda, D. J. Goodman, and U. Timor, “Performance of PRMA: A Packet Voice Protocol for Cellular Systems”, IEEE Trans. on Vehicular Technology, vol. 40, no. 3, August 1991, pp. 584-598.
TDMA
0.1 0.05
TDMA 1
2
3
4
5
6
7
Channel reuse factor K
Fig. 7. Maximum throughputs of CDPA and FDMA/TDMA for different reuse fac. tors, propagation conditions and
=6
sents the maximum value of G allowed by the control mechanism. corresponds to the case in which MTs are The value C=c Kb/s and the channel rate is Mb/s, whereas voice sources at C=c corresponds to the case in which the channel rate is reduced to Mb/s. The values of S , G are derived from the figures shown in this paper. If we consider voice packets with length equal to bit, their inter-arrival time is ms. From the results shown in Table 3, one may conclude that in all the considered system configurations, the delay constraint is satisfied with higher probability than usually requested.
= 31 1 512
[3] G. Wu, K. Mukumoto and A. Fukuda, “Performance Evaluation of Reserved Idle Signal Multiple Access Scheme for Local Wireless Communications”, IEEE Trans. on Vehicular Technology, Vol. VT-43, August 1994, pp. 653658. [4] D. Raychaudhuri, W. Wilson, “ATM-based transport architecture for multiservices wireless personal communication networks,” IEEE J. Select. Areas Commun., vol. 12, pp. 1401-1414, Oct. 1994.
η=4
0.35
[1] D.J. Goodman, “Cellular packet communications”, IEEE Trans. Commun., vol. COM-38, Aug. 1990, pp.1272-1280.
= 156 32
5
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VI. C ONCLUSION In this paper we have compared the maximum efficiency allowed by CDPA and FDMA /TDMA. We have simulated the capture of packets in a detailed cell environment, which includes the propagation effects and a capture model that directly accounts for the receiver structure. This is a more adequate model than those produced in the literature for FDMA/TDMA systems, which are usually based on the reduced description given by the long term
[8] M. Zorzi, S. Pupolin, “Slotted ALOHA for high capacity voice cellular communications,” IEEE Trans. Veh. Tech., vol. 43, pp. 1011–1021, Nov. 1994. [9] F. Borgonovo, L.Fratta, M. Zorzi, “Performance analysis of capture-division packetized access (CDPA) for cellular systems”, Fifth WINLAB Workshop on 3rd Generation Wireless Information Networks, New Brunswick, Apr. 1995. Also in: J.M. Holtzman, ed., Wireless Information Networks, Boston: Kluwer, 1996. [10] F. Borgonovo, M. Zorzi, L. Fratta, V. Trecordi, G. Bianchi, “The CaptureDivision Packet Access (CDPA) for Wireless Personal Communications”, IEEE J. Selected Areas Comm., Vol. 14, No. 4, May 1996, pp 609-622. [11] G. Bianchi, F. Borgonovo, M. De Marco, V. Trecordi, “Capture-Division Packetized Access (CDPA) for cellular systems: an assessment by simulation” in Proc. IEEE ICC’95, Seattle, WA, June 1995. [12] Moe Rahnema, “Overview of the GSM System and Protocol Architecture”, IEEE Commun. Mag., vol. 31, no. 4, pp. 92-100, April 1993. [13] F. Borgonovo, L. Fratta, M. Zorzi, A. Acampora, “Capture-Division Packet Access — A new cellular access architecture for future PCNs”, IEEE Communications Magazine, Vol. 34, No. 3, September 1996. [14] M. Zorzi, F. Borgonovo, “An analytical model for Wireless Packet Access in the presence of slow propagation impairments”, Internal report, Politecnico di Milano, Dec. 1995. [15] M. Zorzi, F. Borgonovo, L. Fratta, “Packet access for cellular systems: the capacity of S-ALOHA and CDPA with slow shadowing”, in Multiaccess, Mobility and Teletraffic for Personal Communications, Paris, France, May 1996. [16] M. Zorzi, “On the analytical computation of the interference statistics with applications to the performance evaluation of mobile radio systems,” IEEE Trans. Comm., vol. 45, pp. 103-110, Jan. 1997. [17] G.C. Clark, J.B. Cain, Error-correction coding for digital communications. New York: Plenum Press, 1981.
