PERFORMANCE OF TDD-SDMA/TDMA SYSTEM WITH MULTI-SLOT ASSIGNMENT IN ASYMMETRIC TRAFFIC WIRELESS NETWORK Yunjian Jia1, Yoshitaka Hara2 and Shinsuke Hara3 1 2
Osaka University, 2-1 Yamada-Oka, Suita-shi, Osaka 565-0871 Japan,
[email protected] Mitsubishi Electric Corporation, 5-1-1 Ofuna, kamakura, kanagawa, 247-8501 Japan,
[email protected] 3 Osaka University, 2-1 Yamada-Oka, Suita-shi, Osaka 565-0871 Japan,
[email protected]
Abstract – This paper proposes a TDD-SDMA/TDMA system with a multi-slot assignment to deal with asymmetry traffic in wireless networks, where the packet-based data services is supported. The proposed system assigns more than one time slots for downlink to each terminal so as to deal with the asymmetric traffic. We also present an efficient algorithm to decide the priority of time slots for downlink, by which the maximum of potential transmission rate for downlink can be obtained. The performance of the proposed system is evaluated by computer simulation in terms of the average delay and the average transmission rate and compared with those of a conventional system with a symmetric time slots assignment. It is verified that a significant improvement on system performance can be achieved if the TDD-SDMA/TDMA system employs the proposed multi-slot assignment scheme. Keywords – TDD, SDMA/TDMA, multi-slot assignment, asymmetry traffic I. INTRODUCTION Due to the popularity of mobile communications, future wireless communication system is required to have a larger system capacity to support high-speed and high-reliable services. Space Division Multiple Access (SDMA) is regarded as a promising technique for increasing the system capacity, where terminals in different angular positions can share the same channel reducing the power of other terminal’s signals with the use of adaptive arrays at base stations (BS) [1]-[2]. SDMA can be combined with other multiple access schemes such as time division multiple access (TDMA) and code division multiple access (CDMA) to obtain larger system capacity. In our pervious works [3][4], we showed that the SDMA/TDMA system can improve the system capacity compared with a sectorized antenna TDMA system with the same number of antenna elements. Thanks to the fact that wireless communication system have shifted from voice transmission to multimedia transmission, it is predicted that the packet-based data services including electronic mail exchange, instant message, the Internet access and Web browsing will increase largely in future wireless communication systems,
0-7803-7589-0/02/$17.00 ©2002 IEEE
which have asymmetric traffic characteristics, namely, heavier traffic flow in downlink (from a base station to a terminal) than in uplink (from a terminal to a base station). Thereby, conventional systems with symmetric resources allocation are inefficient to support the advanced services. We need design new resource management techniques to deal with the asymmetric traffic. In TDD system, we can change the amount of resource allocated to uplink and downlink by just change the number of time slots assigned to each link so that it is easy to support asymmetry resource allocation. In this paper, we propose a TDD-SDMA/TDMA system with a multi-slot assignment for the asymmetric traffic handling and investigate the performance of the proposed system in a multi-cell packet-switched wireless network. In the proposed system, only one time slot can be assigned to each terminal in uplink with a fixed transmission rate, whereas multiple slots in downlink to obtain higher transmission rate. We also present an efficient time slot assignment algorithm for downlink to maximize the transmission rate. Moreover, in order to maximize the system capacity, a closed-loop power control based on the quality of each link is employed for both uplink and downlink to obtain the optimal power allocation and beam pattern in the entire network. In our study, the average delay is used as an important performance measure, which is defined as the average time from call generation to the end of transmission. In other words, we evaluate the system performance in terms of “How long does it take to finish a service from the service generation? ” that can be regarded as a performance criterion of packet-switched system. We also use the average transmission rate as the performance measure. In this paper, the performance of the proposed system is evaluated by computer simulation, and compared with that of a conventional system with a symmetrical time slot assignment where one time slot is assigned to each user in uplink and downlink. The numerical results will clearly show that the proposed system with the multi-slot assignment can achieve significantly better performance than the conventional system with the symmetrical time slot assignment in the asymmetric traffic network.
PIMRC 2002
II. SYSTEM MODEL
K
Min
A. Service Model We consider a wireless network, where the system is demanded to provide packet-switched data services with asymmetric information flow including Web browsing, file download and so on. For these services, the information files are transmitted from base stations to terminals. In [5], WWW traffic characteristics for wired network are analyzed, and it is shown that the distribution of the file size can be well modeled as a log-normal distribution. In our study, we assume the wireless network has the same traffic characteristics as the wired network. Therefore, the probability density distribution of the file size (Bytes) can be expressed as follow:
p( x) =
(log 2 x − ς ) 2 1 exp[− ], (1) 2σ 2 x 2π σ ln 2
Where p (x) is the probability density function. We set ς = 11.61 and σ = 2.26 [2] for the mean file size of 10.7 [KBytes]. B. Traffic Model In our study, the arrival process of a new service request of terminal is assumed to be Poissonic. The system holds a new service request of each terminal in a queue, and all of service requests that are held in the queue are processed according to the order of FIFO (First In First Out). In the downlink-heavy service, the uplink is used to transmit ACK (Acknowledgement) signal, which is transmitted by a fixed transmission rate of 8 [Kbps], and the downlink is used to transmit information data. If a time slot of both uplink and downlink to communication can be assigned to a terminal, the service is allowed to begin. The processing time of each service is determined by the transmission rate for downlink and the file size to be transmitted. The service request of a terminal that cannot be assigned to time slots is continuatively maintained in the queue. For each active terminal, the system monitors the quality of each signal in service continuously, and adjusts them to use the time slots where the required quality of signal can be maintained. The active terminal that cannot maintain the signal quality is forced terminated, and its service request is held into the queue to wait to be processed again. C. Power Control and Beamforming The power control is am important system requirement for SDMA system to compensate for inequality of data quality among near-end and far-end terminals. In our proposed system, we employ a closed-loop power control scheme for both uplink and downlink, where the total transmission power in the network is adjusted to a minimum, where the signal quality in each link is maintained above a required value. For uplink, that is,
J 1 = ∑ P k , Subject to Γ k ≥ γ k
(1)
k =1
J 1 is the total transmission power of all active terminals in the network, Pk is the transmission power of Where
each terminal,
Γk is the SINR of each link and γ k is the
desired SINR threshold of each link. The proposed scheme converges to the optimal solution by the following iteration: 1) The base station calculates the SINR in each link with a current power allocation and beam pattern, and then compares the result with desired SINR threshold. 2)
The transmission power of each active terminal is adjusted so as to satisfy the desired SINR threshold.
3) Repeat the above steps 1) and 2). The above algorithm starting from an arbitrary transmission power allocation converges to the optimum power allocation and the optimum beam pattern for the entire network [6]. Due to the asymmetry of uplink and downlink, we cannot use the beam pattern for uplink as the transmission beam pattern for downlink. For downlink, we use virtual uplink to calculate the transmission beam pattern. The time slot allocation of virtual uplink is identical with that of downlink. We firstly calculate the optimum power allocation and the optimum beamforming vector for virtual uplink by the closed-loop power control. Then we use the optimal beamforming vector for virtual uplink as the transmission beamforming vector for downlink, and the transmission power allocation at base stations is adjusted through the downlink power control iteration according to the steps of the uplink power control scheme. Through the closed-loop power control and beamforming, the minimum power allocations for both uplink and downlink in the entire network are obtained since each signal is transmitted with minimal power that can maintain the required signal quality. Therefore, the system capacity is maximized. III. MULTI-SLOT ASSIGNMENT ALGORITHM In this section, we describe how to assign available time slots to a new terminal for downlink in the proposed system, where each terminal is assigned to one available time slot for uplink and assigned to all of available time slots for downlink. A. Priority of Time Slots After finding an available time slot that can be assigned to a new terminal for uplink, the system examines the possibility assigning time slots to the new terminal for downlink. In our proposed multi-slot assignment algorithm, the system will examine all of free time slots by the order of
current priority of time slots. We present an efficient way to decide the priority, where the time slot that has more free spatial channels is given to a higher priority. As shown in the Figure 1, thanks to the use of SDMA, a maximum of L spatial channels can be provided in each time slot (L is the number of adaptive array antenna elements at each base station). For example, if there are K (K < L) signals in a time slot, the number of free spatial channels in the time slot is (L−K). S pace
diverse wireless communications systems. The simulator can simulate complex multi-cell and multimedia environments, taking account of the channel conditions such as path loss, shadowing, and multipath fading, interference among users, employed transmission format, QoS of each link, and so on. In this paper, the performance of the proposed system is evaluated by the MONSTER and compared with the result of a conventional system with a single time slot assignment. A. Network Structure and Simulation Parameter
T im e F requency
Figure 1 Channels of SDMA/TDMA System The system will search for all empty time slots by the order of this priority to examine the possibility assigning them to the new terminal for downlink. This algorithm can maintain the free spatial channels equably in each time slot for downlink, so that the maximal transmission rate can be provided. B. Procedure of Time Slot Assignment In the proposed system, we employ the time slot assignment algorithm of [3] for both uplink and downlink where a time slot is assigned to a new terminal only if the required signal quality can be maintained not only for the new terminal, but also for the active terminal in the same time slot. The procedure of examining the possibility that a time slot can be assigned to a new terminal is as follow:
: base station : signal
Table 1 Example of a table and its caption.
2) Calculate the interference and beamforming with power control
4) Assign the time slot to the new terminal only if all of results of estimated SINRs are above the required SINRs For uplink, the above steps are repeated to find a time slot that can be assigned to the new terminal. For downlink, all of time slots with free spatial channels are examined by the order of priority, and the terminal is assigned to all of available time slots for downlink in the proposed system. IV. COMPUTER SIMULATION In our works, we analyse system performance by a simulator MONSTER (Multimedia mObile Network Simulator for Test and Evaluation of Radio resource managements) that we have developed in order to simulate
: inter cell interference
Figure 2 Network structure
1) Search for a time slot with free spatial channel
3) Calculate the estimated SINRs of both the new terminal and the active terminals in the same time slot
: terminal
Cell strunctrue
Multiple cells (7 cells)
Number of time slots / frame
Uplink: 3 ; downlink: 9
(the total is 12 slots)
Uplink: 4 ; downlink: 8
Transmission rate per time slot
8 [Kbps]
Path loss factor
3.5
Shadow fading
Log-normal distribution
Fading
Rayleigh distribution
Terminal distribution
Uniform distribution
Call arrival
Poisson distribution
File size distribution
Log-normal distribution
In order to evaluate the performance of the proposed multi-slot assignment scheme, we simulate a TDD-
SDMA/TDMA system in a multi-cell wireless network as shown in Figure 2. A base station is placed at the center of each cell and equipped with a 3-element circular adaptive array with element spacing of half wavelength. The frequency band is reused by fixed channel allocation (FCA) with reuse factor 3 [7]. The packet-switched services are provided, which result in asymmetrical traffic. Table 1 lists some simulation parameters. Figure 3 shows the flowchart of the simulation procedure.
verified that the proposed system has significantly better performance than the conventional system with the same total number of uplink and downlink time slots.
A new service request generation
Enter the queue
Respond to the service requests in queue with order of
Figure 4 Average delay
Examine the possibility providing the time slot (both uplink and downlink) No
C. Average Transmission Rate
A time slot for uplink and no less than one time slot for downlink are available?
We also evaluate the average transmission rate, which is defined as the ratio of the total of transmitted data file size in all processed services to the required time. Figure 5 shows that the proposed system has much higher average transmission rate compared with the conventional system.
Yes In service
Monitor the signal quality of each link Yes
The signal quality is maintained above the required threshold? No Intra-cell handover is available? Yes New channel reassignment
No No
A time slot for uplink and no less than one time slot for downlink is held? Yes The processing time is
Figure 3 Flowchart of simulation procedure B. Average Delay For the packet-switched service, the average delay of “How long does it take to finish a service from the service generation? ” is important to each user. Therefore, the performance of the system is evaluated by computer simulation in terms of the average delay that is defined as the average time from a service request generation to the service termination. This includes the waiting time when the service request is held in queue and the processing time when the data file is transmitted. Figure 4 shows the average delay of the proposed systems and of the conventional system with symmetric time slot assignment, where the total number of time slots for uplink and downlink is 12 in each system. As shown in Figure 4, the average delay is significantly reduced in the proposed system compared with the conventional system. Moreover, among the proposed systems, the average delay is reduced along with the increase of the number of time slots for downlink, though the total number of time slots is the same. It is clearly
Figure 5 Average transmission rate V. CONCLUSIONS This paper presented a TDD-SDMA/TDMA system with a multi-slot assignment for downlink in packet-switched wireless network. The proposed system can assign more than one time slots for downlink to each terminal so as to deal with the asymmetric traffic in packet-switched services. In the proposed system, we presented an efficient algorithm to decide the priority of time slots for downlink, by which the maximum of potential transmission rate for downlink can be obtained. We simulated a multi-cell wireless network where packetswitched services were provided. The performance of the proposed system was evaluated by computer simulation in terms of the average delay and the average transmission rate
and compared with those of a conventional system with a symmetric time slots assignment. Numerical results showed superiority of the proposed system over the conventional system. The proposed system makes use of channel resource more effectively, so that better performance can be achieved in the asymmetric traffic network. REFERENCES [1]
[2]
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[7]
D. Tanaka, T. Ogane, and Y. Ogawa, “A criterion of channel assignment for SDMA with an adaptive array,” IEICE Trans., vol.J82-B, no.11, pp.2133-2149, Nov.1999. F. Piolini and A. Rolando, “Smart channel-assignment algorithm for SDMA system,” IEEE trans. Microwave Theory & Tech., vol.47, no.6, pp.693-699, June 1999. Y. Hara, T. Nabetani and S. Hara, “Time Slot Assignment for Cellular SDMA/TDMA Systems with Antenna Arrays,” Proceedings of IEEE Vehicular Technology Conference (VTC) 2001-Spring, Rhodes, Greece, in CD-ROM, 6-9 May 2001. Y. Hara, T. Nabetani and S. Hara, “Performance Evaluation of Cellular SDMA/TDMA Systems with Variable Bit Rate Multimedia Traffic,” Proceedings of IEEE Vehicular Technology Conference (VTC) 2001Fall, Atlantic City, USA, in CD-ROM, 7-11 October 2001. M. Nabe, K. Baba, M. Murata, and H. Miyahara, “Analysis and Modeling of WWW Traffic for Designing Internet Access Network,” IEICE Trans., vol.J80-B-I, no.6, pp.428-437, June 1997. F. Rashid-Farrokhi, L. Tassiulas, and K. J. R. Liu, “Joint optimal power control and beamforming in wireless networks using antenna arrays,” IEEE Trans. on Commun., vol.46, pp.1313-1324, Nov. 1998. I. Katzela and M. Naghshineh, “Channel assignment schemes for cellular mobile telecommunication systems: a comprehensive survey,” IEEE Personal Communications, pp.10-31, June 1996.
