3.3 Relay Selection Rules And Time-Slot Allocation Schemes
3.3.2 Time-Slot Allocation Schemes
Rbrm , if SignalRS > SignalBS
Rbm , Otherwise (3.11)
3.3.2 Time-Slot Allocation Schemes
In this thesis, we also consider two time-slot allocation schemes: equal time-duration and equal user-throughput allocation schemes.
• Equal Time-Duration (ETD) Allocation:
In this scheme, each user is allocated with the same fraction of time for data transmission no matter data are transmitted directly from BS or by two-hop com-munication. As depicted in Fig. 3.2, we consider a cell with N users. The effective transmission rate for each user is Re(n). Since all the user evenly share the radio resource, the system capacity are given
C = PN n=1
Re(n)
N . (3.12)
• Equal User-Throughput (EUT) Allocation:
The main concept of this scheme is to allocate the time-slot so that all the users have the same throughput. Clearly, this scheme can achieve the fairness for each user in terms of throughput. Suppose that during a time interval, each user sends a packet with the same size P as shown in Fig. 3.3. The total transmission time for
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Figure 3.2: Frame structure of cell for the equal time-duration allocation scheme with N users
N packets is equal to PN
n=1 P
Re(n). Therefore, the system capacity can be expressed as
C = T otal transmitted data bits T otal transmission time
= N · P PN n=1
P Re(n)
= N · ( XN n=1
1
Re(n))−1 . (3.13)
Figure 3.3: Frame structure of cell for the equal user-throughput allocation scheme with N users
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CHAPTER 4
Capacity Enhancement by Optimal Relay Location
In this chapter, we compare the achieved system capacity according to the throughput-oriented and signal strength-throughput-oriented relay selection rules. In the numerical results, we assume that there are a few RSs in a cell and the users are uniformly distributed in the cell. We consider the seven modulation coding schemes (MCSs) in the IEEE 802.16 standard, as shown in Fig. 4.1. The figure shows that the transmission rate and the adopted modulation coding schemes are determined according to the sepa-ration distance between transmitter (BS/RS) and receiver (RS/MS). Table. 4.1 lists the required SINR and net data rate for the seven MCSs [21]. Simulation parameters are listed in Table. 4.2. We can estimate system capacity with the outage probability is 0.1.
Fig. 4.2 shows the flow chart, for finding the optimal relay location. From the flow chart, we can obtain some interesting numerical results as follows:
Figure 4.1: The transmission range of the modulation coding schemes in a cellular network.
Table 4.1: The Required SINR And Net Data Rate With Different Modu-lation Coding Schemes
MCS Modulation Code Rate SINR Net Date Rate
1 BPSK 1/2 0.0 dB 1.29 Mbit/s
2 QPSK 1/2 2.5 dB 2.59 Mbit/s
3 QPSK 3/4 6.0 dB 3.88 Mbit/s
4 16-QAM 1/2 9.0 dB 5.18 Mbit/s
5 16-QAM 3/4 12.0 dB 7.77 Mbit/s
6 64-QAM 2/3 16.0 dB 10.37 Mbit/s
7 64-QAM 3/4 21.0 dB 11.66 Mbit/s
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Table 4.2: Essential Simulation Parameters
Parameter Value
BS transmit power (Pt) 40 dBm
RS transmit power (Pt) 37 dBm
Noise power (N0) -102 dBm
Outage requirement 0.1
BS radius (R) 1750 m
BS antenna height (ht) 30 m
RS antenna height (ht/hr) 15 m
BS/RS antenna Omni-directional
MS antenna height (hr) 2 m
Carrier frequency (fc) 3.5 GHz
System bandwidth (BW ) 3.5 MHz
Standard deviation (σ) 8 dB
Modulation coding scheme BPSK 1/2, 4-QAM 1/2 and 3/4,
(MCS) 16-QAM 1/2 and 3/4, 64-QAM 2/3 and 3/4
Figure 4.2: Flow chart for determining the optimal relay location.
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(a)
(b)
Figure 4.3: Achieved system capacity according to various relay selection rules, for (a) the equal
4.1 Impact of Relay Selection Rule on System Ca-pacity
Fig. 4.3(a) shows the achieved system capacity versus the separation distance between BS and RS, where the equal time-duration allocation scheme is used. Obviously, the throughput-oriented two-hop transmission can achieve higher system capacity. Fur-thermore, if employing the throughput-oriented relay selection rule, there exists an optimal relay location to maximize the system capacity. In this example, the opti-mal relay location is at 1273 meter away from the center of a cell. Compared with the signal strength-oriented rule, the system capacity of the optimal relay location increases 5.5%. Compared with the average value of throughput-oriented rule, the system capacity of the optimal relay location is 12.6% higher. Compared with the network without RS, the system capacity with the optimal relay location improves 25.4%. In addition, this figure also shows that the signal strength-oriented two-hop transmission may yield lower system capacity than the one-hop transmission at some locations, where at most 29.8% capacity reduction is possible. Compared with the average value of signal strength-oriented rule, the system capacity of the optimal relay location increases 18.5% . This phenomenon is due to the fact that transmis-sion through a relay station requires two transmistransmis-sion phases. By using the signal strength-oriented relay link selection, some users exploit the two-hop communications to improve link reliability at the cost of lower capacity.
Fig. 4.3(b) illustrates the system capacity for the equal user-throughput al-location scheme. In the figure, one can see that the optimal relay al-location is at 1305 meter according to the throughput-oriented relay selection rule. Compared with the signal strength-oriented rule, the system capacity of the optimal relay location is 5.1% higher. Compared with the average value of throughput-oriented rule, the system capacity of the optimal relay location improves 31.3%. Compared with the
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network without RS, the system capacity of the optimal relay location is increased by 64.3%. In addition, it is also shown that the signal strength-oriented two-hop transmission causes lower system capacity than the one-hop transmission at some lo-cations. At most 8.2% capacity degradation occurs. But compared with the average value of signal strength-oriented rule, the system capacity of the optimal relay loca-tion improve 18.5%. From Fig. 4.3(a) and Fig. 4.3(b), it is also shown that the equal user-throughput allocation scheme results in a lower system capacity. According to the equal user-throughput allocation scheme, the user with lower transmission rate will be allocated with more radio resource to achieve the same throughput for each user, thereby lowering the overall system capacity.