Target SINR & interfered threshold effect

In this part, the user number of each femtocell is three, and every femtocell has 12

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subchannels. Users have the same minimum transmission rate requirement, 128kbps, which is a general requirement of data traffic. There are 3 kinds of system performance evaluated to show the difference between methods.

5.1.1 System throughput

Figure 17 System throughput variation with target SINR or interfered threshold values.

The system throughput ratio indicates the system throughput improvement compared to the limited power control methods. The formulation is as following,

(6)

From the result in Figure 17, for the limited power control method, when the

“target SINR” value is lower than the minimum data transmission threshold, it makes the interfering users reduce their serving power until below the transmission threshold that whichever interfering or interfered users are unable to receive the data, so the system throughput reduces about 5%. For the “target SINR” value is higher than the minimum data transmission threshold, the system throughput seems at most only 1%

improved if the proper “target SINR” value is chosen. This result is because the

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decreasing throughput of good channel condition users. If the value of “target SINR”

is set too high, there are no any good channel condition users can match the power control requirement, and no power adjustment happens. The figure also shows that proposed method, no matter what kind of modes, brings about 19% improvement of system throughput if proper value of “interfered threshold” is selected. The system throughput increases with the parameter value below 8dB, but when the value becomes higher than 8dB, the system throughput starts to decrease. The reason is that higher “interfered threshold” value makes more orthogonal channels helping poor users, but when value is above 8dB, more users which don‟t interfere with each other are allocated in orthogonal channels decreasing the throughput of good users. For the channel selection modes, the mode counting interfering number get a little bit better system throughput when “interfered threshold” value is about 8dB because of utilizing the less occupied subchannels, but it decreases fast when “interfered threshold” value goes bigger than 8dB for the reason that poor users are mislead by the count of interfering times but don‟t notice which are the real interference. In general the strongest interference decides the channel conditions of users here. The maximum interference mode has better performance than random selection method even when the “interfered threshold” value is too high. Power adjusted mode provides a little improvement on system throughput because the channel quality is improved.

5.1.2 Outage probability

Figure 18 Variation of user outage rate with target SINR or interfered threshold values.

0

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The outage probability is calculated with the number of users who can‟t reach their requirement of transmission rate divided by total number of users. Following is its formulation.

(7)

Figure 18 shows that original control method has about 32% outage rate, and the best of limited power control method is about 24% outage rate, only 1/4 of the original outage user saved. But both are above the 10% outage rate which is the requirement of standard systems. The “target SINR” value could lead to a misunderstanding of capability of saving users when value is low and underestimate the number of users could be rescued when value is high, so the outage rate increases when the “target SINR” value is lower than 4dB, and the outage rate is back to the original when the “target SINR” value is high. Proposed method makes the user outage rate become only about 4% at “interfered threshold” value 6dB, and this result proves the ability of orthogonal subchannel assignment of mitigating interference. For the channel selection mode by the interference count, the graph shows that outage rate not only slightly higher when low “interfered threshold” value which is intuitive because of underestimate the impact of interference, but also when “interfered threshold” value is high for the reason that the femtocells include more interfering sources into consideration and neglect the weighting of the strongest interference effect. The maximum interference mode doesn‟t increase the user outage rate with

“interfered threshold” value. The power adjustment helps little on user outage rate because the orthogonal subchannel partition has done the interference mitigation.

5.1.3 Power consumption

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Figure 19 Power consumption ratio with target SINR or interfered threshold values.

Because the limited power control method and proposed method without power adjustment don‟t fully utilize the power budget of femtocell, this part wants to see the power saving difference between methods. The power consumption of random channel selection method is used as comparison objective. The formulation is:

From the Figure 19, although the limited power control method uses the power reduction to mitigate interference, it saves only 8% power when the “target SINR”

value is 3dB where the method decreases the system throughput and doesn‟t help reducing the user outage rate. For the proposed method without power adjustment, the higher “interfered threshold” value brings more power saving effect. But considering the other two effects before, the value of “interfered threshold” about 8dB should be chosen. At “interfered threshold” value 8dB, both of the channel selection modes use 68% of original system power consumption. This part shows that proposed method without power adjustment has a little less system throughput improvement compared to the proposed method with power adjustment but provides about 32% power saving.