2 Related Work 7
4.7 System Flow chart 27
Fig17. System flow chart of proposed CR MAC
Fig.18. The association procedrue of SUs
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Fig19. Specturm sensing and sensing report procedure
Fig20. Resource request issue and data transmission procedure
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5. Throughput Analysis
We firstly base on the proposed CR MAC architecture to analysis the CR network throughput. Futher, we extend the throughput analysis equation of WIFI network include the impaction of SUs. That, we derivate the throughput equation from the throughput analysis of “ Performance Analysis of IEEE 802.11 Distribution Coordination Function”. The author proposes throughput analysis for saturation WIFI user throughput. Finally, we base on the throughput influence threshold of WIFI network and the accuracy threshold of spectrum sensing evalution algorithm to calculate the maximum system throughput that includes WIFI network and CR
R: the number of acquiring resource request in this SUPERFRAME
: the number of frame per data period
: the data rate of user j on the channel i : the number of sensing samples of channel i
( ) : the accuracy bases on the sensing time and sensing report number
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, j = 1,2,…,K
∑ ≤ , Then, CR Throughput can be written as below:
( ) ∑ ( ) ∑
M: the number of WIFI users in the WIFI network K: the total number of SUs in the network
k: the number of SUs in the WIFI network
r: the transmit probability of user in the WIFI network Ts: the transmit time of data packet
Tas: the transmit time of associated packet
Tc: the collision time caused by users data transmission at the same time E[P]: the expected user data packet size
: the slot time
: the probability that at least one user transmits his data : the probability that only one WIFI user transmits his data : the probability that only one SU transmits his data
: the system throughput of WIFI network with k SU in the network : the system throughput of WIFI network
In the following, we would introduction how to calculate the WIFI throughput.
We assume there exist some WIFI users and both of them always have date to transmit (saturation) in the network. Then, there also exist some CR users that would send association request in the network. We derivate the throughput equation from the throughput analysis of [13] that the author proposes a throughput analysis equation for saturation WIFI users throughput.
Then, the WIFI throughput can be written as below:
= ( )
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= ( )
= ( )
( )
= ( )
= ( )
( )
= (
) ( )
The WIFI throughput is according to the probability of that there are k SUs in the WIFI network
= ∑ (
) (
)
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5.3
System Throughput
Notation
M: the number of WIFI users K: the number of SUs
N: the number of data channels
: the throughput influence threshold for WIFI network : the accuracy threshold of spectrum evaluation algorithm α: the sensing time to acquire one sample.
g: the number of channel and SU group : the number of SUs in the group
: the number of sensing channels in the group : the number of available TVB channels
m: the number of sensing sample per data channel s : the probability of report error
: the duration of DCF mode
: the duration of PCF mode
: the maximum duration of PCF mode
: the residual duration of PCF mode
: the maximum number of polling SUs in PCF mode
: the minimum number of polling SUs in PCF mode
: the maximum coexistence number of polling SUs in PCF mode W( ,M): the throughput of WIFI network
( ): the throughput of cognitive radio network z: the throughput of all the networks
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Problem description:
We try to find the maximum throughput of CR network and WIFI network, and there is a tradeoff between CR throughput and WIFI throughput because of the PCF mode duration. The other tradeoff is between the channel sensing time and sensing report time. If more users sense the channels simultaneously, it would decrease the QP duration but increase the CP duration. Because the sensing time per sample (10ms) is greater than report time per user (500us), we have to decrease the QP duration as possible to acquire maximum throughput.
Assume:
We assume that we can query cloud for the number of sensing sample per channel m, the number of sensing channel , and the number of SU in every group. Without loss the generality, we also assume the number of sensing channel is less than the number of user in every group and the maximum number of polling SUs in PCF mode would less than the number of SUs per group:
Calculate Procedure:
We first calculate the maximum duration of PCF mode and the system throughput that is based on the influence threshold .Then, decrease the maximum duration of PCF mode, calculate the new system throughput, and check if increase the throughput or not till the duration is equal to zero. Finally, we can acquire the maximum system throughput and the duration of every period that is based on the sensing accuracy threshold and throughput influence threshold .
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Fig21. flow chart of calculating procedure Input: , , α, N, K, M, , T
1) query cloud for the sensing samples per channel m, the number of sensing channels , and the SU groups by
2) the WIFI throughput
(M, K ) = ∑ (
) (
)
3) calculate the maximum duration of PCF,
, and T =
4) calculate the maximum polling number, and
=
⌊ ⌋ If ≥m, go to (5)
Else if m, = or decrease , go to (5)
35 Else, find the minimum number of polling users
⌈
36
6. Analysis Result
In this section, we would show the analysis result. We divide into three parts (1) SU coexistence (2) WIFI throughput with SUs (3) System throughput. The duration of SUPERFRAME is 400ms. The required sensing sample per channel is 5 to achieve the accuracy 0.9. The transmission rate is 11Mbps, basic rate is 1Mbps, and the average packet size of WIFI user is 1500 bytes. The TVB channel bandwidth is 6MHz, the channel SNR is 10dB. The packet size of BEACON is 464bits, CF_END is 160 bits, POLL is 160 bits, resource request is 32 bits, and sensing report is 32+sensing channel*4*8 bits.
The following is the residual WIFI parameter setting:
Fig22. 802.11 parameter setting
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6.1 Maximum Coexistence SUs
We first give the value of WIFI user, timeout, and average packet size, and find the maximum coexistence SUs. The analysis result show coexistence number decrease with increasing packet size and increase with increasing timeout.
Fig23. Maximum coexistence SUs with different packet size and timeout Then, we want to know the coexistence SUs with different access scheme of WIFI users. The analysis result show the coexistence SUs in RTS/CTS scheme is smaller than basic scheme, but is large than basic scheme with lots of WIFI users.
Fig24. Maximum coexistence SUs with different access scheme
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6.2 WIFI Throughput with SUs
We want to know when there exist some SUs in the WIFI network, the impact to the WIFI throughput. We first show the traditional throughput that doesn’t exist SUs.
Then, calculate the throughput when give the different value of WIFI users, SUs, and timeout. The analysis result show the impact to WIFI throughput increase when decrease timeout and increase coexistence SUs. But the impact is not obvious to WIFI throughput.
Basic Access
M traditional SU=50,t=1 SU=50,t=10 SU=100,t=1 SU=100,t=10 10 5.82198 5.82154 5.82194 5.8211 5.8219 30 5.07402 5.07388 5.074 5.07374 5.07399 50 4.6889 4.68882 4.6889 4.68874 4.68889
Fig25. WIFI throughput with coexistence SUs by basic access
The throughput impact in RTS/CTS is also not obvious to WIFI throughput RTS/CTS Access
M traditional SU=50,t=1 SU=50,t=10 SU=100,t=1 SU=100,t=10 10 4.70566 4.7053 4.70559 4.7053 4.70559 30 4.48614 4.48602 4.48612 4.48602 4.48612 50 4.35001 4.34994 4.35 4.34994 4.35
Fig26. WIFI throughput with coexistence SUs by RTS/CTS access
Through the analysis result, we know the impact of coexistence SUs is little for WIFI throughput no matter we adopt basic access scheme or RTS/CTS access scheme.
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6.3
System Throughput
We calculate the system throughput in this section. Give M=20, K=20, timeout=10s, packet size =1500 bytes. The report error probability s =0.1, and we want to know the impact to system throughput when we give different value of WIFI throughput impact. When the throughput threshold increases, we can poll more SUs for collecting sensing report. That is, we just need fewer sensing time of QP, and increase the CR throughput. When we increase the impact threshold, we can find the suitable PCF duration for sensing report and request issue.
Fig27. System throughput1
40
Then, Give M=20, K=20, timeout=10s, packet size =1500 bytes, and impact threshold is 0.1. We want to know the impact for system throughput when the report error probability increases. Through the analysis result, we can see that we need to poll more SUs to acquire the require number of sensing report and it would cause more PCF duration. That is, decrease the WIFI throughput. In the other hands, we also need more sensing time of QP for sensing report and decrease the CR throughput.
Fig29. System throughput2
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Then, Give M=20, timeout=10s, packet size =1500 bytes, impact threshold is 0.1, and report error probability 0.1. The sensing channel is 8 and available channel is 1.
We want to know the impact to throughput with different SU number. When SU number increases, more SUs could sense channel at the same time. It needs fewer channel sensing time, and increase the CR throughput.
Fig31. System throughput with different SU number
Fig32. WIFI throughput with different SU number
SU 10 15 20 25 30 35 40
CP(s) 0.05615 0.06707 0.089 0.13288 0.13288 0.13288 0.13288 RP(s) 0 0.01152 0.01347 0.01152 0.01347 0.01541 0.01736
QP(s) 0.05 0.04 0.03 0.02 0.02 0.02 0.02
Fig33. The duration of every period with different SU number
0
42
Then, give K=20, timeout=10s, packet size =1500 bytes, impact threshold is 0.1, and report error probability 0.01. The sensing channel is 8, and PCF duration is 0.011339s. We use the ns-2 simulator to validate the WIFI analysis result. The simulation result doesn’t match the analysis result because the analysis result doesn’t consider the packet transmit error probability and other control message. The simulation result is close to analysis result, so we could consult the result to design our real CR-MAC duration.
Fig34. The comparison with different number of WIFI users
We set the queue length 50 packets, and all WIFI users are saturation. The time of queue delay increase with the number of WIFI users, because more and more user content and use the channel.
Fig35. Queue delay with different number of WIFI users
0 0.5 1 1.5 2 2.5
2 4 6 8 10 12 14 16 18 20
queue delay(s)
The number of WIFI users
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7 Conclusion and Future Work
In this paper, we use the ISM band for control channel realization for CR network, and we analysis the impaction to the WIFI network. We discuss the coexistence problem that includes interval consistence and maximum coexistence user number. The analysis result is close to simulation result, and we can use the analysis result to design the real CR MAC protocol.
In the future, we would discuss more efficient cooperative spectrum sensing and sensing report policy. In the other hand, we also discuss more efficient resource request acquire policy. Then, not only extend the SUs to use the ISM band, but also extend the WIFI users to use the TVWS for the heterogeneous network. That, design a hybrid MAC protocol coexists between CR network and WIFI network.
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