2011 17th Asia-Pacific Conference on Communications (APCC) 2nd – 5th October 2011 | Sutera Harbour Resort, Kota Kinabalu, Sabah, Malaysia
A Novel operating and backup channel allocation scheme in IEEE 802.22 systems Seung-Hoon Hwang, Senior Member IEEE and Cha-Eul Jeon Div. of Electronics and Electrical Engineering Dongguk University-Seoul, Korea E-mail:
[email protected] the protection of the incumbent user, the MAC layer needs to consider the following techniques. Firstly, it includes an efficient spectrum sensing scheme for the primary user and a sensing report to the system. Secondly, it also includes new design of the MAC layer and frame structure. Finally, it covers self-coexistence schemes, interference avoid skills among the CPEs, and channel management schemes for the incumbent user.
Abstract—In this paper, novel operating and backup channels allocation schemes for IEEE 802.22 system are introduced and their performances are investigated. Interference level and the probability of appearance for incumbent user are taken into account in order to optimally update the backup channels list. Additionally, the operating channel is allocated based on Carrier to Interference and Noise Ratio (CINR) so as to meet Quality of Service (QoS) requirement. Simulation results illustrate that performance is significantly enhanced by employing the proposed channel allocation schemes in the IEEE 802.22 system.
Multiple 802.22 BSs and CPEs may operate in the same vicinity and unless appropriate measures are taken at the air interface level, self-interference may render the 802.22 system useless. The MAC layer addresses the self-coexistence using a mandatory mechanism including two elements such as spectrum etiquette and on-demand frame contention (ODFC) [5]. The channel selection in a cell follows the spectrum etiquette rule so that the chosen channel may be interferencefree or interfered with minimum number of channels. The available channels are classified in [5] as follows. Operating channel is the current channel used for communication between BS and CPEs within the WRAN cell. Backup channels are the channels which have been cleared to immediately become the operating channel in case the WRAN needs to switch to another channel. Candidate channels are the channels that are candidates to become a backup channel.
Keywords-IEEE 802.22, WRAN, operating channel, backup channel, channel allocation.
I.
INTRODUCTION
For next generation mobile communication, the demand for wider bandwidth is substantially increased in order to support high-speed data services. Meanwhile, the resource of frequency spectrum is limited because most of radio spectrum has already been allocated. Furthermore, since new wireless access and communications technologies appear continuously, it has been very attractive to efficiently utilize the frequency spectrum. According to the Federal Communications Commission (FCC) spectrum policy task force report, the usage of allocated spectrum varies 15% to 85% depending on temporal and geographic situation [1], [2]. Therefore, Cognitive Radio (CR) technology has been introduced in order to use the spectrum more efficiently.
The channel classification is described in the specification of [1]. However, the specific algorithm for selection the operating channel and defining how the backup and candidate channels are prioritized is outside the scope of the standard since it is implementation issue. Therefore, in this paper, the specific channel allocation schemes for both operating and backup channels are proposed and their performances are investigated in terms of probability as well as throughput.
The CR technique is included in the specification of IEEE 802.22 wireless regional area networks (WRAN), where TV spectrum band between 54 MHz and 862MHz is used to provide broadband wireless internet access in rural areas without interference to licensed TV broadcasting. The CR was initially proposed by Mitola in [3] and [4], where the spectrum utilization was significantly improved by allowing the unlicensed 802.22 (secondary) users to access the unoccupied spectrum band for an incumbent (primary) user in a certain location and time without interfering with incumbent user. The incumbent users refer to the TV broadcasting listeners or Part 74 devices (wireless microphones). The unlicensed wireless users refer to IEEE 802.22 entities such as base station (BS) and Consumer Premise Equipments (CPEs).
II.
A. Spectrum etiquette [5] This section describes the spectrum etiquette in the specification of IEEE802.22. Figure 1 illustrates the procedure of spectrum etiquette. The central cell has six neighbor cells. The cell has one operating channel (bold), one backup channel (underlined), and candidate channel (double underlined). The underlined number in parenthesis (bold) indicates the channel occupied by the incumbent user. Two incumbents appear in the central cell on the operating and backup channel, and thus the central cell is forced to change both its operating and backup
In order to share wireless channel resources, both PHY and MAC layers for the IEEE 802.22 are specified to coexist with the incumbent systems. The MAC layer protects the primary user and at the same time communicates WRAN signals. For
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PROPOSED CHANNEL ALLOCATION SCHEMES
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channel. As a result of incumbent discovery, the spectrum etiquette is triggered. The former candidate channel 7 is promoted to the operating channel, and channel 5 is promoted to backup channel, which is only used by one neighbor cell as the operating channel. All the neighbor cells perform spectrum etiquette after receiving the channel set update information from the central cell accordingly. The table 1 illustrates the resultant channel sets for the central cell. The backup channel set is determined by one or multiple channels from Local Priority Set 1; If Local Priority Set 1 is empty, update the backup channel set from Local Priority Set 2 which are backup channels used by the least number of neighbor cells. If Local Priority Set 2 is also empty, choose channels from Local Priority Set 3 which are operating channels occupied by the least number of neighbor cells.
I / N = ∑ 10
cn _ edge 10
×d
− alpha i
× 10
sigma × randn 10
i
where ‘cn_edge’ is the carrier to noise ratio at the cell edge, ‘alpha’ and ‘sigma’ are constant values to calculate path loss and shadowing respectively, ‘randn’ is a random number which has normal distribution of which mean is zero and standard deviation is 1. The measured CINR is used to decide whether the operating channel is allocated to the users or not. In other words, only if the measured CINR meets a required CINR of user which depends on the Quality of Service (QoS) level, then the operating channel is allocated to the user.
Figure 1. The procedure of spectrum etiquette
TABLE I.
Figure 2. CINR measurement
CHANNEL STATUS FOR CENTRAL CELL
Central Cell Status
After Spectrum Etiquette
Before
Operating Channel Set
1
7
Backup Channel Set
2
5
Candidate Channel Set
7
-
WRAN-Occupied Channel Set
3,4,5,6,8
3,4,5,6,8
Neighbour Backup Channel Set
4,5,6,7,8
4,5,6,7,8
Local Priority Set 1:
2
-
Local Priority Set 2:
2,7
-
Local Priority Set 3:
3,4,5,6,8
3,4,5,6,8
Figure 3. Example of backup channel allocation
Figure 3 shows the example of backup channel allocation when the backup channel is updated based on the interference level which is measured by equation (1). The backup channel in the central cell needs to be updated because there is no backup channel. We assume that there are four available channels. Therefore, the backup channel can be chosen among channels 2, 3, and 4 which are occupied by the same number of neighbor cell users. Thus, each channel’s CINR is measured and then the best channel is allocated. In the figure, channel 4 is selected as backup channel because the users occupying channel 4 in the neighbor cells are relatively far from the BS in the central cell.
B. Proposed operating and backup channel allocation schemes Figure 2 shows the example of Carrier to Interference and Noise Ratio (CINR) measurement. We can consider the CINR is calculated when the operating channel is assigned using the following equation.
CINR =
C 1 1 (1) = = −1 N + I N / C + I / C (C / N ) + (C / I ) −1
C / N = 10
cn _ edg 10
× d −alpha × 10
An example of cell status is shown in Table 2, when the appearance probabilities (number in parenthesis) of the incumbent user are provided to the backup channels in Local Priority Set 1 and Local Priority Set 2. Additionally, the measured interferences (number in parenthesis) are given to the backup channels in Local Priority Set 3. The reason why the
sigma×randn 10
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The performances are investigated in terms of forced termination probability, system throughput and drop probability. Let NF denote the number of forced termination, NC denote the total number of channel allocation call, and NB indicate the number of block. The forced termination probability (FTP) is defined as
criterion is different in Local Priority Set 3 is that the channels in Local Priority Set 3 are occupied by the neighbor cells. For example, when one backup channel is needed after spectrum etiquette, channel 8 with lower appearance probability is chosen out of channel 8 and 9 in Local Priority Set 2. If three backup channels are needed, then two channels 8 and 9 from Local Priority Set 2 are chosen and then channel 7 from Local Priority Set 3 is selected because of lowest interference level. TABLE II.
AN EXAMPLE OF CELL STATUS
Central Cell Status
Before 1
4
3
Backup Channel Set
3, 4
-
8 or 8, 9, 7
2, 5, 6, 7
2, 5, 6, 7
2, 5, 6, 7
5, 6, 7, 8, 9
5, 6, 7, 8
5, 6, 7, 8, 9
3(0.6), 4(0.3)
-
8(0.5)
8(0.5), 9(0.7)
-
2(15), 5(4), 6(12), 7(1)
2(15), 5(4), 6(12), 7(1)
4
3
Local Priority Set 1: Local Priority Set 2: Local Priority Set 3: Operating Channel Set
3(0.6), 4(0.3), 8(0.5), 9(0.7) 2(15), 5(4), 6(12), 7(1) 1
III.
NF NC − N B
(2)
The system throughput (T) is defined as
After Backup channel allocation
After Spectrum Etiquette
Operating Channel Set WRAN-Occupied Channel Set Neighbor Backup Channel Set
pFTR =
T=
NC − N F − N B × 100 NC
(3)
The forced termination occurs for three cases. The first case is that there are no available channels for 802.22 users, when the 802.22 user should switch the operating channel to one of backup channels due to the incumbent user. The second case happens when the user consecutively loses several times at the contention. The third case is that there are no backup channels to meet the CINR requirement. The block occurs when there are no available channels for 802.22 users and when there are no backup channels to meet the CINR requirement. The block number is only taken into account when the 802.22 user initially request an operating channel.
NUMERICAL RESULTS
Simulation parameters are listed in Table 3, where there are nineteen cells and each cell has two backup channels and two incumbent users at maximum. Average call arrival rate, average incumbent user arrival rate, and average call holding time is 100 times/hour, 50 times/hour and 60 seconds, respectively. Total number of channels is assumed to be ten to fourteen. The number of consecutive fail is defined to consider the case that is terminated by force. The conditions such as the incumbent and 802.22 user’s status are checked every 0.01 second. TABLE III.
SIMULATION PARAMETERS
Simulation Parameters
Values
Average call arrival rate
100 times/hour
Average incumbent user arrival rate
50 times/hour
Average call holding time
600 second
Number of cell
19
Number of channels
10~14
Number of maximum incumbent user per cell
2
Number of backup channel per cell
2
Time step of condition check
0.01 second
Number of consecutive fail (NCF)
16
Figure 4. FTP (CINR Threshold= 5dB)
The FTR performances for various numbers of channels are shown in Fig. 4, when the CINR threshold is 5dB. The proposed backup channel allocation schemes are compared with the conventional IEEE 802.22 scheme. It is observed that the proposed channel allocation based on the interference, probability, or their combination improves the FTP performance, regardless of the number of channel. Furthermore, it is shown that performance gain by the probability based channel allocation is more significant than that by the interference based channel allocation, since both
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NF and NB are further decreased by the probability based channel allocation in Eq. (2). Note that the performance of the proposed channel allocation is the best among them. Figure 5 presents the throughput performance when the CINR threshold equals to 5dB. The performances are improved by the proposed channel allocation schemes. For example, the proposed channel allocation may achieve the throughput gain of 2% when the number of channel is ten. Note that the throughput gain by the interference based channel allocation is higher than that by the probability based channel allocation. This is because the throughput increases, as both NF and NB are further decreased in Eq. (3).
Figure 5. Throughput (CINR Threshold=5dB)
IV.
CONCLUSION
In this paper, the novel operating and backup channel allocation scheme have been proposed to optimally update the backup channels and efficiently allocate the operating channel in IEEE 802.22 systems. Numerical results show that the proposed channel allocation based on the interference, probability, or their combination improves the FTP and the throughput performance, regardless of the number of channel. REFERENCES [1] [2]
[3]
[4]
[5]
FCC, "Notice of proposed rule making and order", ET, December 2003, Docket No 03-222. C. M. Cordeiro, K. Challapali D. Birru, and N. S. Shankar, “IEEE 802.22: The First Worldwide Wireless Standard Bsed on Cognitive Radios,” IEEE DySPAN, Nov. 2005. Joseph Mitola III, "Cognitive Radios: Making Software Radios More Personal," IEEE Personal Communications, vol. 6, Issue 4, pp. 13-18, August 1999. Joseph Mitola III, “Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio” Ph.D. dissertation, Royal Inst. of Technol., Stockholm, Sweden, 2000. IEEE P802.22/DRAFTv2.0 Draft Standard for Wireless Regional Area Networks Part 22 : Cognitive Wireless RAN MAC and PHY.
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