LOGO
Abstract
The aggregated data rate denotes the rate of transmitting an amount of data through the channel in a specific period of time. Figure 4-5 bellow shows the aggregated data rate in channel 1 and channel 2 before implementing the backup channel procedure BCR. Figure 4-6 illustrates the improvement in the aggregated data rate in channel 2 after
implementing the backup channel procedure. The transmission between A and B that take place in channel 1 will be
transfer to channel 2 due to reappearance of primary user in channel 1. This interprets the noticeable increase in channel 2 aggregated data rate. The data size used to simulate this fig is [500 250 150 100] byte with data rate [10 5] Mbps.
However, the plotted data is represented the payload data packets, the control frames overhead is not plotted in this figure because our focus is to compare the amount of transmitted payload
the throughput versus the packet size. The throughput in this chapter is defined as the mean value of the amount of data packets transmitted during a specific period of time. It is calculated as the mean of the total transmitted data; figure 4-7 shows the relation between the throughput and the packet size. A variable packet size form [50 - 250] bytes with two data rate [10 5] Mbps used to simulate this figure, its clear that the throughput increases with the increase of the data packet size till a certain point then it starts to degrade.
the collision rate of the reference protocol without BCR where no backup channel reservation, so when the primary starts sending the data, its data collide with the secondary user’s data transmitted in that channel. In the case of our proposed protocol, we assume that the channel will be vacated before the primary start sending its data, so no collision happened.
Comparing with the reference in [51], spectrum scanning and reserve channel methods for link maintenance in cognitive radio system, the protocol dedicates the reserved channel during the scanning process. This may affect the number of free channels that might be used for establishing a new session, which may be considered as a waste of resources. In our protocol, the sensed free channel could be used to initiate new SUL or for maintaining a suspended SUL. The
protocol in [51] has a reference to the candidate free channel called pointer, the pointed channel could be the nearest available free channel or least interfered channel found during routine scanning. The come out drawbacks of this protocol highlighted when the SU does not have a pointer or when the pointer is not updated. Then the SU temporarily occupy the nearest reserved channel for a short period until new channel is sensed free, then consequently switched to the newly found channel, this make the SU link not stable, due to frequent switching and higher data packet loss during multiple switching. In addition, the protocol obtains higher communication overhead and highly dynamic network conditions. In contrast, our protocol reduces the collision percentage by 0.11.
This protocol is suitable for a large packet size that accesses the channel for long time to accomplish its transmission.
Because the channel is susceptible to primary user appearance, our protocol makes it easy to reserve a backup channel for emergency use.
Results
References Methods and Materials
Figure_1. BCR-MAC protocol packets exchange
Figure_1Throughput vs. Packet size
Figure_1.baggregated data rate before implementing channel reservation in both channel 1 &2
The proposed efficient multiple channel cognitive radio MAC protocol with backup channel reservation (BCR-MAC), to facilitate quick and smoother shift to a reserved channel before the secondary link degrades due to primary user arrival.
The number of the reserved channels is not fixed in advance; it diverges proportionally to spectrum occupancy and interference levels. The reserved channels are distributed over several spectrum bands to facilitate nearer switching of channels and to have least change in reserved channel conditions. BCR-MAC protocol is implemented in decentralized network with common control channel and multiple data channels. The protocol significantly improves the network
performance; keeps the privilege to the licensee and utilizes the available free channel perfectly with less packet
exchange overhead. Although the primary detection time is a critical metric of this protocol, in our protocol we assume that primary arrival time is known in prior, this assumption will be handled in our future work.
Reserving an available idle channel to be used when the secondary link degrades due to the primary arrival or link degradation is significantly mitigates collision, reduces the packet loss ratio and reduces channel-switching time. In addition, using two transceivers, one dedicated to monitor the control channel, significantly alleviates hidden terminal problem. Furthermore, using overhearing mechanism will significantly assist in distributing sensing and network
information. The prominent advantage of this protocol is that, no synchronization is required to implement the protocol.
The shortcomings of the proposed protocol are discussed in details with the appropriate solutions. On the future work, the primary behavior’s study is included to estimate its arrival time. More study for channel selection incorporated additional factors such as physical characteristics and the residual free time. Furthermore, the channel reservation could be used for other application such as reserving the next hop in a multi-hop network.
Conclusions Introduction
A cognitive radio regularly searches for a free channel during initial channel selection and when decides to
evacuate the current channel. Under dynamic channel conditions, every channel is vulnerable to channel degradation or link termination due to PU reappearance. Therefore, the secondary user monitors the channel for a minimum period called detection time to estimate the occupancy nature and ensures the fitness of the channel for selection. Under such scenarios, an active CR user may not have enough time to sense, and access the available free channel during the
transmission. The latency in sensing and detecting the free-channels and use them for SU link maintenance, might cause significant loss in QoS. Since the available number of free-channels may vary considerably in this dynamic shared spectrum, it is difficult to isolate a fixed number of channels as reserved. Hence, To build its status based channel list SCL, each node stores in a local table the state of each channel (busy /idle) including the release time of the currently used channels [37]. In a contention-based algorithm, each node needs to overhear all traffic sent by nearby nodes within its transmission range. Control packets, such as RTS, CTS and ACK, specified in IEEE 802.11, are received by all neighbors within the transmission range of the sender, and then discarded by non-destination
nodes. The overhearing feature of the contention-based algorithms could be used to exchange information between nodes without introducing too much overhead. Since, both RTS and CTS messages in IEEE 802.11 contain the
remaining duration of the current transmission, other active nodes update their NAVs based on it, and they will not try to transmit packets in that channel until the current transmission is completed. Besides the duration of the current
transmission, more information such as backup channel identification could be piggybacked for low overhead
information distribution. In This chapter, we propose a backup channel-based protocol to improve the cognitive radio network performance such as throughput, connectivity and reliability.
Although abundance of work carried out on channel sensing and allocation mechanism to improve spectrum utilization, Solutions for spectrum mobility (the capability of a distributed cognitive radio to scan the spectrum and adaptively switch channels to mitigate secondary user’s link degradation) is less explored to attain its optimum
solution. In this work, we focus on this point and propose reservation methods to compensate the loss in secondary link performance.
Recently, collision has been a major fear in wireless networking. Various MAC protocols proposed to handle this issue are in two categories, either contention-based or reservation-based. CSMA/CA is one of the popular
contention based wireless MAC protocols, which becomes basis to the IEEE 802.11 DCF MAC protocols, that uses a back off timer techniques performed in each node. However, most of MAC protocols still suffer from low
throughput due to high packet loss and collision when network traffic increases. However, in this work, we intended to solve a new type of collision related to the properties of cognitive radio networks, by proposing a backup channel reservation procedure.
This protocol proposed to keep the privileges to primary user and maintain the secondary user link when degrades due to collision with the primary user while reclaiming its channel. It also improves the network connectivity as one of its functionality.
We consider the following assumptions
1) A number of C + 1 non- overlapping channels are available for use, all of them have the same bandwidth. One of the channels is a dedicated as a common control channel (CCC), this channel assumed to be free of PU
interference. The CCC is more preferable to be selected from the unlicensed band to ensure its availability for long time
2) A Decentralized network with nodes equipped with two transceivers, one for monitoring the common control channel, and the other is capable of dynamically switching among multiple data channels.
3) Each node equipped with the sensing capability that enables the node to detect the presence of the primary user, in the same time it could record the running length of the last duration periods in a buffer with a specific length (500 durations). Channel scanning and sensing features are carried-out independently of the current communication, and causing no interrupt for signal transmission or reception.
4) A primary user location, bandwidth and arrival time in the current used channel, are known in prior (arrival time is either statistically estimated due to the primary previous behavior or detected by using energy detection
techniques) with ignorance error probability. Each node, say X, maintains a sensed channel list (SCL) that included the data channels sensed in a specific time and it is regularly updated. Each list entry CL[i] (i = 1, 2, ..., n) has two fields: Statei,X and RleaseTi X. Statei,X indicates whether data channel i is available for node X or not at the
current time.
If channel i is used by X or by X’s neighbors, then, Statei,x = {busy}. Otherwise, Statei,X = {idle}. RleaseTi,X indicates the exact time in which channel i will be released by node X or by X’s neighbors. If Statei,x = {idle}, then RleaseTi,x = .
Channel ID The channel states Channel release time
i Statei,X RleaseTi,X
j Statej,X RleaseTj,X
: : :
n Staten,X RleaseTn,X
Table 4 1‑ sensed channel list (SCL) in node X
1.First scenario: select an idle channel from the SCL
•A network initialized when node A wants to transmit data to node B and node A has at least one idle data radio, A will send an SRTS to B on the control channel. The SRTS frame carries node A’s SCL and the duration time required to transmit the data.
•After receiving the SRTS, node B compares A’s SCL with its own SCL to check if there is an idle common data channel for
subsequent communication. If there is no common data channel i.e. If data channel i (i = 1, 2, ..., n) satisfying Statei,A = Statei,B = {busy}, node B replies with a NCTS frame to A on the control channel. The NCTS frame indicates that node B has all the
channels busy.
•After receiving the NCTS, node A stops sending frames to B, waits for a while, then restarts the back-off procedure and attempts to send an SRTS to B again.
•Conversely, if there is at least one common data channel i (i = 1, 2... n) satisfying State i, A = Statei,B = {idle}, node B will reply with SCTS to A on the control channel and will tune its data radio to the selected data channel i for the data transmission.
•The SCTS message includes the information of the selected data channel i, and the duration time copied from SRTS. Thus, SCTS hinders B’s neighbors from using the selected channel for the expected duration time of the transmission between A and B.
•For a channel reservation, the sender seeks for a free channel that is available for both communicating nodes, If there is at least one data channel j (j = 1, 2, ..., n) satisfying Statej,A = Statej,B = {idle}. Then, channel j is chosen as backup channel that is to say;
channel j is candidate as reserved channel to be used in case the current communication is affected by the primary arrival.
•At the same time node A sends the SRES on the control channel, a data packet will be transmitted from node A to B on the selected data channel i.
•A reservation message SRES, which includes the candidate backup channel information and the new duration time according to the primary arrival time. Similarly, the SRES will restrain neighbors of A from using the selected data channel i and in the same time announce the reserved data channel j.
•Since the primary’s return time on channel i is known in prior, the transmission on channel i should be terminated before PU reclaim.
Both communicating nodes will tune to the reserved channel j to reconstruct the current communication before collision with primary data occurs. In this case our protocol follows the same approach as the FRC-MAC scheme
0 1000 2000 3000 4000 5000 6000
0 0.5 1 1.5 2 2.5x 104
Time in sec
Aggregated data rate (bps)
Aggregated data rate in CH1 and CH2 before BCR
CH1DATA without CR CH2DATA without CR
50 100 150 200 250
0.5 1 1.5 2 2.5 3 3.5 4
packetsize (B)
throughput (Mbps)
throughput vs. packet size
the proposed protocol without BCR protocol
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