3.1 System Model
The system model of the developed DCSS is illustrated in Fig. 1. Let αi represent the BS of the considered WRAN scenario. Here, i represent the number of BS, which can vary from1 to N. In this investigation, it is restricted to1.
Let βj represent the number of PUs and ℽk represent number of SUs served by αi. Let j varies from 1 to 3 while k from 1 to 2.The maximum coverage distance offered by αi be represented as di. Let Ttm be the different traffics transferred between pair of SUs (ℽk) through αi at tm instances. Here Ttm varies from 1 to 3 (tm ϵ Tt1, Tt2, Tt3). Let ℽ1 and ℽ2 be the pair of SUs, then the CBR data traffic transmitted between γ1 and γ2 be denoted as Tt1. Similarly the voice be denoted as Tt2 and the video traffic be denoted as Tt3 (Tt1 = CBR, Tt2 = Voice and Tt3 = Video). Every αi possess a functional entity known as the SM while every γk has a lighter version of the SM known as a SA to assist αi in sensing the availability status of the spectrum.
Table 1
Notations Used | Description |
αi | No of Base Station (BS) |
βj | No of Primary User (PU) |
γk | No of Secondary User (SU) |
Ttm | Types of Traffics |
τsi | Departure time at source node |
τdi | Arrival Time at destination node |
di | Coverage Distance offered by BS |
Δσ | Delay when operation channel is allocated |
ΔK | Delay when backup channel is allocated |
ΔC | Delay when candidate channel is allocated |
\({\sigma }_{N}\) | Set of operational channels |
\({K}_{N}\) | Set of backup channels |
\({C}_{N}\) | Set of candidate channels |
The SM has the responsibility to provide connectivity and resource allocation to γk that are within the coverage of corresponding αi. Every γk, that are present within the coverage of αi, execute the process of initialization to get associated with αi The SA module present in the γk does continuous spectrum sensing mechanism to obtain the availability status of the channel and report this information periodically to the SM module of αi.
This functionality is executed by the SA module mainly to avoid the γk experiencing interference with the other channel. The detail of developed DCSS is discussed in the subsequent section.
Channel Classification
In this research, DCSS mechanism is executed to realize efficient spectrum management. It is accomplished by exploring the association between SM of αi and SA of γk. Upon initialization of γk with αi, the authorized γk served by αi are instructed to execute spectrum sensing. Based on the duration of spectrum sensing, the available channels served by αi are classified into operational channel, backup channel and candidate channel.
The primary function of αi is to assign the available channels to the βj. As long as the βj are effectively utilizing the available channels, spectrum efficiency can be ensured. However in real time scenarios, the presence of βj is stochastic which leads to underutilization of the available spectrum. Hence the concept of DCSS is introduced in this research to improvise the spectral utilization. The working mechanism of DCSS is illustrated in the Fig. 2.
Let C be the total number of channels that are available in the database of αi. The SM of αi instructs SA of γk to observe the ideal duration of the available channels. Based on the ideal duration, the channels are classified into operational, backup and candidate. The available channels can be either idle or busy. Whenever it is not occupied by βj, it is idle. Then, αi can allocate the idle channel to γk. Thus the available channel is utilized effectively.
The available operational channels are used for the communication between the αi and γk. The SA of γk senses the operational channel at least once every two seconds in order to safeguard from βj interference. When the βj detection is done for every 6 sec, then it is labelled as backup channel. If the case is up to 30secs, it is categorized as candidate channel. When the channel is idle for more than 60 seconds, it is reported as protected channel as indicated in Fig. 3. The SA of the γk executes the above operation and provides the detail periodically to the SM of αi. The channel classification mechanism detailed above is dynamic in nature, since the idle status of the channel depends on the traffic flow of βj. Hence the reassessment of the above channels is performed by γk and αi periodically [15].
Channel allocation to SU (γ k )
From the previous section, it is found that the available channels maintained by the database are categorized as operating, backup, candidate channels and protected respectively. Whenever the γk request αi for resource allocation, the operating channels are initially allocated to the authorized γk. However, when the βj resumes its transmission with this operating channel, immediately αi issues UCS notification to the γk. This notification indicates that αi indicate the process of relocation of resource assigned to γk from operating channel to backup channel, as the available operating channels are re-assigned to the licensed βj. Thus, the γk will experience certain level of latency due to this switching operation. However the packet delivery is ensured by the above operation. Based on the resource availability and circumstances, this switching mechanism is repeated with respect to backup and candidate channels. Thus the spectral efficiency and throughout is enhanced by the developed DCSS mechanism.
From the perspective of DCSS, the capability of γk in spectrum sensing is limited within its coverage distance (d1). The developed DCSS mechanism utilizes the inputs obtained from both SA and SM. Energy detection technique [16] is adopted in spectrum sensing.
Two Way Handshaking
Let σN represent set of operating channels as expressed in Eq. 1. Let KN denote the set of backup channels represented in Eq. 2 and\({C}_{N}\) be the set of candidate channels represented in Eq. 3.
\({\sigma }_{N}\) = [\({\sigma }_{1},{\sigma }_{2},{\sigma }_{3\dots }{\sigma }_{N}\) ] (1)
\({K}_{N}\) = [\({K}_{1},{K}_{2}\dots {K}_{N}]\)(2)
\({C}_{N}\) = [\({C}_{1},{C}_{2},{C}_{3\dots }{C}_{N}\) ] (3)
In execution of conventional Binary Hypothesis Test (BHT) over σN,, whenever σ1 is measured below threshold TH, it is declared as idle (available) and allotted to γk through Two Way RTS / CTS handshake procedure. However, with the advent of UCS notification, γk abrupt its ongoing transmission with the corresponding channel and begin the process of searching for other free channels.
On the other hand, in the developed DCSS, when UCS notification is witnessed, the ongoing transmission of γk is buffered, and the energy level next operational channel σ2 is measured and the process is repeated for all other channels in σN. If all the channels are being utilized, then the above mentioned process is extended with back-up channels (KN) and resource reallocation is ensured. All the above activities are executed by γk in coordination with SM mechanism of αi.
In order to acquire the channel availability at any given instant of time, the γk has to register itself with the αi and then depending on the presence / absence of the βj activity, the channel is allocated to the γk by the αi. The Two Way handshaking procedure involved in this process is represented in Fig. 3. The commands REG_REQ indicates the request made by γk to register with αi. The acknowledgement to this request is provided by REG_RSP respectively.
After successful registration, γk contends for the channel by sending CHA_REQ packet to the αi which cross-checks with the βj for its activity. If βj is idle, then αi acknowledges the request with CHA_RSP packet, after which the data transmission is carried out. In case of appearance of βj, the UCS packet is realised making the γk to switch to next available channel. But if βj is active, αi sends a busy signal to γk, SM in association with SA searches the availability of channels among the back-up channels. Upon release of UCS packet indicating the return of βj, the γk is switched to the next available channels. This process is repeated until resource reallocation is accomplished.
Traffic and Modulation Schemes
Upon allocation of channels from either σN / KN / CN, γk begins transmission. CBR, voice and video are three different traffic applications considered in this research. CBR is a basic application with ON/OFF traffic pattern, while voice traffic follows constant and exponential inter arrival time. Video traffic consumes more bandwidth than voice. However is dependent on the codec and kind of video being streamed. Thus, the data rates of each application differ, demanding the usage of different modulation schemes in order to meet out their required QoS. QPSK, 16QAM and 64QAM are the three modulation schemes specified by IEEE 802.22 to achieve the above requirement [17]. In this research, three types of modulation schemes are analysed to determine the suitable scheme to provide application specific end to end delay.