Radio Resource Optimization Criteria

RRA involves strategies or schemes of allocating radio resources such as frequency bands,

channels, and transmit antenna to the channel state information based on some optimiza- tion criteria. Optimizing these radio resource criteria will go a long way in improving the overall performance of the CRSN in SG environment. Hence, the aim is to utilize the limited spectrum, constraints power and network infrastructure efficiently. The summary of literature with respect to various resource optimization criteria used in different context

of CRSN has been tabulated in Table 3.2. This table highlights each resource optimiza- tion criterion used in the different context of CRSN including CRSN based SG. One can infer from the table that the utilization of optimization criteria for RRA in CRSN based SG is limited. In this scenario, resource optimization criteria such as energy efficiency, throughput maximization, and adaptive modulation are yet to be applied in CRSN based SG. Hence, research attention should be drawn in this direction.

Table 3.2: Summary of Resource Optimization Criteria for CRSN based SG Resource optimization criterion CRSN CRN based SG CRSN based SG References for various optimization Energy efficiency Yes Yes No [41], [39], [147],

[140], [73] QoS guarantee No Yes Yes [133], [104],[142] Throughput Yes No No [143], [142] Interference mitigation Yes Yes Yes [142], [144], [145] Fairness Yes No Yes [142], [141] Priority scheduling Yes Yes Yes [142], [145] Adaptive modulation Yes No No [146] spectrum handoff Yes Yes Yes [140], [147]

The following optimization criteria are considered:

3.3.1.1 Energy Efficiency

Realizing energy efficiency with power algorithm schemes is usually required to extend the lifetime of the battery power of the sensor node. Energy efficiency criterion is highly necessary for CRSN in SG, because the sensor nodes have limited power battery con- straints. However, the schemes used for this criterion are based on energy preservation and power consumption minimization which cannot achieve maximum power performance. Energy/power efficient schemes for CRSN related applications in general and in SG have been widely studied in literature [73], [140]. Since SG applications are mission critical, it is essential to incorporate an energy harvesting scheme in the energy efficiency so as to provide a perpetual lifetime to the sensor node.

3.3.1.2 QoS Guaranteed

SG has various applications with different and stringent QoS requirements. Hence, the design of RRA schemes should put into consideration different QoS support for the SG applications. Resource allocation schemes involving CRSN applications that considered the QoS requirements are found in [141], [104]. [142] considered QoS guarantee for hetero- geneous traffic of SG applications such that each traffic type has an associated priority with specific QoS support. QoS support is very imperative especially for SG surveillance and multimedia applications including distribution automation [102].

3.3.1.3 Maximizing throughput scheduling

Giving scheduling priority to data flows in terms of consumed network resources per amount of information transferred will help to maximize total throughput of the CRSN based SG. Scheme utilizing throughput maximization scheduling based criterion in CRSN applications have been investigated in [143], [142].

3.3.1.4 Interference Mitigation and Avoidance

Destructive interference from the external network to the CRSN based SG network should be avoided. Also, co-channel interference within the network as well as interference to the primary networks should be mitigated or cancelled. This interference avoidance and minimization criterion improves both the primary and secondary networks. RRA scheme that utilized this criterion in protecting the links of both the primary users and the secondary network has been well studied in [144], [142]

3.3.1.5 Fairness Scheduling Criterion

Fairness among Secondary Users in opportunistic spectrum access and scheduling and fairness in transmitting power allocation to Secondary Users are essential in the design of RRA schemes for CRSN based SG. Since there is trade-off among QoS guaranteed, maximum throughput and fairness, consideration of fairness among multiple sensor node in prioritizing traffic should be done in such a way that throughput maximization and QoS support must be maintained as well as in the CRSN based SG network. Work that utilized this fairness criteria in SG is in [142] which considered QoS guaranteed for heterogeneous traffic of SG applications such that each traffic type has an associated fairness. RRA strategies that utilized fairness criterion are also found in [142].

3.3.1.6 Priority Scheduling Criterion

The need to prioritize various SG applications traffic is essential in the capability to adapt to varying network conditions in real time [145]. A typical traffic type in smart grid applications is the control commands having small packet size [145]. Hence, prioritizing traffics types per their order of importance, bandwidth/spectrum demand, real time, and power of consumption is highly beneficial in CRSN based SG domain. Prioritizing traffic in CRSN based SG was also considered in [142].

3.3.1.7 Reduced Adaptive Modulation Overhead

The adaptive modulation scheme in CRSN based SG dynamically adapt to other modulation type due to the DSA capability. This leads to overhead as well as supplemental energy consumption, which results from the adaptation or switching to other modulation types

[146] at the sensor node. Hence, there is need to design RRA scheme in CRSN based SG that has reduced complexity of the adaptive modulation mechanism.

3.3.1.8 Reduced Spectrum Hand-offs

Spectrum hand-off occurs too often in CRSN applications. This leads to overhead as well as extra energy consumption at the sensor nodes. Incurred in hand-off, the buffer overflows and results in packet losses which affects the transmission reliability. Studies employing this criterion for RRA in CRSN applications have been well reported in [140], [147]. The authors in [140] presented a reduced handoffs technique using a home gateway (HGW) for home area network in a cognitive radio based SG. Whereas [147] investigated a resource allocation scheme involving reduced spectrum handoff for CRSN applications.