Table 1 Description of RA problem formulations in CRN

[55, 70, 71, 90]

Optimal RA in MIMO-based CRN.

Maximising the achievable data rate (or total capacity) of SUs.

Transmit power limit of SUs, interference limit to PUs, total transmission time of SUs must be equal to the time slot duration.

Number of SUs served.

[50, 91, 92]

Efficient RA for CRN with cooperation.

Maximising the sum rate of all SUs.

Transmission power budget of the SUs and the relays, interference to PUs within its tolerable threshold, each subchannel can only be allocated to one SU.

Achievable rate over a subchannel, power allocated to each subchannel, integer variables of time slot and a binary allocation indicator

[49, 83, 84]

Energy-efficient RA for CRN with imperfect sensing and/or femtocells.

Maximising bandwidth capacity for SUs.

Power contraint on SUs network, minimum rate guarantee for some SUs and best effort service for the remaining SUs, each subchannel can only be allocated to one SU.

Transmit power for each SU, the binary channel allocation indicator.

[45, 61–63]

Optimal RA in MIMO-OFDMA based CRN.

Maximising sum throughput of the SUs.

Interference leakage to PUs always below a threshold, each SU must achieve the minimum required data rate, total transmit power of all SUs must be below the available power at base station, no more than one SU is allocated to each subchannel.

Data rate on each subchannel.

[67]

RA for CRN with opportunistic access

Minimising the symbol error rate of the SUs’ network transmission.

Constraint on the maximum individual powers of each SU, a minimum number of symbols must be sent within a time frame, constraint on the minimum acceptable throughput of the network, interference power to PUs must be below a certain threshold.

Total power available to the system, transmitted symbol time,