System model:In the D2D communication underlaying the cellular networks,D2D users coexist with the cellular users. For the improvement of the system performance ,resource sharing is done between the D2D users and the cellular users, only uplink resource sharing is considered since the Uplink spectrum is underutilized compared to that of the Downlink spectrum .Furthermore uplink resource sharing in the d2d communications only affects the Base station and the incurred interference can be mitigated by base station coordination. System model assumes the fully loaded cellular network such that N cellular users occupy the N orthogonal channels in the cell and there is no spare spectrum .We also assume both the cellular units and the d2d pairs have minimum SINR requirements and the base station has the perfect CSI information of all the linksThe distance based pathloss model we are using considers both fast fading due to multipath propagation and slow fading due to shadowing .Hence the channel gain between the cellular users i and the Base station can be given by Where K is constant determined by system parameters,is fast fading gain with exponential distribution ,is slow fading gain with log normal distribution , is path loss exponent and L is the distance between the Cellular user i and the base station .Similarly we can calcaulate the path gain between the d2d pair j and the base station. The noise is considered to be additive white Gaussian noise .

Problem Definition:To improve the performance of the fully loaded cellular networks ,a D2D pair is set up only if the minimum SINR requirement can be guaranteed and the incurred interference to the cellular units is below threshold. We call D2D pair as the admissible pair and the cellular user to be reuse candidate .In the fully loaded cellular networks all the orthogonal resources are used by the cellular users, so if a D2D connection is to be established, the same orthogonal resources which are used by the cellular users is allocated to the D2D pair. The problem arises in which orthogonal resource should be allocated to the D2D pair. If we allocate the resource of the cellular user which is very near to the D2D pair , then that cellular user will suffer from interference and system throughput will suffer. Also the transmit powers of the cellular units and the D2D pairs should not be maximum as they increase the interference and decrease the system throughput. Therefore to maximize the throughput while limiting the interference, we propose the resource allocation based upon the distance between the d2d pairs and the cellular units .

Proposed Methods:To maximize the overall throughput of the system we are proposing a method which consists of Qos-aware admission control, power control for single d2d pair and then finding the overall throughput of the system. In Qos aware admission control we determine whether a d2d pair with a targeted SINR can be admissible or not,in power control we allocate the transmit power to cellular units and d2d pairs and then find the overall throughput implementing the above methods.1.Qos aware admission controlIf the D2d connection mode is to be estlablished ,d2d pair cann share resource with a cellular user only if the SINR rerquirements are satisfied and the transmit power of cellular units and d2d pairs must be less than the maximum transmit power

Where the is the SINR of cellular user,is SINR of d2d pair,is transmit power of cellular user,is transmit power of d2d pair.If the above requirements are satisfied ,d2d mode can be established,now we have to allocate the same resource which the cellular user is using to the d2d pair, so the cellular user become a reuse candidate of the d2d pair.To avoid the interference problem as discussed in the problem definition, we determine the distances between all the d2d pairs and the cellular users and find the minimum distance which is denoted by . Then we allocate the same resource of the cellular user to the d2d pair whose distance is greater than the calculated minimum distance, and the cellular unit becomes the resued candidate of the d2d pair. The formula for distance between d2d pairs and the cellular units is given by

Then we define access rate as ratio of number of reused candidates to the d2d pairs.

2.power control The transmit power of the cellular units and the d2d pairs should be less than the maximum transmit powers ,and should be greater than minimum transmit power.The minimum transmit power of cellular unit i is The minimum transmit power of d2d pair is

So we choose the transmit power which are greater than minimum value and less than the maximum values such that it maximizes the throughput keeping the interference minimum.

3.ThroughputThe maximum throughput of the cellular unit when it does not shares it resource with d2d pair is

When it shares it resource with the d2d pair, the maximum achievable sum throughput is

Where and are channel gains of the interference links from transmitter of d2d pair j to the base station and from cellular unit I to the receiver of d2d pair j respectively., are transmit powers of cellular units and d2d pairs which we select from above power control method.Hence the d2d throughput gain can be expressed as