Public Safety Network (PSN) is the network for critical communication when disaster occurs. As a key technology in 5G, Cloud-Radio Access Network (C-RAN) can play an important role in PSN instead of LTE-based RAN. This paper firstly introduces C-RAN based PSN architecture and models the OFDM resource allocation problem in C-RAN based PSN as an integer quadratic programming, which allows the trade-off between expected bitrates and allocating fairness of PSN Service User (PSU). However, C-RAN based PSN needs to improve the efficiency of allocating algorithm because of a mass of PSU-RRH associations when disaster occurs. To deal with it, the resources allocating problem with integer variables is relaxed into one with continuous variables in the first step and an algorithm based on Generalized Bender’s Decomposition (GBD) is proposed to solve it. Then we use Feasible Pump (FP) method to get a feasible integer solution on the original OFDM resources allocation problem. The final experiments show the total throughput achieved by C-RAN based PSN is at most higher by 19.17% than the LTE-based one. And the average computational time of the proposed GBD and FP algorithm is at most lower than Barrier by 51.5% and GBD with no relaxation by 30.1%, respectively.
With the rapid upgrading of emergency application, Public Safety Network (PSN) is getting increasingly popular. It is being recognized that wireless-based PSN is a key to a successful response to emergency situations. Article [
There are many PSN issues that needs to be settled by new B4G or 5G technologies. Article [
However, the PSN Service Users (PSUs) will produce a large amount of traffic in times of disaster. Hence, the ability of bandwidth allocation algorithm is a key to assure the critical communication. Article [
The contributions of this paper include two key aspects. The first is we introduce C-RAN based PSN architecture and propose the OFDM resources allocation model in this network. To the best of our knowledge, C-RAN based PSN has not been investigated in the previous works. We formulate OFDM resources allocation problem in C-RAN based PSN as a quadratic programming (QP) with integer variables of OFDM resource block that aims to maximize the emergency system performance. In the formulated problem, the objective system performance is affected by the expected bit-rates revenue and the allocation fairness among different PSUs. By analyzing the wireless channel model, we propose an objective function of the C-RAN based PSN system performance that allows trade-off between the expected bit-rates revenue and the PSU’s fairness when allocating resources.
Second, we present an efficient solution for the problem facing the centralized resource allocation in C-RAN based PSN. We firstly relax the integer variable in this QP problem into the continuous one. And a GBD-based algorithm is proposed to address this relaxed problem. The traditional Bender’s Decomposition (BD) algorithm is used to solve the Mixed-Integer Nonlinear Programming (MINP) problem by decomposing the original problem into a master and a subproblem [
The rest of the paper is organized as follows: In Section
Figure
C-RAN based PSN architecture.
Our C-RAN based PSN architecture allocates PSN spectrum resources centrally to improve the whole network performance. But to meet the critical communication requirement, this raises one important issue that needs to be considered: the assignment algorithm efficiency, especially for that there are a lot of PSU-RRH associations when disaster occurs. This is the key problem we want to solve in this paper. Since the OFDM is the dominant resource-multiplexing technology currently, in this paper, we focus on the OFDM resource allocation in C-RAN based PSN with a view to maximizing the system performance by optimally scheduling the centralized OFDM bandwidth resources.
Firstly, it needs to establish an association model between PSU and RRH. Assume that in a C-RAN based PSN system,
To facilitate analysis, we leave the reusable resource case to our further study, and temporarily in this section, the resource cannot be reused in the same C-RAN based PSN system. Then we get one of the constraints of
Let
Then
In Figure
Bit-rate capacity is a key feature to the performance of future wideband PSN rich-content services. The typical aim of the resource allocation scheme is to maximize the total bit-rate capacity in the traditional cellular. Hence, in this paper we use the achievable bit-rate capacity of all PSUs as the C-RAN based PSN system’s revenue. This section analyzes the characteristics of the wireless fading channel to derive the outage probability of the system, based on which the achievable bit-rate capacity of a PSU in unit bandwidth can be resolved. Finally, the achievable PSU bit-rate per unit bandwidth is defined to represent the system revenue.
Let
Assume that the PSU uses Maximal Ratio Combining (MRC) scheme to achieve full diversity gain, and the fast fading
then the received power of UE
Supposing the threshold of the received power guaranteeing the basic communication service is
In this paper, we take the impacts of two different scales of fading into consideration, which results in an outage probability resolving problem in elaborately modeled fading channel. Let
The cumulative distribution function of a gamma distributed variable with scale parameter being 1 can be written as
Substituting (
Then we can use the Gauss-Hermite quadrature to solve the numerical result of (
As
The PDF of SNR in (
Let random variable
Then (
Then we can adopt the Gauss-Laguerre quadrature to solve the numerical result of (
Let
In addition, the variance of
The correlation coefficients of
As complete joint transmission technology is adopted in C-RAN and a PSU can be served by multiple RRHs simultaneously, in the case of
This paper takes into account both expected revenue of the system and PSU’s fairness when allocating the RBs in C-RAN based PSN. Hence, for the sake of maximizing the system performance, the following quadratic programming (QP) is derived:
Because of the integer decision variables of PSU, when having a large amount of PSU-RRH associations when disaster occurs, the time complexity of solving this NP problem will increase distinctly. Hence, this paper proposes a corresponding Generalized Bender’s Decomposition method on the basis of integer variable continuation, which satisfies the demand on efficiency of algorithm in PSN service.
Firstly, relax problem (
It can be seen that (
Let
Problem in (
From (
Let
Lemma
Each iteration in Generalized Bender’s Decomposition will narrow the gap between the upper bound
Firstly, we prove the upper bound of
So with the optimal solution of problem in (
Then, we prove the lower bound of
According to Lemma
( ( ( ( ( ( ( of the objective function
The proposed GBD algorithm takes the difference between
The Generalized Bender’s Decomposition-based Algorithm
As an optimization problem with continuous variables and linear constraints [
GBD algorithm can solve the solution of relaxation problem (
The principle of FP adopts a heuristic way to find the feasible integer solution
It is clearly observed that if
Thus, this paper proposes the allocation algorithm on the basis of taking the OFDM RB as basic allocated unit, which is shown in Algorithm
( ( ( ( ( ( ( ( ( of the objective function
It can be seen that
As for problem (
For problem (
Because
Since
Because the operator
This paper evaluates the performance from both C-RAN based PSN architecture and proposed GBD algorithm. The throughput and PSU’s allocation fairness in C-RAN based PSN are investigated in Section
Simulation parameters.
Parameter | Value (unites) |
---|---|
Transmitting power of each RRH |
20 W |
Antenna gain |
15 dBi |
Slow fading mean |
3 dB |
Slow fading standard deviation |
1.5 dB |
Number of UE’s antenna |
2 |
Gauss-Laguerre phase |
5 |
Gauss Hermite phase |
5 |
Path loss exponent |
4.5 |
Bandwidth of each resource block |
0.2 MHz |
Fairness factor |
0.3 and 0.6 |
Our experiment assumes a C-RAN based PSN architecture of 10 RRHs of 20 W transmitting power with a fixed position placed in a 2000 m by 2000 m area. Each RRH serves 20 PSUs. These PSUs are randomly placed in each run and satisfies that 50% of them have average SNRs lower than 3 dB. In the simulation a variable bandwidth value per RRH is assumed (5, 10, 15, 20) MHz in both downlink and uplink. These bandwidths per RRH are equivalent to 25, 50, 75, and 100 OFDM RBs, respectively. The fairness factor of Algorithm
From Figures
System total throughput versus total resources with C-RAN based and LTE-based PSN.
Jain’s Fairness Index versus total resources with C-RAN based and LTE-based PSN.
In Section
Upper and lower bound value versus iteration step with different algorithms.
Figure
Upper and lower bound value versus iteration step with different-sized networks.
The average computational time of proposed GBD algorithm is expressed against the different-sized network in Figure
Computational time versus size of C-RAN based PSN.
The similar results are obtained from the perspective of the jitter of the computational time which is shown in Figure
Positive jitter of computational time versus size of C-RAN based PSN.
Algorithm
Objective function value versus terminated parameter
This paper researches on the C-RAN based PSN architecture and OFDM resources allocation problem in it. Firstly, we design the C-RAN based PSN and then by formulating the OFDM RB resources allocation problem into a QP with integer variable, we work out a solution considering both the expected system revenue and the PSU’s allocation fairness. To solve this complicated QP with integer variable, the continuation of integer variable is used to relax the original problem into a general QP problem. The relaxed problem is optimally solved by a proposed Generalized Bender’s Decomposition algorithm after finite iterations. Then FP standard is adopted to obtain the actual integer solution on the original problem. The trade-off between the system degradation and the error tolerance of the proposed algorithm is given theoretically. The numerical results finally show the C-RAN based PSN obtains a good throughput performance without a cost of PSU’s allocation fairness and the proposed GBD and FP based algorithm has the fine convergence and low computational time.
One achievement of this paper is to propose a PSN architecture based on C-RAN and model the resource allocation problem in this centralized allocation system. In addition, OFDM resource allocation problem in C-RAN based PSN is complicated because of the integer variables and a large amount of PSU-RRH associations when disaster occurs. However, the allocating efficiency is more important in PSN than commercial network. Hence, another achievement is that we use a corresponding GBD and FP method to solve resources allocating problem efficiently in the proposed C-RAN based PSN, which is referential to the future works on the similar problem.
The authors declare that they have no competing interests.
This paper is supported by the 863 Program (2015AA01A705) and NSFC (61271187).