Position: Analysis of WiMAX FFR
I. Overview
According to WiMAX Forum, there are 455 WiMAX networks put into business operation in 135 countries around the world, and many countries will issue the licenses of WiMAX. As the bandwidth of the license for WiMAX issued by each country is different, some licenses comprise broad bandwidth resources. For example, Sweden issued TDD frequency points of 50 MHz in the frequency band of 2.5 GHz and Japan issued the licenses for the bandwidth of 30 MHz, but some licenses issued by some countries contain limited bandwidth resources. For example, the license issued by India contains a bandwidth of 20 MHz and an operator of Singapore has only a bandwidth of 12 MHz. As the narrow bandwidth resource of WIMAX is limited, the operators expect ZTE to provide a solution which can best utilize the frequency resources and provide mobile cellular network with best capacity and coverage.
The limited frequencies resources will result in inter cell interference (ICI) which is an inherent problem of mobile cellular telecommunication system and an inevitable outcome of frequency reuse. Like other cellular telecommunication systems, the frequency reuse mode and frequency of mobile WiMAX has to be determined in the stage of network plan.
In mobile cellular system, the frequency reuse mode is defined as (C×N×S), of which C stands for the number of the base sites, N stands for total number of the channels (or channel group) where the frequency is reused and S stands for the number of the sector of each base site. When the operators have rich resources of WiMAX, their major frequency reuse mode is 3 (1×3×3) or 6 (2×6×3) . When there is limited resource of frequency, i.e. only one of 10 MHz, the cellular networks are formed in the following ways:
♦ Co-channel network deployment, 1×1×3
♦ PUSC 3 Segment, 1×3Segment×3
♦ Fractional Frequency Reuse (FFR)
In 1×1×3 mode networking, it has too much interference at the boundary of the cell. In PUSC 3 Segment mode networking, it is hard to ensure a certain throughput because it is inefficient in using the frequency spectrum.
However, FFR can meet the requirement for both coverage and capacity. The following is the analysis of the WiMAX FFR of ZTE.
II. Fractional Frequency Reuse (FFR)
Fractional Frequency Reuse (FFR) is a technology that adopts different frequency reuse coefficient in different reuse groups. If the terminal is suitable to work in the condition where the reuse coefficient is 3, the reuse factor with coefficient 3 is provided for such terminal. If the terminal is suitable to work in the condition where the reuse coefficient is 1, the reuse factor with coefficient 1 is provided for such terminal. More flexible approach is to control all the sub-carrier groups with different transmission power to coordinate different cells radius. FFR can be realized in static mode and dynamic mode. In static mode, the frequency coefficient used by the terminal, which is associated with the terminal location, is relatively regular. When the terminal measures the interference in the adjacent downlink cell (sector) and reports the result to the base site, the base station shall allocate appropriate frequency coefficient to such terminal in response to the interference. The problem with the static mode approach is that it is not capable of dealing with different conditions in various deployment sites and business flow distribution because each reuse group has definite number of sub-carriers. In this case, the dynamic mode of FFR shall be used. That is to say the reuse group where the terminal locates is Continuously updated. Such dynamic dispatch gives general considerations to the QoS requirement of the clients, the avoidance of inter cell interference and the principle of equity. Dynamic FFR is rather a complicated technology because it requires that the base station obtain the channel quality information of each reuse group in a timely manner.
Figure 1 Fractional Frequency Reuse
The figure above is a typical FFR mode, in which F1, F2 and F3 stands for the subsets of different sub-channels respectively in the same frequency. In such frequency configuration, the users at the center of the sector adopt the reuse factor 1 to maximize the spectrum efficiency and the users at the boundary of the sector adopt the reuse factor 3 to ensure the quality of service and throughput. The system that uses FFR aims at achieving a better overall spectrum efficiency than that of reuse coefficient 1 and reuse coefficient 3.
FFR aims at reducing interference and improve the SNR of the signal by effectively allocating the system resources and reduce the time and frequency interference of the resources used at the boundary between adjacent cells so as to improve the service at the boundary of the cells in the system and even the service of the whole system.
III. Solution of ZTE WiMAX FFR
Solution of ZTE WiMAX FFR is focused on two approaches, namely frequency division FFR and time division FFR, as described below:
♦ Frequency division FFR
In frequency division FFR, the frequency resources are divided as follows:
Figure 2 Division of Frequency Resources of frequency division FFR
In frequency division FFR, the frequency resources are divided into a reuse set named reuse 3 including sub-band W1, W2 and W3. The adjacent sector 1, sector 2 and sector 3 respectively select W1,W2 and W3 in reuse 3 and are transmitted with high power Phigh, and the other two sub-bands are transmitted with lower power Plow .
Figure 3 The frequency resources allocation
The frequency resources allocation in adjacent sectors in frequency division FFR is indicated in the figure above, in which each sector is divided into inner ring section and outer ring section before allocating the sub-band resources corresponding to Phigh in Reuse 3 to the outer ring section and finally allocating the sub-band resources corresponding to Plow in Reuse 3 to the outer ring section.
In frequency division FFR, the base station shall distinguish the users of the inner ring section from those of the outer ring section by the SINR value reported by the users.
♦ The time division FFR
The time-frequency resources division in the time division FFR is indicated in the figure below, in which the down-link sub-frame supports two Zones. One is PUSC 1/3 and the other is PUSC all. The users at the center of the cell are dispatched at PUSC all and the users at the boundary of the cell are dispatched at PUSC 1/3. Therefore, the network deployment of single frequency point can be used to improve the frequency spectrum efficiency to some extent.
Figure 4 The time frequency resources allocation in the time division FFR
In the time division FFR, the time frequency resources are divided into Reuse3Zone and Reuse1Zone. Reuse1Zone uses the mode of PUSC all sub-channels and the adjacent Cell A, Cell B and Cell C and use Reuse1Zone, while Reuse3Zone uses segment network deployment of PUSC, where the frequency resources of Reuse1Zone allocated by the adjacent Cell A, Cell B and Cell C are orthogonal to each other.
The transmission power of the sub-carrier of Reuse3Zone shall be notably higher than that of Reuse1Zone. Like frequency division FFR, the base station shall distinguish the users of the inner ring section from those of the outer ring section by the SINR value reported by the users.
IV.Analysis of FFR Performance
The FFR achieved the network deployment with frequency coefficient 1. The result of emulation and test has showed that the solution of ZTE WiMAX FFR is better than the network deployment of 1×1×3 and 1×3Segment×3 in the following ways:
♦ The time division FFR has the same coverage as that of 1×3Segment×3.
♦ The frequency division FFR is better than the network deployment of 1×1×3 in coverage, but not as good as the network deployment of 1×3Segment×3. As the transmission power of the inner ring significantly decreases, the coverage of frequency division FFR will be the same as that of the network deployment of 1×3Segment×3.
♦ In the mode of the time division FFR and the frequency division FFR, the overall frequency spectrum efficiency is higher than that of the network deployment of 1×3Segment×3 and similar to that of the network deployment of 1×1×3.
In conclusion, considering coverage and throughput, the performance of FFR is better than that of the mode of 1×3Segment×3 and 1×1×3.
By enabling the users of inner and outer rings to reuse different frequencies, ZTE WiMAX FFR has avoided interference, improved the performance at the boundary of the cell, achieved the network deployment of frequency reuse coefficient 1 and helped the operators improve the coverage and throughput in the conditions of limited frequency resources.
By supplying the products of high performance, ZTE WiMAX has established its position as a leader in the Industry. By 2009, ZTE has established more than 40 commercial and experimental offices for mobile WiMAX in the global market covering America, Africa, Asia and Europe. ZTE E2E WiMAX solution is the best choice of the operators.
