Broaden Your Way with Resilient WiMAX Network

In recent years, as the ARPU value of voice services continues to decline, wireless broadband data services have become the new growth magnet that network operators are looking for. According to Cisco’s 2009 analytical data, wireless broadband data services will show a hundredfold growth trend and a tremendous potential during the next five years. It has been proved that large-screen terminals like downsized Laptops, UMPCs, IPhones, and Smart Phones that are emerging one after another, have lead to a sharp increase in the demand for wireless broadband data services. 3G networks have now been overwhelmed and wireless bearer networks with higher speeds are needed.

WiMAX emerges to meet this demand, serving as an edged tool for emerging operators to rapidly deploy new networks, seize market opportunities, and gain an upper hand in the competition; and also as an effective means for traditional large fixed-line operators to retain and expand existing user groups, improve ARPU value, and strengthen their leading positions.

Different from the previous data value-added technology, WiMAX is designed from the ground up to provide broadband data services. Its very nature in network construction follows the characteristics of broadband data services. Firstly, capacity is a KPI to assess network quality and its macrocell construction focuses on balancing network capacity with network coverage. WiMAX can stand out from the market competition only if it provides a DSL-like rate that is several times higher than HSPA. Secondly, major services (like Internet) and terminal modes (like laptops and PCs) have determined that the dominant service models for WiMAX are fixed and nomadic access, which allow a discontinuous and uneven traffic distribution. In-depth hotspot coverage has become the foremost issue to be considered when designing a WiMAX network. Based on these two characteristics of data network construction, ZTE put forward its innovative concept of building a resilient WiMAX network that has an insight into the operators’ needs from the perspective of hybrid macrocell networking and layered micro/pico/femtocell networking.  

Hybrid Networking Adaptive to Various Scenarios

From the above analysis, we can see that one characteristic of wireless broadband data network is the uneven geographical distribution of traffic demand, namely 70% of traffic is concentrated in 30% of the geographical area, such as the CBD, residential areas, hotspots, etc. Therefore, that 30% of the geographical area is a major source of profit for the operator. We call it the value area and the remaining 70% is the non-value area. Figure 1 shows the site distribution of an HSPA+ network in Hong Kong (yellow dots represent sites). It is very clear that 50% of its sites are deployed and distributed in the Golden Bowl area, covering less than 5% of the whole networking area, which can ensure adequate competitiveness by providing an average flow of 6Mbps per user.

Clearly, traffic features are different in the value area and the non-value area. The value area enjoys a high intensity of users and a gathering of high-end users. What the operator has to resolve is Internet quality. Therefore, large capacity is the key to meeting the users’ needs in this area. However, as the non-value area is scarcely populated, it is more important for users to get access to the Internet than to enjoy high transmission rates. Therefore, ensuring wide network coverage is the core factor in this area. Considering different core needs in the value and non-value areas, it is necessary to adopt the hybrid networking to enable the optimum balance of coverage and capacity.

For densely populated or valued urban areas, we can use capacity density, the ratio of network capacity against coverage area, to accurately reflect end-user service experience. The simulation results show that with the same multi-antenna technology and signal path environment, MIMO 2T2R boasts about 72% higher capacity intensity than MIMO 4T4R, meaning that MIMO 2T2R can provide a higher average throughput per unit area while offering a higher download flow (34.3% higher) to edge users. Therefore, within the value area, the 2T2R networking can achieve the optimum balance of capacity and coverage to meet the key needs of users for high capacity.

For scarcely populated suburban and rural areas, or non-value areas, network construction aims at maximizing the coverage area, reducing the number of sites deployed and lowering the Total Cost of Ownership (TCO). Because of the gap in transmit power between Base Stations (BSs) and terminals, the bottleneck that restricts network coverage lies in the uplink. It has been proved that the best way to improve uplink coverage and achieve a balance between the uplink and downlink coverage is to adopt asymmetric 2T4R (4Rx against 2Tx provides 4.5dB antenna gain and 40% reduction of sites) or 4T8R (8Rx against 4Tx means 2–3dB antenna gain and 30% reduction of sites). In addition, high-gain terminals and ultra-far area coverage solutions are also used to effectively enhance network coverage.

The hybrid networking can not only ensure large capacity for microcells in the value area, which helps to improve user experience, but also achieve wide coverage for macrocells in the non-value area, which helps to reduce TCO. Therefore, it is the preferred solution for operators to optimize the whole network planning and increase their Return On Investment (ROI).

Layered Architecture for Smooth Capacity Unloading

Though the hybrid networking balances macrocell capacity and coverage, another problem emerges ― how to extend the hotspots coverage against the uneven distribution of data services. Layered architecture is the optimal solution for this problem.

Layered network architecture is similar to the “heterogeneous network” advocated in the industry. Its main idea is that network capacity, especially large capacity in hotspot areas, can not be solely carried by macrocells, but be unloaded gradually at different layers through the layered stack of micro/pico/femtocells. As shown in Figure 2, micro/pico/femtocells are layered and complement one another to build a resilient network capable of flexible capacity expansion.

The layered network architecture has advantages over traditional macrocell capacity unloading. Firstly, it is flexible. As wireless data network services are unevenly distributed, the network capacity can by no means be increased evenly across the whole network. Instead, hotspot areas would become the first to have insufficient carrying capacity; micro/pico/femto BSs could be deployed on demand to make full use of every space. The BS size (micro/pico/femto) is flexibly chosen according to the traffic intensity and coverage. Secondly, it generates less interference. Micro/pico/femto BSs have small, easy-to-design transmit power that can greatly reduce the interference to macro BS. Thirdly, it has low deployment. The cost of micro/pico/femto new BS sites is far lower than macro BS sites.

Indoor coverage has attracted more and more attention from operators because it is the most important hotspot coverage. According to the statistics of Japanese operator NTT DoCoMo, although the indoor coverage occupies only 20% of the entire coverage area, over 70% of data services, especially high-speed data services occur indoors. The quality of indoor coverage directly influences operators’ competitiveness. Because WiMAX utilizes higher frequency bands, it has more blind spots in indoor coverage that can not be easily solved through outdoor macro BSs when compared to 2G and 3G networks. Considering that indoor vertical capacity intensity is several or even dozens of times higher than that of outdoors, and that the wireless environment is complicated, it is practical to deploy several pico BSs at every layer to fully absorb capacity. The three-dimensional in-depth coverage of buildings can be achieved through the co-frequency reuse among different layers.

Conclusion

ZTE has been committed to developing a full range of custom-made WiMAX product series including 2T2R macro, 2T4R macro, 4T8R macro, micro, indoor and outdoor pico BSs, and delivering end-to-end WiMAX solutions. Using the hybrid and layered networking, it helps operators build WiMAX resilient networks that can effectively improve their ROI.