Macro/Micro Cellular Systems Combining Multiple Access | NETWORK ARCHITECTURES

The semi hierarchical cellular systems can be extended for solving the mobility problem. Generally cells are categorized into macrocells, microcells and picocells depending on its size. Macrocells and microcells are usually deployed in rural and urban regions, respectively, while the picocells are in a building. In some region such as a hot-spot zone, an MS can access both macrocells and microcells as in Figure 1. A service model is proposed  where macrocells and microcells cover high speed and low speed MSs, respectively. This structure is effective, because a high speed MS has to change cells frequently if covered by microcells.

 

Figure 1: The model of a hierarchical cellular network

We extend the hierarchical cell structure by integrating multiple access techniques. Some systems under development are based on OFD-MA (e.g., WiMAX) that combines OFDM and frequency division multiple access (FDMA). Since OFDMA systems have lots of channels in a frequency domain, it has higher allocation granularity than OFDM system. It also has the ability of taking advantage of adaptive modulation and coding (AMC), but its application is limited to low mobility. If an MS using OFDMA has high mobility, it cannot perform coherent detection properly due to the long symbol.

Meanwhile, a FH-OFDMA system, which combines frequency-hopping and OFDMA, has the advantage of exploiting diversity (Y. Kim et al, 2003). Though it experiences difficulty in supporting high data rates and AMC, it can overcome channel fading and multiuser interference through a FH pattern. Accordingly, it is a viable combination that microcells for low mobility use OFDMA that has fine granularity, while macrocells for high mobility use FH-OFDMA that is robust to channel fading and interference.

Each cell plane can handle traffic classes differently as well. High rate data services are suitable for OFDMA that has high spectral efficiency and supports various data rates by AMC. In contrast, low rate services like voice are adequate for FH-OFDMA that is easy to use diversity. If an MS has the capability of supporting dual modes, it can switch cells according to mobility and traffic type in a manner of using vertical handoff that offers an additional merit of load balancing.

In summary, the hierarchical network that consists of OFDMA microcells and FH-OFDMA macrocells has the ability of supporting various users with different mobility and traffic types. Since the considered network is based on a common OFDMA platform, it is more manageable than other heterogeneous networks. Also, it provides the flexibility in network planning. 4G networks will be most probably overlaid with 2G or 3G cellular networks. As existing cellular networks are basically designed for circuit-switched voice service, in some regions, they will keep undertaking voice users and high mobility users like the FH-OFDMA macrocell system, while 4G networks focus on data traffic users by using the OFDMA microcell system.