【正文】 ated with handoff. Macrocells provide widearea coverage beyond microcell service areas and ensure better intercell provide capacity due to greater frequency reuse and cover areas with high traffic density (called hot spots). Examples of hot spots include an airport,a railway station, or a parking lot. In less congested areas (., areas beyond a city center or outside the main streets of a city) traffic demand is not very high, and macrocells can provide adequate coverage in such areas. Macrocells also serve highspeed MSs and the areas not covered by microcells (., dueto lack of channels or the MS being out of the microcell range). Also, after the microcellular system is used to its fullest extent, the overflow traffic can be routed to of the important issues for the overlay/underlay system is the determination of optimum distribution of channels in the macrocells and microcells. Reference evaluates four approaches to sharing the available spectrum between the two tiers. Approach 1 uses TDMA for microcell and CDMA for macrocell. Approach 2 uses CDMA for microcell and TDMA for macrocell. Approach 3 uses TDMA in both tiers, while approach 4 uses orthogonal frequency channels in both overlay/underlay system has several advantages over a pure microcell system:? The BSs are required only in high traffic load areas. Since it is not necessary to cover the whole service area with microcells,infrastructure costs are saved.? The number of handoffs in an overlay system is much less than in a microcell system because fastmoving vehicles can be connected to the overlay macrocell.? Both calling from an MS and location registration can easily be done through the microcell are several classes of umbrella cells. In one class, orthogonal channels are distributed between microcells and another class, microcells use channels that are temporarily unused by macrocells. In yet another class,microcells reuse the channels already assigned to macrocells and use slightly higher transmit power levels to counteract the interference from the macrocells.Within the overlay/underlay system environment, four types of handovers need to be managed [19]: microcell to microcell, microcell to macrocell, macrocell to macrocell, and macrocell to describes bined cell splitting and overlaying. Reuse of channels in the two cells is done by establishing an overlaid small cell served by the same cell site as the large cell. Small cells reuse the split cell’s channels because of the large distance between the split cell and the small inner cell, while the large cell cannot reuse these channels. Overlaid cells are approximately 50 percent more spectrally efficient than segmenting (the process of distributing the channels among the small and largesize cells to avoid interference).A practical approach for implementation of a microcell system overlaid with an existing macrocell system is proposed in . This reference introduces channel segregation (a selforganized dynamic channel assignment)and automatic transmit power control to obviate the need to design channel assignment and transmit power control for the microcell system. The available channels are reused automatically between microcells and macrocells. A slight increase of transmit power for the microcell system pensates for the macrocelltomicrocell interference. Simulation results indicate that the local traffic is acmodated by the microcells laid under macrocells without any significant channel management effort. The methodology of the Global System for Mobile Communications (GSM)based system is extended to the macrocell/microcell overlay system in. The use of random frequency hopping and adaptive frequency planning is remended,and different issues related to handoff and frequency planning for an overlay system are discussed. Four strategies are designed to determine a suitable cell for a user for an overlay system. Two strategies are based on the dwell time (the time for which a call can be maintained in a cell without handoff), and the other two strategies are based on user speed estimation. A speed estimation technique based on dwell times is also CDMA cellular system can provide full connectivity between the microcells and the overlaying macrocells without capacity degradation. Reference analyzes several factors that determine the cell size, the soft handoff (SHO) zone, and the capacity of the cell clusters. Several techniques for overlayunderlay cell clustering are also outlined. Application of CDMA to microcell/macrocell overlay have the following major advantages:? A heterogeneous environment can be illuminated uniformly by using a distributed antenna (with a series of radiators with different propagation delays) while still maintaining a highquality signal.? SHO obviates the need for plex frequency studies the feasibility of a CDMA overlay that can share the 1850–1990 MHz personal munications services (PCS) band with existing microwave signals (transmitted by utility panies and state agencies). The results of several field tests demonstrate the application of such an overlay for the PCS band. The issue of use of a CDMA microcell underlay for an existing analog macrocell is the focus of. It is shown that high capacity can be achieved in a microcell at the expense of a slight degradation in macrocell finds that transmit and receive notch filters should be used at the microcell BSs. It shows that key parameters for such an overlay are the powers of the CDMA BS and MS transmitters relative to the macrocell BSs and the MSs served by the macrocells. Reference [25] studies spectrum management in an overlay system. A new cell selection method is proposed, which uses the history of microcell sojourn times. A procedure to determine an optimum velocity threshold for the proposed method is also outlined. A systematic approach to optimal frequency spectrum management is described.Special Architectures There are several special cellular architectu