Cellular Networking and Mobile Radio Channel Characterization
2.2 The Crux of the Cellular Concept
2.2.2 The Core of the Cellular Notion
Consider the“precellular” system just reviewed. The crux of the cellular idea is the replacement of each of the large zones with several smaller hexagonal-shaped areas, called cells, and the replacement of high-power zone transceivers with as many low-power cell transceivers as needed, in fact one for each cell, each providing coverage to the corresponding cell area and not beyond. A set of duplex radio channels are then carefully divided into subsets of radio channels, and each cell is provided with one of these subsets. Cells that are“far away” from each other may use the same radio channel subsets. In this manner, the spectrum is reused and spectral efficiency of the cellular network is dramatically improved relative to that of the precellular system.
Before presenting the details of cellular concept, a brief description of cell classifications is provided. Cells may be categorized according to the size of their coverage area. A macrocell covers a relatively large area with an antenna that is mounted on a high tower. A macrocell serves a large cell in the ubiquitous mobile phone network with a cell radius of greater than 1 km and a transceiver antenna output power ranging from 40 to 160 W. A microcell is a smaller cell in a mobile phone network, which typically provides coverage for small sections of urban areas such as shopping malls and airport terminals. The microcell radius is between 250 and 1000 m with the output power of 2–20 W. A picocell provides coverage for a smaller area than that of a microcell and is normally used for indoor applications. The radius of a picocell is about 100–300 m with an output power that ranges from 250 mW to 2 Watts. Finally, the femtocell that is used for in-building applications. The radius of femtocells is in the range of 10–50 m with antenna power output of 10–200 mW [1,2]. Cells in AeroMACS network are of either macrocell or microcell type.
According to the classical cellular network design principals, for a given geographical area over which cellular service is contemplated, a number of steps shall be taken and careful planning is deliberated before the network is laid out. In what follows, the cellular architecture is briefly described. The treatment is brief and in outline form, as the intent is a quick review. For more extensive discussion on cellular concept, the reader is referred to Ref. [3–5].
1) Each service area is divided into a number of hexagonal-shaped areas called cell. Each cell has a transceiver system called base station (BS), base transceiver station (BTS) or cell site.
2) The available spectrum is divided into a set of duplex radio channels.
The set is further divided into a number of nonadjacent radio channel subsets. Different subsets of radio channels are assigned to neigh-boring cells’ base stations. This is to reduce adjacent channel interference, which is the interference between spectrally adjacent radio channels in the cellular network.
3) The cell coverage area conceptually has a hexagonal shape. The actual cell coverage area, which is generally different from the conceptual one, is called cell footprint as shown in Figure 2.2. Cell footprints are determined byfield measurements and signal propa-gation models. Figure 2.2 also illustrates that in actual cellular networks, there might be areas in which the received signal is so weak that coverage is not available, these are the so-called blind spots.
4) Cells with a single BS located at their centers are called center-excited cells. Cells with transceiver stations on their vertices are called edge-excited cells. Each station provides coverage for a section of the cell.
5) The power output of each BS is adjusted for the cell coverage and not beyond. This provision is carried out by power control protocols.
6) Directional antennas may be selected for base stations. The under-lying cell is divided into, normally, three or six sectors. Each sector is served by a single-directional antenna. This configuration reduces the interference, thus increases the network capacity. This process is called sectorization.
7) As the MS moves away from the BS, the signal loses its strength; the cellular network may automatically interface the MS to a new BS with a new assigned radio channel. This process, called handover or handoff, sustains the quality of communications at a predefined level. Handoff procedures constitute an important subset of cellular network mobility protocols. Depending on the architecture of the network, handoff may occur between adjacent cells, adjacent sectors, smaller and larger cell, and so on. The entire handoff process is transparent to the mobile user.
Figure 2.2 (a) Conceptual cellular plan with hexagonal-shaped cells. (b) Actual cellu-lar layout with cell footprints and blind spots shown by darkened areas.
8) When the traffic increases in a cell, the cell is divided into four or more smaller cells, each new cell will be able to handle almost as much traffic as the original cell. This is known as cell splitting, which is another key feature of cellular communications. Cell splitting may be implemented in static (permanent) or dynamic fashion.
9) The same set of radio channels are used in more than one cell. This is one of the key distinguishing features of cellular communications compared to conventional mobile communications, and is called frequency reuse. Cells with identical allocated radio channels are called co-channel cells. Reusing an identical radio channel in different cells in the cellular network creates co-channel interference (CCI).
CCI and adjacent channel interference (ACI) eventually limit the cellular network capacity for a given allocated spectrum.
10) In order to assemble an operational network that may also be nected to other telecommunications networks, several BSs are con-nected to a Mobile Switching Center (MSC) or Mobile Telephone Switching Office (MTSO). MSC-BS connection is formed through stationary links such as coaxial cables, terrestrial microwave, orfiber optic lines. MSCs are essentially software machines that act like the nerve centers of the network. MSCs are in charge of channel assign-ment to BSs and MSs, handoff processes, and a host of other protocols.
MSCs are connected to other MSCs and to public telecommunication networks such as PSTN and ISDN (integrated services digital network) as shown in Figure 2.3.
11) Signaling data and control information in cellular networks are trans-mitted through a small set of dedicated radio channels that are separate from traffic channels. This technique is known as common channel signaling. In other words, radio channels are divided into traffic channels and control channels.
12) Smart antenna systems with multiple-input multiple-output (MIMO) configuration are commonly incorporated into modern cellular net-works as a standard feature. MIMO techniques improve system throughput and performance at the expense of computational intensity and hardware/software complexity.
It is evident that the cellular concept, supported by frequency reuse notion, represents a major breakthrough in resolving the spectral scarcity and consequently has substantially increased the system capacity with a limited spectrum allocation. In summary, frequency reuse, mobility management, trunking, and the MIMO concept are fundamental ideas that form the foundation of modern wireless networks. We now further investigate key aspects of cellular network, which is the architecture adopted by AeroMACS as well.