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Position location strategies, applications (Part 1)

Posted: 02 Nov 2011     Print Version  Bookmark and Share

Keywords:CDMA  TDMA  Position location  Cellular network 

The first U.S. cellular telephone system was deployed by Ameritech in Chicago in late 1983. It used the standard advanced mobile phone system (AMPS), for which the FCC had just allocated 40MHz of spectrum in the 800- MHz band. Each channel, one-way, has a bandwidth of 30kHz, so the duplex channel has a total bandwidth of 60kHz; therefore, the total 40MHz contain 666 duplex channels. The modulation technique used is FM with a multiple access scheme of FDMA. Hence, this is an analogue system.

The channels in the 800MHz band are numbered from 1 to 1023, excluding the numbers 800 to 989. If the reverse channel uses channel n, from the reverse channel band, the forward channel will use the nth channel from the forward channel band. The forward and reverse channel centre frequencies are separated by 45MHz.

Cellular network
The cellular scenario is based fundamentally on the cellular concept, where an area is divided into cells with antennas transmitting at a lower power. This scenario has at its heart the concept of using the same carrier frequency in different areas or regions separated by distances such that the co-channel interference does not keep them from using the same frequency in those regions. Frequency reuse has been common for many years;we find it in the AM/FM radio broadcast stations around each country where the same frequency band can be used in two distant cities for two different radio stations as long as their signal satisfies the co-channel interference criterion.

The basic cellular scenario consists of these areas or cells being serviced by low power antennas and coordinated by a switching centre. Figure 1 shows the basic architecture of a cellular network with hexagonal cells.

Figure 1: Basic cellular network scenario.

It contains a mobile switching centre (MSC) with connectivity to the public switched telephone network (PSTN). Several cells can be connected to an MSC in high-density areas where the number of MSCs needed increases. The MSC is in charge of functions such as mobile registration, mobile paging, call establishment, handoff management, channel and resource assignment, signalling, connectivity, and so on. It also has two databases that contain information about current users registered in the network: the home location register (HLR) and the visitor location register (VLR) that register customers whose service contract is with the home network or those that are roaming in this area, respectively. The information on each user with a contract in an area is registered in the HLR, which contains information such as the type of services to be provided.

The network is basically divided into regions called location areas;these areas are cells or groups of cells where users register in the network. Thus, the HLR and VLR registers contain user information such as the electronic serial number (ESN), the mobile identification number (MIN), the user's profile, service and restrictions, the user location given by the location area, and the base station servicing the user. The MSC together with the HLR or VLR stores the location area of the user. If a mobile station crosses the boundary between two different location areas, an update is initiated by the network MSC to register the new location area. This updating can be done periodically by the network regardless of location area changes.

The first generation of cellular communication networks was provided mainly by the standardAMPS among others. A brief reviewof some of these standards and their basic characteristics appears in Table 5.1. As shown in the table, we can see common denominators, such as the FM modulation and the medium access control (MAC), determined by FDMA. In addition,we see that depending on regions of the world, the frequency bands where the systems work vary slightly.

Table 1: First-generation characteristics of cellular standards.

2G and 3G review
When one thinks about the evolution from the first generation to the third generation (3G), one can describe such technologies by a key characteristic. For example, first generation was characterized by analogue modulation, while 2G is typified by the idea of digitalising the transmission—that is, the use of digital modulation,which requires information to be digitized. 3G has been characterized by the term multimedia, although one may find that it has some limitations in order to achieve quality of service for each application (figure 2). One of the most important facts of this evolution is that it has provided the base for incredible growth in terms of user numbers, unimaginable at the beginning of the 2G era.

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