Cellular Network Scenario
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 center. Figure 5.1 shows the basic architecture of a cellular network with hexagonal cells.
FIGURE 5.1 Basic cellular network scenario.
It contains a mobile switching center (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, signaling, 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 5.1 First-Generation Characteristics of Cellular Standards
5.1.2 2G and 3G Technology Review
The evolution of wireless personal communications was determined basically by the solutions to two problems. First, a new technology that did not have the same problems as the 1G system in terms of roaming and area code numbers was designed in Europe. Due to interests in evolving technologies with more potential growth, digital technologies were the principal option. The solution in this case was GSM. Second, when bandwidth demand in the United Stateswas higher than what service providers could offer, the FCC granted more bandwidth in exchange for a long-term solution. Digital technologies were considered due to their evolutionary potential, and the U.S. TDMA and CDMA were the technologies chosen.
The fundamental concepts defining each digital technology are the modulation and MAC method used. There are several options, but only a few of these concepts were suitable for integrating the circuits in portable and mobile handsets.
FIGURE 5.2 Evolution toward 3G.
Table 5.2 Second-Generation Characteristics of Cellular Standards
Table 5.2 presents a summary of the modulation, channel bandwidth, and multiple access used by 2G technologies. The main standard for 2G is GSM, especially in Europe. In America, GSM1900 (a variation of GSM) and CDMA (IS-95) are both used. One of the limitations of these technologies is data rate and capabilities of handling data applications such asWeb browsing. For example, in the beginning GSM and CDMA could only offer data rates of up to 9.6 Kbps.
Figure 5.3 shows a visualization of the concept of FDMA compared toTDMA; in the latter, the time dimension was added to the frequency dimension already used in FDMA. Figure 5.4 displays CDMA, which adds the code dimension to the frequency.
FIGURE 5.3 FDMA and TDMA concepts. (a) One user per channel. (b) One user per channel per slot.
FIGURE 5.4 CDMA concept with N codes and N users per channel.
The most important parts of CDMA that explain its popularity are power control and orthogonal codes. Even though signals from users are not necessarily synchronized, the codes have properties of quasizero cross-correlation that allow them to eliminate multiple access interference (MAI). Power control is necessary in order to extract the information from the superposition of the signals from all the users transmitting, since it allows a fair comparison of these signals by not letting any of them be stronger to the extent of eliminating the desired information.
Since increased demand for service and for new applications is a market constant, 2G systems needed to evolve. Such upgrades were termed 2.5G. The enhanced systems have as many features as their 3G evolution has. The enhancements of 2G toward 2.5G were concentrated in the area of data services; technologies such as general packet radio services (GPRS) became available. Other technologies included enhanced data rates for global evolution (EDGE), originally defined as enhanced data rates for GSM evolution, and high-speed circuit-switched data (HSCSD).
HSCSD is the system that allows handling the frequency channels the same as in a normal GSM system,but it lets the network assign several time slots to the same user,hence increasing data rate. The upgrade did not require hardware changes in the network, only new mobile handsets that could use HSCSD. The problem with this technology is that frequency and time are used in a circuit-switched fashion, which makes some resources unavailable to other terminals even if the resources are not being used at specific times. A point in favor of HSCSD is that since information and resources are handled according to a circuit-switched criterion, it works with real-time services.
In contrast, GPRS, which is the most used, is not well suited for real-time applications. When GSM is upgraded with GPRS, the eight time slots are used and the data rate increases up to 115 Kbps. This is achieved by using a packetswitching service,which allows the occupancy of the resource by one user only when there is a packet to be transmitted. The most important applications in GPRS are email andWeb browsing, which do not require real-time delivery of information.
GSM uses a modulation known as GMSK where information symbols are processed by a Gaussian filter before modulation. EDGE, in contrast, changes the modulation scheme to 8PSK, which allows transmission of three bits per symbol in the same bandwidth; thus, the data rate is three times higher than for GSM. EDGE can coexist with GSM, allowing users to keep their handsets when not requiring the higher data rate services. Due to distortion caused in the channel and path loss, 8PSK is not suitable for long distance; hence, the combination of data transmission systems of GPRS and EDGE, known as enhanced GPRS or EGPRS, can be deployed to service data and voice. The maximum data rate achieved with EGPRS using the eight time slots of the frequency channel is 384 Kbps, which is the standard data rate handled in 3G systems.
The standard IS-95 CDMA started providing a data rate of 9.6 Kbps, and later incremented to 14.4 Kbps in the first version. The standard was reviewed and in its B version incremented its data rate to 64 Kbps. Version IS-95C works with data rates of up to 144 Kbps. This last version represents a smooth transition to 3G toward the standard cdma2000; in brief, there is a direct evolution of IS- 95 to 3G with cdma2000, but not in the case of GSM, which needs to evolve towardWCDMA. The set of protocols together with the standard IS-95 is known as cdmaOne. The power control mechanism adjusts the transmission power in 1-dB increments 800 times per second.