12.3 Wireless Local Area Networks

Figure 12.4 INTELSAT-6 satellite.

is called a wireless wide area network (WWAN) or a wireless metropolitan area network (WMAN). A network covering only a small range is called a wireless personal area network (WPAN).

Computers, printers, robots, and so on can be connected to a wireless network by infrared or radio links. Most WLANs use radio waves and are also called radio local area networks (RLANs). Radio waves can penetrate most walls and floor surfaces, whereas these solid objects block infrared. WLANs offer many advantages over traditional wired networks, for example, user mobility, installation speed, and flexibility.

Many standards are available for WLANs. IEEE 802.11, Bluetooth, and HomeRF operate in the unlicensed industrial, scientific, and medical

(ISM) band, 2,400 to 2,483.5 MHz. Bluetooth and HomeRF are more like WPANs than WLANs. HiperLAN/2 is a high-performance standard operating in the 5-GHz band that was developed by the European Telecommunications Standards Institute (ETSI). DECT is a standard for cordless phones that can also be used for WLANs.

316 Radio Engineering for Wireless Communication and Sensor Applications

Figure 12.5 Coverage areas of an INTELSAT satellite over the Atlantic Ocean.

No license is required for low-power transmitters operating in the 2.45-GHz band. In addition to WLANs, several other applications, including microwave ovens and radio frequency identification (RFID) systems, use this band. To reduce interference between different users, spread spectrum techniques can be used. There are two types of spread spectrum techniques: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum

(FHSS) [2].

In DSSS, a digital bit stream representing the source data is multiplied by a pseudorandom (PN) code with a bit or chip rate much higher than that of the data. Thus this product has a much higher symbol rate than the original data, causing the spectrum to spread. In the receiver, the original data can be recovered by multiplying the received signal with the same PN code. Only the signal with the same PN code despreads. To an unintended receiver, a DSSS signal appears as wideband noise.

In FHSS, the transmitter changes carrier frequency in a pattern known to the receiver. To an unintended receiver, an FHSS signal appears to be short-duration impulse noise.

12.4 Mobile Communication

Mobile communication [3–5] has grown faster during the last decade than any other application of radio engineering. Optical fibers and copper cables cannot compete with freely propagating radio waves in this application. In addition to cellular mobile systems, which are the main topic of this section, a moving person can use satellite systems or pager systems for voice or data transfer. The geostationary satellites of International Mobile (earlier Maritime) Satellite Organization (INMARSAT) offer communication services for ships, airplanes, and trucks. The operating frequencies between the satellites and mobile users are 1.6/1.5 GHz (uplink/downlink). Several other systems based on satellites on low Earth orbit (LEO) have been launched since 1998. Compared to the GEO systems, many more satellites are needed in the LEO systems, but shorter path lengths allow the use of handheld terminals.

In a cellular network, the coverage areas of the base stations form a ‘‘cellular’’ structure. To offer a good availability both in densely and sparsely populated areas, cells of different sizes are needed. To avoid interference, adjacent cells use different frequencies. The same frequencies can be reused in cells, which are far enough from each other. Also, the power levels of transmitters should be controlled to reduce interference. When a mobile phone moves from one cell to another, the network has to take care of this handover without interruption. (Cellular systems can be used also for rough locating because the cell, in which the mobile unit locates, is known. The accuracy may be enhanced by using measurements performed by more than one base station.)

In the development of cellular mobile systems, three generations can be distinguished: first generation analog systems, second generation digital systems, and third generation wideband systems. In addition to voice transfer, the second generation systems are usable also for low bit rate data services. Third generation systems support wideband multimedia services. The following introduces an example of each generation.

Nordic Mobile Telephone (NMT) represents the first generation. It was developed by the Nordic countries: Denmark, Finland, Norway, and Sweden. The NMT 450 network, operating in the 450-MHz band, was launched in 1981. In response to congestion, the upgraded NMT 900 network, which had a larger capacity, started in 1986. NMT networks are still in operation in many countries outside Scandinavia. Several other analog systems have been in use around the world, including AMPS in the United States and TACS in the United Kingdom. However, due to emerging digital systems, many first generation networks have been closed down.

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In NMT 450, the transmitting frequencies of mobile phones (receiving frequencies of base stations) are 453 to 457.5 MHz, and the transmitting frequencies of base stations (receiving frequencies of mobile phones) are 463 to 467.5 MHz. Thus, the duplex frequency is 10 MHz. In NMT 900, the transmitting and receiving frequencies are 890 to 915 MHz and 935 to 960 MHz, and the duplex frequency is 45 MHz. A frequency channel is assigned for each user in a cell, that is, the access method is the frequency division multiple access (FDMA). The channel spacing is 25 kHz. Thus, NMT 450 has 180 channels and NMT 900 has 1,000 channels. The modulation method is FM. The mobile telephone exchange (MTX) is a central component of the NMT network. Each MTX is responsible for a group of base stations. The MTX determines the frequencies and transmitting powers of mobile phones, and takes care of the handovers from one base station to another. One base station covers an area with a radius of 0.5 km to 20 km.

Global System for Mobile Communications (GSM) represents the second generation. It is a digital cellular standard developed at first by CEPT and later by ETSI. Originally, the acronym GSM came from the name of the study group Groupe Spe´cial Mobile, which was formed to develop a common European standard. The first GSM networks were launched in 1991. GSM was rapidly accepted worldwide. Other second generation standards include Personal Digital Cellular (PDC) operating in Japan, American standards IS95 and IS-136, and Trans-European Trunked Radio (TETRA), which is a standard for private mobile networks.

GSM (or GSM 900) operates in the same frequency range as NMT 900 (i.e., 890–915 MHz and 935–960 MHz). The access method is a combination of the FDMA and TDMA. There are 124 carrier frequencies, which are spaced 200 kHz apart. Each frequency channel is divided into eight time slots for different users. Mobile phones and base stations transmit short bursts of data. The bit rate during a burst is 270.833 kbit/s. The length of a burst is 0.577 ms or 156.25 bit periods. Consequently, a TDMA frame of eight bursts lasts 4.615 ms. Effects of multipath fading are alleviated by using slow frequency hopping, that is, the carrier frequency changes from one frame to another. The modulation method of the carrier is a Gaussian minimum shift keying (GMSK). To reduce the number of bit errors, channel coding and interleaving is used. In channel coding, redundancy bits are added in order to detect and correct errors. Interleaving disperses a series of consecutive bit errors to several blocks, making error correction easier.

Figure 12.6 shows the architecture of a GSM network. The mobile station is connected to a base transceiver station (BTS) via a radio link. The base station controller (BSC) controls a group of BTSs and manages their

Figure 12.6 Architecture of GSM network. (MS = mobile station; HLR = home location register; VLR = visitor location register; EIR = equipment identity register; AuC = authentication center; PSTN = public switched telephone network; ISDN = integrated services digital network.)

radio resources. The mobile-services switching center (MSC) performs the switching functions of the network and provides a connection to other networks. MSCs also take care of the registration, authentication, and location updating of subscribers. Received power levels are continuously monitored. MSCs and BSCs make decisions about handovers using these received signal strengths. Transmitter power levels are also controlled according to the signal strengths; the maximum transmitter power of a handheld phone is 2W, and thus the maximum average power is (2W)/8 = 0.25W.

Digital Cellular System, DCS 1800 or GSM 1800, is an upgraded version of GSM 900. Mobile units transmit at frequencies from 1,710 to 1,785 MHz and base stations at 1,805 to 1,880 MHz. The number of frequency channels is 374. GSM 1800 is especially useful in metropolitan areas where the cells are small.

GSM supports data transfer at the speed of 9.6 or 14.4 kbit/s. An extension to the GSM standard, High-Speed Circuit Switched Data (HSCSD), permits using three time slots per frame, allowing a data speed of 43.2 kbit/s.

General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) are further extensions of GSM, which may operate in the existing GSM networks. GPRS is better suited for data transfer than GSM or HSCSD, which are circuit-switched systems. In GPRS, data is transmitted in packets and more than one timeslot per TDMA frame may be allocated for a user. In EDGE, the eight-level phase modulation, 8PSK, is used. One 8PSK symbol contains 3 bits of data, allowing a higher transfer rate than the GMSK modulation of GSM.

The standardization work for the third generation systems is ongoing. Within ITU these systems are called International Mobile Telecommunications