Electronics · Ch 13 — Modern Communication Systems
Chapter Summary
Chapter Summary
- A cellular mobile telephone system links a mobile user into the standard telephone network. Early analog AMPS has been replaced by digital 2G/2.5G/3G/4G systems, which use the spectrum more efficiently, resist noise better and allow compact, feature-rich handsets. The 2G systems are GSM, IS-136/TDMA and CDMA.
- A 2G handset (Figure 13.1.1) has an RF section (LNA, mixers, frequency synthesizer, power amplifier), a baseband section (vocoder, A/D and D/A converters, DSP) and an embedded controller running the display, keyboard and user features.
- The service area is broken into hexagonal cells by cell splitting, which raises channel capacity and shrinks cell radius. Every cell links to the master control centre, the MTSO. Frequency reuse lets clusters of distant cells share the same frequencies, and call hand-off switches an ongoing call from a weaker to a stronger cell as the user moves.
- Cell phones use the UHF/microwave bands (originally 824–849 MHz uplink and 869–894 MHz downlink, 832 channels of 30 kHz; later the PCS band, 1850–1990 MHz). Setting up a call follows a fixed nine-step sequence from powering up to termination.
- The Internet is a public, worldwide network of networks reached through an ISP. Communication follows protocols (TCP, IP, FTP, HTTP), and every resource has a unique URL. (Per the official corrigendum, a URL has the form scheme//www.username.domain(host)/filename, e.g. http://www.google.com/index.html.) Domains such as .com, .edu, .net, .in identify the organisation type.
- Wi-Fi (IEEE 802.11) is a wireless LAN using the 2.4 GHz and 5.8 GHz bands up to ~100 m, with CSMA/CA access (see Table 13.2.2 for the 802.11a/b/g/n standards). Bluetooth is a short-range PAN technology in the ISM band (2400–2480 MHz) using FHSS and FSK; one master plus up to seven slaves forms a piconet.
- Optical fibre communication carries information on a beam of light through a glass/plastic fibre, giving enormous bandwidth — a light source (usually an LED) is switched on and off by digital pulses and a light detector recovers the signal (Figure 13.2.4).
- A communication satellite is a relay/repeater: an Earth station's uplink (~6 GHz) is received, amplified and frequency-translated by a transponder and retransmitted as a lower-frequency downlink (~4 GHz). …