CHAPTER 5 WIRELESS COMMUNICATIONS Introduction to Telecommunications by Gokhale Introduction • Wireless – Communications system in which electromagnetic waves carry a signal through atmospheric space rather than along a wire – Most systems use radio frequency (RF, which ranges from 3 kHz to 300 GHz) or infrared (IR, which ranges from 3 THz to 430 THz) waves – IR products do not require any form of licensing by the FCC 2 Timeline of Major Developments • Mobile Telephone System (MTS) – Introduced in 1946 – Simplex (one-way transmission) and manual operation • Improved Mobile Telephone System (IMTS) – Introduced in 1969 using a 450 MHz band • Advanced Mobile Phone Service (AMPS) – Introduced in 1983 – First system to employ a “cellular” concept 3 Cellular Topology • Cellular network: – Series of overlapping hexagonal cells in a honeycomb pattern • Cellular network components – Base Station:Transmitter, Receiver, Controller, Antenna – Cell: Base station’s span of coverage – Mobile Switching Center: Contains all of the control and switching elements to connect the caller to the receiver, even as the receiver moves from one cell to another 4 Personal Communications Systems (PCS) • PCS is also called Personal Communications Networks (PCN) • Goal of PCS is to provide integrated voice, data and video communications • Three categories of PCS: – Broadband: cellular and cordless handsets – Narrowband: enhanced paging functions – Unlicensed: allows short distance operation 5 Hierarchical Cell Structure • Key features of PCS – Variable cell size – Hierarchical cell structure (picocell, microcell, macrocell, supermacrocell) 6 Cells Analog Access • Analog Cellular Systems – First generation system – Based on FDMA (Frequency Division Multiple Access), where frequency band is divided into a number of channels. Each channel carries only one voice conversation at a time. – AMPS operates on 800 MHz or 1800 MHz – Advantages: • Widest coverage – Limitations: • Inadequate to satisfy the increasing demand • Poor security 8 • Not optimized for data FDMA 9 Digital Access • D-AMPS (Digital-AMPS) • TDMA (Time Division Multiple Access) • CDMA (Code Division Multiple Access) Digital wireless technologies provide greater system capacity. 10 TDMA • TDMA – Second generation system – Enables users to access the whole channel bandwidth for a fraction of the time, called slot, on a periodic basis – Has applications in satellite communications – Advantages • Improved capacity 11 TDMA 12 CDMA • CDMA – Third generation system – Separates users by assigning them digital codes within a broad range of the radio frequency – First technology to use soft-handoff – Employs spread spectrum technique – Advantages • Improved capacity, coverage, voice quality, and immunity from interference 13 An Overview of Cellular Technologies 14 Spread Spectrum Technique: FHSS • Frequency Hopping Spread Spectrum (FHSS) – Resists interference by jumping rapidly from frequency to frequency in a pseudo-random way – Advantage • Increases the total amount of available bandwidth through the assignment of multiple hopping sequences within the same physical area • More flexible than DSSS – Application • In large facilities especially with multiple floors 15 Spread Spectrum Technique: DSSS • Direct Sequence Spread Spectrum (DSSS) – Resists interference by mixing in a series of pseudo-random bits with the actual data – Advantage • If bits are damaged in transmission, the original data can be recovered as opposed to having to be retransmitted – Application • Is substituted for point-to-point or multi-point connectivity to bridge LAN segments – Limitation • Roaming capabilities are less robust 16 Spread Spectrum Technique: CDPD • Cellular Digital Packet Data – Allows for a packet of information to be transmitted in between voice telephone calls – Enables data specific technology to be tacked onto existing cellular telephone infrastructure 17 Wireless Applications • Cellular Phone – High mobility and narrow bandwidth (20 to 30 kHz) • Cordless Phone – Low mobility and narrow bandwidth (20 to 30 kHz) • Wireless LAN – Low mobility and high bandwidth (typically 10 Mbps) – Wireless Application Protocol (WAP) is a standard for wireless data delivery, loading web pages, and navigation 18 The Wireless Spectrum Narrowband, Broadband, and Spread Spectrum Signals • Narrowband - a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies. • Broadband - a type of signaling that uses a relatively wide band of the wireless spectrum. • Spread spectrum - the use of multiple frequencies to transmit a signal. Cellular Communications • Mobile telephone service - a system for providing telephone services to multiple, mobile receivers using twoway radio communication over a limited number of frequencies. • Mobile wireless evolution: – First generation – Second generation – Third generation Cellular Call Completion • Components of a signal: – Mobile Identification Number (MIN) - an enclosed representation of the mobile telephone’s 10-digit telephone number. – Electronic Serial Number (ESN) - a fixed number assigned to the telephone by the manufacturer. – System Identification Number (SID) - a number assigned to the particular wireless carrier to which the telephone’s user has subscribed. Cellular Call Completion Call Completion Advanced Mobile Pone Service (AMPS) • A first generation cellular technology that encodes and transmits speech as analog signals. Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) • Each voice signal is digitized and assigned a unique code, and then small components of the signal are issued over multiple frequencies using the spread spectrum technique. Global System for Mobile Communications (GSM) • A version of time division multiple access (TDMA) technology, because it divides frequency bands into channels and assigns signals time slots within each channel. • Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common in the United States. • Makes use of silences in a phone call to increase its signal compression, leaving more open time slots in the channel. Emerging Third Generation (3G) Technologies The promise of these technologies is that a user can access all her telecommunication services from one mobile phone. • CDMA2000 - a packet switched version of CDMA. • Wideband CDMA (W-CDMA) - based on technology developed by Ericson, is also packetbased and its maximum throughput is also 2.4 Mbps. Wireless Local Loop (WLL) • A generic term that describes a wireless link used in the PSTN to connect LEC central offices with subscribers. • Acts the same as a copper local loop. • Used to transmit both voice and data signals. Local Multipoint Distribution Service (LMDS) • A point-to-multipoint, fixed wireless technology that was conceived to supply wireless local loop service in densely populated urban areas and later on a trial basis to issue television signals. • A disadvantage is that its use of very high frequencies limits its signal’s transmission distance to no more than 4km between antennas. Multipoint Multichannel Distribution System (MMDS) • Uses microwaves with frequencies in the 2.1 to 2.7 GHz range of the wireless spectrum. • One advantage is that because of its lower frequency range, MMDS is less susceptible to interference. • MMDS does not require a line-of-sight path between the transmitter and receiver.