Wireless Technologies Wireless refers primarily to the cellular telephone industry. Wireless is also used to refer to some other radiolike services, including wireless local-area networks (LANs) and personal- area networks (PANs). Other special wireless systems are radio frequency identification (RFID) systems and numerous infrared wireless devices. Ultrawideband (UWB) is a technology used in wireless networking and low-cost short-range radar systems.
Topics Covered in Chapter 20 Cellular Telephone Systems Digital Cell Phone Systems Wireless LANs and PANs PANs and Bluetooth Infrared Wireless Radio Frequency Identification Ultrawideband Wireless
Cellular Telephone Systems A cellular radio system provides standard telephone service by two-way radio at remote locations. Cellular radios or telephones were originally installed in cars or trucks, but today most of them are available in handheld models. Cellular telephones permit users to link up with the standard telephone system, which permits calls to any part of the world.
Cellular Telephone Systems (Continued) Cellular radio telephone service is available nationwide. The original cell phone system, known as the advanced mobile phone system, or AMPS, was based on analog technologies. Although AMPS is still in use, it has gradually been phased out by second- (2G) and third-generation (3G) digital cell phone systems.
Cellular Concepts The basic concept behind the cellular radio system is that rather than serving a given geographical area with a single transmitter and receiver, the system divides the service area into many small areas known as cells. The typical cell covers only several square miles and contains its own receiver and low-power transmitter. The coverage of a cell depends upon the density (number) of users in a given area.
Cellular Concepts (Continued) Each cell is connected by telephone lines or a microwave radio relay link to a master control center known as the mobile telephone switching office (MTSO). The MTSO controls all the cells and provides the interface between each cell and the main telephone office. As the vehicle containing the telephone passes through a cell, it is served by the cell transceiver.
Cellular Concepts (Continued) The telephone call is routed through the MTSO and to the standard telephone system. As the vehicle moves, the system automatically switches from one cell to the next. The cellular system operates primarily in the 800- to 900-MHz range. A newer cellular band, designated the personal communications system (PCS) band extends from 1850 to 1990 MHz and is called PCS-1900.
AMPS Handset Although AMPS cell phones are gradually being phased out, millions are still in use. An AMPS unit consists of five major sections: transmitter, receiver, synthesizer, logic unit, and control unit. Mobile radios derive their operating power from the car battery. Portable units contain built-in rechargeable batteries. The transmitter and receiver share a single antenna.
Digital Cell Phone Systems Most new cell phones and systems use digital rather than analog methods. All-digital systems were developed primarily to expand the capacity of existing cell phone systems. Digital techniques provide several ways to multiplex many users into the same spectrum space. Digital systems are more reliable in a noisy environment.
Digital Cell Phone Systems (Continued) Digital circuits can be made smaller and more power- efficient and therefore handsets can be more compact and can operate for longer periods of time on a single battery charge. Digital cell phones greatly facilitate the transmission of data as well as voice so that data services like email and Internet access are possible with a cell phone. Most modern digital phones are referred to as second- and third-generation (2G and 3G) phones.
2G Cell Phone Systems Three basic second-generation (2G) digital cell phone systems are in wide use today. Two of them use time division multiplexing (TDM), and the third uses spread spectrum (SS). The TDM systems are the Global System for Mobile Communications (GSM) and the IS-136 standard for time division multiple access (TDMA). The SS system is code division multiple access (CDMA).
Vocoder To use digital data transmission techniques first requires that the voice be digitized. The circuit that does this is a vocoder, a special type of analog-to-digital (A/D) converter and digital-to- analog (D/A) converter. With voice frequencies as high as 4 kHz, the minimum Nyquist sampling rate is 8 kHz. The A/D in a vocoder should sample the voice signal every 125 μs and generate a proportional binary word.
Vocoder (Continued) This serial data signal, representing the voice, is now used to modulate the carrier and the composite signal transmitted over the assigned channel. The other technique is data compression. Data compression techniques process the digitized voice signal in such a way as to reduce the number of bits needed to represent the voice reliably. In modern cell phones a variety of vocoding data compression schemes are used.
IS-136 TDMA IS-136 (IS means interim standard) is the Telecommunications Industry Association (TIA) standard that fully describes the time division multiple access (TDMA) cell phone system. TDMA is also known as digital AMPS (DAMPS), American digital cellular (ADC), or North American TDMA (NA-TDMA). IS-136 operates concurrently on the same 800- to 900-MHz band channels used by AMPS and is also used in the PCS-1900 bands.
Global System for Mobile Communications The most widely used 2G digital system is GSM. GSM originally stood for Group Special Mobile but has become known as Global System for Mobile Communications. GSM is widely implemented in both the 800- and 1900-MHz personal communication system band. It is gradually replacing the IS-136 systems in the United States.
IS-95 CDMA The IS-95 CDMA TIA cell phone standard is called code division multiple access (CDMA) and is also known as CDMA One. CDMA uses direct sequence spread spectrum (DSSS) with a 1.2288-MHz chipping rate that spreads the signal over a 1.25-MHz channel. Up to 64 users can use this band simultaneously with little or no interference or degradation of service. The CDMA system uses FDD for duplexing.
Digital Cell Phone Circuits Digital cell phones are quite different from analog phones. Because they use digital techniques and pulse modulation methods, and since massive growth in cellular usage has caused spectrum crowding and interference problems, new architectures and circuits have been developed. A variety of different circuits have been created to accommodate numerous standards.
Digital Cell Phone Circuits (Continued) Three major trends dominate the cell phone evolution: increased digital processing, increased integration of circuitry on a few chips, and multimode/multiband phones. Most new digital phones also contain AMPS circuitry. If a subscriber roams into an area lacking a carrier that uses digital technology, the phone reverts to analog, which is still supported in most areas.
2G Digital Cell Phone The RF section contains the transmitter and receiver circuits including mixers, local oscillators or frequency synthesizers for channel selection, the receiver LNA, and the transmitter power amplifier. The baseband section contains the vocoder with its a/D and D/A converters plus a DSP chip that handles many processing functions. An embedded controller handles all the digital control and signaling, handoffs, and connection and identification operations. The controller also runs the display and keyboard and all other user functions such as number storage, auto dialing, and caller ID.
Direct Conversion Superheterodyne designs are still used, however, variations such as direct-conversion and very low IF designs have been implemented. The direct-conversion or zero IF design sets the LO frequency to the incoming signal frequency so that the translation is directly to the baseband signal. Since direct conversion works only with double- sideband suppressed (DSB) AM signals, changes have been made to accommodate FSK, BPSK, QPSK, and other forms of digital modulation.
Direct Conversion (Continued) Direct conversion eliminates the need for an expensive and physically large selective IF filter. Direct conversion eliminates the imaging problem so common in superheterodyne designs, especially in the crowded multiband cellular spectrum. With direct conversion, baseband filtering can be accomplished using simple low-pass RC filters and/or DSP filters.
Low IF When an IF is used near the baseband frequencies, filtering is simple and very effective. Most 2G and later phones are multiband phones that can operate in two or three bands, thereby permitting widespread roaming. The signal passes through one of three SAW filters and feed into a mixer. An image reject mixer uses a technique similar to the phasing method of generating a single sideband (SSB) signal.
2.5G Cell Phone Systems The designation 2.5G refers to a generation of cell phones between the original second-generation (2G) digital phones and newer third-generation (3G) phones. 2.5G phones bring data transmission capability to 2G phones in addition to normal voice service. A 2.5G phone permits subscribers to exchange emails and access the Internet by cell phone. The two technologies used in 2.5G systems are EDGE and GPRS.
2.5G Cell Phone Systems (Continued) The most popular 2.5G technology is the general packet radio service (GPRS). This system is designed to work with GSM phones. It uses one or more of the eight time slots in a GSM phone system to transmit data rather than digitized voice. A faster 2.5G technology is enhanced data for GSM evolution (EDGE). It uses 8-PSK modulation instead of GMSK to achieve even higher data rates up to 384 kbps.
3G Cell Phone Systems Third-generation (3G) cell phones are true packet data phones. 3G phones feature enhanced digital voice and high- speed data transmission capability. 3G applications include fast email and Internet access. 3G phones are being packaged with personal digital assistants (PDAs). High speed also permits the transmission of video.
Base Stations The most complex and expensive part of any cellular telephone system is the network of base stations that carriers must have to make it all work. Base stations consist of multiple receivers and transmitters so that many calls can be handled on many different channels simultaneously. The most visible feature of a base station is its antenna on a tower. Base station antennas have become directional which helps to increase subscriber capacity.
Horizontal Radiation and Reception Pattern of a Cell Site Antenna
Wireless LANs and PANs Local-area networks (LANs) are still interconnected mainly by CAT5 twisted pair. Wireless extensions and even complete wireless LANs have become more common now that reliable, low-cost wireless modems are available. Wireless personal-area networks (PANs) are being implemented in a variety of applications.
Wireless LANs In a wireless LAN, the computers or nodes are linked to one another by radio. Each computer contains a sophisticated modem that both transmits and receives over a short distance. Each wireless node is linked back to a server that makes the connection between different nodes. The most robust, affordable, and flexible standard is the 802.11b IEEE wireless Ethernet standard. Flexibility and cost make wireless LANs so appealing.
Wireless Gateway Using 802.11b Wireless Ethernet
PANs and Bluetooth A personal-area network (PAN) is a very small network that is created informally or on an ad hoc basis. A PAN typically involves two or three nodes, but some systems permit many nodes to be connected in a small area. PANs can be wired, but today all are wireless. The most popular wireless PAN system is Bluetooth, a standard developed by the cell phone company Ericsson for use as a cable replacement.
Bluetooth Bluetooth is a digital radio standard that uses frequency-hopping spread spectrum (FHSS) in the unlicensed 2.4-GHz ISM band. Three levels of transmission power have been defined depending upon the application. Bluetooth transceivers are available in either one or two chip sets that interface to the PAN. Bluetooth transceivers send out search signals and then listen for nearby equipped Bluetooth devices.
Bluetooth (Continued) If another Bluetooth device comes into range the two Bluetooth devices automatically interconnect and exchange data. These devices form what is called a piconet, the linking of one Bluetooth device that serves as a master controller to up to seven other Bluetooth slave devices. Bluetooth devices can also link to other piconets to establish larger scatternets.
Bluetooth (Continued) The main applications for Bluetooth are cordless headsets for cell phones, wireless connections between PCs, or laptop computers and PDAs. Bluetooth applications include: laptop connections at meetings, wireless printer-to-PC connections, laptop- to-cell phone connections, wireless audio headsets, and wireless digital camera-to-TV set connections. The Bluetooth standard is maintained by the Bluetooth Special Interest Group (SIG) and supported by more than 2000 manufacturers.
Infrared Wireless Perhaps the most widespread wireless system uses infrared (IR) light for short-distance data communication. The most widely used is the wireless remote control on TV sets, VCRs, and DVD players and on most audio CD stereo systems. Infrared has also been used for wireless LANs and PANs.
TV Remote Control Almost every TV set sold these days, regardless of size or cost, has a wireless remote control. Other consumer electronic products have remote controls including VCRs, cable TV converters, CD and DVD players, stereo audio systems, and some ordinary radios. Generic remote controls are available to hook up to any device that you wish to control remotely.
TV Remote Control (Continued) All remote control devices work on the same principle. A small handheld battery-powered unit transmits a serial digital code via an IR beam to a receiver that decodes it and carries out the specific action defined by the code. A TV remote control is one of the more sophisticated of these controls, for it requires many codes to perform volume control, channel selection, and other functions.
IR PANs Besides remote control, the primary application for IR data communication is in short-distance links between computers, computers and printers, or ad hoc PANs. Distance links are typically up to 1 m, however under some conditions, the distance can be extended to 9 m. There must be a clear line of sight between the transmitter and receiver.
IR PANs (Continued) An IR transceiver connects to interface circuitry in the PC or PDA. The interface is typically a small embedded controller inside the computer or PDA. The encoder puts the serial digital data from the PC or PDA into the proper format for transmission. A high-current bipolar transistor or MOSFET drives one or more IR LEDs.
IR PANs (Continued) The receiver consists of the PIN diode that picks up the IR light from a nearby transmitter. The signal is amplified and shaped and then sent to the decoder, which recovers the original data. Although many laptops and PDAs have a built-in transceiver, their use is often restricted by this need for line of sight. A better arrangement is a receiver dongle which consists of a cable attached to the interface in the PC or PDA and to the movable dongle containing the LED and PIN diode.
IrDA System The most widely used IR data communication system was developed by Hewlett-Packard. It has since become an international standard that is maintained by the Infrared Data Association (IrDA). The complete interface and system are referred to as IrDA. The systems are designed for a short range of 20 to 30 cm, but can be used up to 1m.
IrDA System Most systems use data speed rates of 4 Mbps, however, a 16-Mbps version is now available. IrDA does not use a modulated IR beam, but rather baseband transmission that requires encoding and decoding. The standard NRZ serial data is converted into pulses especially encoded for IR operation. The 4-Mbps version uses another encoding scheme, called 4 PPM (pulse position modulation).
Radio Frequency Identification Another growing wireless technique is radio frequency identification (FRID). RFID uses thin, inexpensive tags or labels containing passive radio circuits that can be queried by a remote wireless interrogation unit. The tags are attached to any item that is to be monitored, tracked, accessed, located, or otherwise identified.
Radio Frequency Identification (Continued) RFID tags are widely used in inventory control, container and parcel shipping, capital equipment and other asset management, baggage handling, and manufacturing and production line tracking. Other applications for RFID tags are personnel security checking and access, animal tracking, and theft prevention. As the technology develops, prices drop and new applications are being discovered.
RFID Operation The tag is a very thin labelike device into which is embedded a simple passive single-chip radio transceiver and antenna. The chip also contains a memory that stores a digital ID code unique to the tagged item. For the item to be identified, it must pass by the interrogation or reader unit, or the reader must physically go to a location near the item.
RFID Operation (Continued) The reader unit sends out a radio signal that may travel from a few inches up to no more than a hundred feet or so. The radio signal is strong enough to activate the tag. The tag rectifies and filters the RF signal into direct current that operates the transceiver. This activates a low-power transmitter that sends a signal back to the interrogator unit along with its embedded ID code. The reader checks its attached computer where it notes the presence of the item and may perform other processing tasks associated with the application.
Ultrawideband Wireless Perhaps the newest and most unusual form of wireless is known as ultrawideband (UWB) wireless. The primary application of UWB has been military radar. Also known as impulse, basband, or carrierless wireless, UWB transmits data in the form of very short pulses, typically less than 1 ns. The UWB transmitter circuits use BPSK to generate pulses which are applied directly to the antenna.
Ultrawideband Wireless (Continued) The receiver amplifies the incoming signal and then applies it to a correlator consisting of a multiplier, where it is multiplied by a stream of coded pulses similar to those transmitted. The multiplier output exceeds a specific level, it is considered to be detected and recovered. The recognized signal is then demodulated into the original data. Broadband antennas are used for UWB.
Advantages and Disadvantages of UWB UWB offers many benefits to radar, imaging, and communication applications: Superior resolution in radar and imaging. Immunity to multipath propagation effects. Higher data rates than are possible with other wireless technologies License-free operation
Advantages and Disadvantages of UWB (Continued) No interference to other signals using the same frequency band. UWB signals appear as random noise to conventional radios. Power-efficient. Extremely low-power operation. Peak power levels are in the milliwatt region, and average power in microwatts. Simple circuitry, most of which can be integrated in standard CMOS. Potentially low cost.