Introduction to Telecommunications by Gokhale.ppt

1、Introduction to Telecommunications by Gokhale,CHAPTER 5,WIRELESS COMMUNICATIONS,2,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

2、infrared (IR, which ranges from 3 THz to 430 THz) waves IR products do not require any form of licensing by the FCC,3,Timeline of Major Developments,Mobile Telephone System (MTS) Introduced in 1946 Simplex (one-way transmission) and manual operation Improved Mobile Telephone System (IMTS) Introduced

3、 in 1969 using a 450 MHz band Advanced Mobile Phone Service (AMPS) Introduced in 1983 First system to employ a “cellular” concept,4,Cellular Topology,Cellular network: Series of overlapping hexagonal cells in a honeycomb pattern Cellular network components Base Station:Transmitter, Receiver, Control

4、ler, Antenna Cell: Base stations 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,5,Cellular Network Topology,6,Personal Communications Systems (PCS),PCS is also cal

5、led 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,7,Hierarchical Cell Structure,Key

6、features of PCS Variable cell size Hierarchical cell structure (picocell, microcell, macrocell, supermacrocell),8,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

7、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 Not optimized for data,9,FDMA,10,Digital Access,D-AMPS (Digital-AMPS) TDMA (Time Division Multiple Access) CDMA (Code

8、 Division Multiple Access)Digital wireless technologies provide greater system capacity.,11,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 Impr

9、oved capacity,12,TDMA,13,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 inter

10、ference,14,An Overview of Cellular Technologies,15,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

11、 multiple hopping sequences within the same physical area More flexible than DSSS Application In large facilities especially with multiple floors,16,Spread Spectrum Technique: DSSS,Direct Sequence Spread Spectrum (DSSS) Resists interference by mixing in a series of pseudo-random bits with the actual

12、 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,17,Spread Spectrum Techniq

13、ue: 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,18,Wireless Applications,Cellular Phone High mobility and narrow bandwidth (20 to

14、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,19,Bluetooth,Bluetooth is a uniting technology that

15、 allows electronic devices (like computers, headphones, keyboards) to make their own connections Originated in 1994 when Ericsson formed the Bluetooth Consortium with IBM, Intel, Nokia, and Toshiba Operates in the unlicensed 2.4 GHz band, an open frequency band in most countries, ensuring worldwide

16、compatibility Open standard that works at the two lower layers of the OSI model Includes application layer definitions for product developers to support data and voice applications Uses FHSS technique Bluetooth addressing 48-bit address is divided into 24-bit OUIs and 24-bit device address,20,Blueto

17、oth piconet: Master/Slave setup,21,Transmission Speed,Data rate: raw transmission speed Overhead: transmission rules and protocols Throughput: capacity available to the user,Overheads = Data Rate Throughput,22,Wireless LANs,Advantages of wireless LANs Highly beneficial for mobile professionals Real-

18、time communications improves efficiency, and productivity Recommended for hard-to-wire sites Solve problems like cabling restrictions and frequent reorganizations Disadvantages of wireless LANs Less functional and offer limited coverage More expensive to install than wired LANs Higher error rates du

19、e to interference from outside signals,23,Wireless LAN Specifications,IEEE 802.11 Standards for Wireless LANs 802.11 standards provide for interoperability between different manufacturers equipment Mobility is handled at Layer 2, especially the MAC sub-layer 802.11-compliant solutions consist of: Ac

20、cess points (wireless transceivers), and Wireless PC (PCMCIA) cards,24,IEEE 802.11 Standards,802.11a Uses the 5 GHz spectrum Provides maximum throughput of 54 Mbps Accommodates more users, but has shorter operating range when compared to 802.11b 802.11b Uses the 2.4 GHz unlicensed radio band Typical

21、 throughput of 11 Mbps 802.11g Same high speed as 802.11a and uses the 2.4 GHz band so it is backwards compatible with 802.11b,25,Microwave LANs,Microwave LANs utilize signals above 30 MHz, which requires licensing by the FCC Microwave LAN components Modem, RF unit, Antenna Restrictions on Microwave

22、 LANs Line-of-sight Antennas should not be more than 30 miles apart Communications are affected by atmospheric conditions such as rain and humidity Applications LAN-to-LAN connection,26,Microwave Relay System,27,Radio LANs,Types of Radio LANs Narrow-Band Radio LAN Have a cost advantage Lower data th

23、roughput Applications in warehousing and industrial environments Spread-Spectrum Radio LAN Highly reliable and secure Signal is attenuated by brick and concrete, and metal objects Applications in office environments Wireless LAN technology components PCMCIA cards and roaming-enabled access points,28

24、,Infrared (IR) LANs,Types of IR systems Line-of-sight Point-to-point high-speed connectivity Require line-of-sight Reflective Bounce signals off walls, ceilings and floors Scatter Use diffused signals Low-speed but better coverage,29,Broadband Wireless Systems,Wireless Local Loop (WLL) Used in place

25、 of wire-line local loop Broadband capability (can carry voice, data, and video) Local Multipoint Distribution System (LMDS) Requires line-of-sight Supports transmission over short distances High Capacity, High Cost Multichannel Multipoint Distribution System (MMDS) Wider coverage Low Capacity, Low

26、Cost,30,Comparison Table: Broadband Wireless Technologies,31,Satellite Communications,Components of a satellite system Satellite Earth Station Establishes and maintains continuous communication links with all other earth stations in the system Satellite A wireless transceiver placed in orbit around

27、the earth Each satellite band is divided into separate portions Uplink (earth to space) Downlink (space to earth) Applications of satellite communications Preferred in locations where high-speed wire connections are not an option for geographic or financial reasons Navigation, Weather monitoring, an

28、d Broadcasting,32,Satellite Frequency Allocations for Various Applications,33,Satellite Communications Parameters,Figure of Merit = Gr = receiver antenna gain (dB) Tsys = system noise temperature Standards for INTELSAT systems have set the figure of merit to be equal or higher than 40.7 dB,34,Geosyn

29、chronous Satellite (GEO),The rotational period of a GEO matches that of the Earth and its orbit is without inclination GEO is both geosynchronous and geostationary GEOs must orbit the equator at an altitude of 22,237 miles Use the Ku-band (12 to 14 GHz) frequencies for transmission, but Ka-band (27

30、to 40 GHz) is also practical Compared to Ku-band, Ka-band makes interference less likely, reduces power consumption and antenna size GEOs have a large footprint (about 40% of the Earth) Mainly used for international and regional communications Shortcoming is latency (about 240 ms),35,GEOs Footprint

31、is about 40% of the Earths Surface,36,Global Positioning System (GPS),GPS is a world-wide radio navigation system funded by the US Department of Defense GPS is formed by a constellation of 24 satellites at 11,000 mile altitude Satellites repeat the same track and configuration over any point approxi

32、mately each 24 hours,37,GPS Specifications,Six orbital planes are equally spaced and inclined at 55o with respect to each other, which provides between five and eight satellites visible from any point on the earth Each satellite has its own pseudo-random code so all GPS satellites can use the same f

33、requency without jamming GPS receiver on earth measures distance by timing, but since its timing is not as accurate as an atomic clock, it must make four simultaneous measurements,38,LEO and MEO Satellites,Characteristics of LEO (Low Earth Orbit) Satellites, and MEO (Medium Earth Orbit) Satellites T

34、he system consists of a large fleet of satellites, each in a circular orbit at a constant altitude They are not geostationary Can have problems with jitter or variable latency MEOs operate from an elevation between 1,800 and 6,500 miles while LEOs operate from an elevation between 500 and 1,000 mile

35、s. Therefore, fewer MEOs are sufficient to cover the globe.,39,International Wireless Communications,3G systems combining terrestrial and satellite communications are under development UMTS (Universal Mobile Telecom System) and IMT-2000 Based on W-CDMA (Wideband-CDMA) for wide-area applications and TD-CDMA for low-mobility indoor applications GSM (Global System for Mobile communications) 2G system based on TDMA Operates at 1900 MHz,