Wireless Networks. Why Wireless Networks? Wireless Local Area Network. Wireless Personal Area Network (WPAN)

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Tobias Wade
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1 Wireless Networks Why Wireless Networks? rate MBit/s wired terminals WMAN WLAN CORDLESS (CT, DECT) Office Building stationary walking drive Indoor HIPERLAN UMTS CELLULAR (GSM) Outdoor Mobility CT - Cordless Telephony (analogous predecessor of DECT) DECT - Digitally Enhanced Cordless Telecommunications (standard for cordless telephones within the local range) GSM - Global System for Mobile Communication (cellular radio standard) UMTS - Universal Mobile Telecommunications System (universal radio system which unites many different systems) WLAN - Wireless Local Area Network (standard for wireless connection of (handheld) computers) HIPERLAN alternative LAN to WLAN, wireless ATM extension WMAN - Wireless MAN (technology for crossing the last mile between network and user) beyond that: Satellite systems, Bluetooth/IrDA within the short-distance range Page 1 Characteristics Radio transmission, need for new transmission protocols Advantages Spatially flexibly within the reception area Ad-hoc networks can be realized without previous planning No wiring problems (e.g. historical buildings, fire protection, aesthetics) More robustness against disasters such as earthquake, fire - and also inattentive users, who pull plugs! Disadvantages Generally still very low data rates for larger number of users Often proprietary solutions with higher performance, standards are less efficient Following many national regulations, global regulations are only slowly created Limited frequency spectrum, interference of the frequencies Page 2 Wireless Personal Area Network (WPAN) IrDA (Infrared Association) Infrared standard for the connection of peripheral devices rate up to 4 MBit/s, data rate in the use however only 115 KBit/s A few meters range, only in direct line of view Susceptible to disturbances IEEE (WPAN, Bluetooth): rate up to 723 KBit/s Range up to approx. 10/15 meters (with higher transmitting power: up to 100 meters), creation of small radio cells Ad-hoc networking: spontaneous (automatic) connection of several mobile devices (maximum of 8) to an own small network Used with cellular phones, personal digital assistants (PDAs), Wireless Local Area Network IEEE (Wireless LAN, WLAN) Standard for the support of mobile computers rates: 2 about 100 MBit/s Physical layer and MAC: can be regarded as wireless variant of Ethernet Base stations (Access Point, AP) connect WLAN directly with Ethernet, additionally Ad-hoc networks are possible Physical media: infrared and radio transmission IEEE (Wireless MAN, Wireless local loop) focuses on mobility, on wireless building covering Higher data rate ( MBit/s), larger range HIPERLAN (High Performance Radio Local Area Network) Different variants, of type 1 with 23,5 MBit/s to type 4 with 155 MBit/s Range of approx. 50 meters up to 5 kilometers No products Page 3 Page 4

2 Cellular Telephone Systems Classification of Tasks in OSI-RM Cordless Systems (small range) DECT (Digital Enhanced Cordless Telecommunications) Standard for the cordless telephone Transmission of speech and data in spatially limited areas (home range) Cellular Systems (medium range) GSM (Global System for Mobile Communications) Mainly conceived for speech transmission, in addition usable for data communication (Wide Area Network) Low data rates (9.6 KBit/s) Supplementary protocols for data communication (EDGE, GPRS, HSCSD) UMTS (Universal Mobile Telecommunications System) Also: IMT-2000 (International Mobile Telecommunications) Integration of several data, data rates up to 2 MBit/s and finally: Satellites for world-wide covering Application layer Transport layer Network layer Link layer Physical layer Service localization Adaptive applications Flow control Service quality Addressing, Routing Terminal localization Handover Authentication Multiplexing Medium access control Modulation Interference, absorption Frequencies Page 5 Page 6 Representation of radio signals Frequencies of Radio Networks (MHz) are represented physically as electromagnetic waves: s(t) = A sin(2 π f t + ϕ) A Twisted Pair A: Amplitude f: ϕ: Phase Coaxial Cable ϕ Wave Guide 0 T = 1/f Infrared Optical transmission Visible light Wavelength Cellular Phones Cordless Phones Wireless LANs Europe USA Japan GSM , / , , / , / UMTS (FDD) , UMTS (TDD) , CT , CT DECT IEEE HIPERLAN , AMPS, TDMA, CDMA , TDMA, CDMA, GSM , PACS , PACS-UB IEEE , PDC , , , PHS JCT IEEE Page 7 Page 8

Transmission of Digital Signals The digital signals (0 resp. 1) can be transferred as electrical signals (e.g. with Coaxial cable or Twisted Pair) very often used within Ethernet or other LANs optical signals (e.

transmission of digital information, the signals can be represented on the medium directly as defined voltage tension combinations (cable codes: NRZ, Manchester code, ).

Not modulated signal Carrier frequency (sin) modulated signal Page 9 Modulation of Digital Signals Bit value 1 0 1 1 0 The conversion of the digital signals can take place in various ways, basing on

3 Transmission of Digital Signals The digital signals (0 resp. 1) can be transferred as electrical signals (e.g. with Coaxial cable or Twisted Pair) very often used within Ethernet or other LANs optical signals (e.g. optical fiber) common for WANs, sometimes also within LANs electromagnetic signals (e.g. satellites, wireless networks or mobile phones) WLAN, GSM, If electrical signals are used for the transmission of digital information, the signals can be represented on the medium directly as defined voltage tension combinations (cable codes: NRZ, Manchester code, ). With radio signals the digital signal must be modulated on an analogous carrier signal. Not modulated signal Carrier frequency (sin) modulated signal Page 9 Modulation of Digital Signals Bit value The conversion of the digital signals can take place in various ways, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Amplitude Phase Amplitude Modulation (Amplitude Shift Keying, ASK) Technically easy to realize Needs little frequency range Not very robust due to signal weakening Used in optical transmission Resulting signal (frequency range): Page 10 Modulation of Digital Signals Modulation of Digital Signals Bit value Bit value The conversion of the digital signals can take place in various ways, basing on the parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Amplitude Phase Modulation ( Shift Keying, FSK) The conversion of the digital signals can take place based on different parameters of an analogous wave: s(t) = A sin(2 π f t + ϕ) Amplitude Phase Phase Modulation (Phase Shift Keying, PSK) Needs larger frequency range Used for telephony Resulting signal (frequency range): 180 phase shift Complex demodulation process Robust against disturbances often preferred in wireless communication Resulting signal (frequency range): Page 11 Page 12

Minimum Shift Keying (MSK) Minimum Shift Keying (MSK) Modification of the FSK procedure Used to increase bandwidth efficiency (data transmission rate in relation to needed frequency range) and at the

4 Minimum Shift Keying (MSK) Minimum Shift Keying (MSK) Modification of the FSK procedure Used to increase bandwidth efficiency (data transmission rate in relation to needed frequency range) and at the same time to avoid disturbances on the adjacent channels Within FSK, the needed frequency range depends on the distance between the carrier frequencies The distance can be reduced by preprocessing and special demodulation MSK procedure Each bit is doubled to two channels (even, odd), the bit duration thereby is doubled Higher carrier frequency makes half an oscillation more during a bit Combination rules of the two channel signals for the generation of the end signal Further bandwidth efficiency by Gaussian low-pass filters before modulator GMSK (Gaussian MSK), used e.g. for GSM, DECT Page 13 Even bits Odd bits Low frequency f 1 High frequency f 2 MSK signal No phase shifts! even odd Signal value h: high resp. n: low frequency +: positive resp. -: negative frequency phase Even = 0, odd = 0: invert f 2 Even = 1, odd = 0: invert f 1 Even = 0, odd = 1: f 1 Even = 1, odd = 1: f 2 Page 14 Advanced PSK Procedures The phase shift can also cover more than two phases: shift between M different phases, whereby M must be a power of two. Thus at the same time more information can be sent. Example: QPSK (Quaternary Phase Shift Keying) Shifting between 4 phases 4 phases permit 4 states: code 2 bits at one time Thus doubled data rate Q = A sinϕ Advanced PSK Procedures Quadrature Amplitude Modulation (QAM) Combination of ASK and QPSK n bit can be transferred at the same time (n=2 is QPSK) Bit error rate rises with increasing n, but less than with comparable PSK procedures ϕ I = A cosϕ A = amplitude of the signal I = in phase, signal component (in phase with carrier signal) Q = quadrature phase, quadrature component (perpendicular to the carrier phase) Page QAM: 4 bits per signal: 0011 and 0001 have same phase, but different amplitude 0000 and 0010 have same amplitude, but different phase Page 16

5 Signal propagation In free area straight propagation The receiving power is proportionally to 1/d α (d = distance between sender and receiver, α = 2 in free area, 2,7-5 in cities, 4-6 in buildings) Values for α larger than 2 are a result of absorption, reflection, dispersion, diffraction, Based on the receiving power different areas result: Transmission area Normal communication Low error rate Identification range Signal identification is still possible Too high error rate for interpretation of the signal Interference range No signal identification Disturbance of other transmissions Interference Identification Transmission Sender Distance Page 17 Multipath Transmission Factors which affect the propagation: Natural environment: Mountains, water, vegetation, rain, snow Artificial environment: Buildings etc. Propagation mechanism: Absorption, shading (rain, vegetation) Diffraction/diversion: sharp and round edges Reflection at large surfaces Dispersion at small obstacles Refraction: Refractive index becomes smaller with increasing height Diffraction Absorption direct way Dispersion Effect: - The receiver gets a signal on several ways at the same time - Weakening of the transmitting power with reflection, diffraction, - The same signal is received with different phases, Interference takes place - The signal is strewn temporally, interference with neighbor symbols Reflection Page 18 Effects of the cellular network channel Channels Cause Multipath transmission Movement Signal absorption Shading Effect Fast Fading Delay Spread ( dispersion) Doppler shift (frequency dispersion) Path loss Slow Fading Meaning Strongly varying signal strength over the distance Received signal consists of many differently delayed components Change of the distance between sender and receiver leads to a change of the phase of the received signal Change of the propagation conditions of waves by fog or rains in dependence of the frequency Slow fluctuations of the average value of the signal strength Page 19 The entire frequency spectrum is divided into fixed ranges (channels) which can be used for transmission. In order to avoid interference, protection distances between those ranges must be kept. To avoid collisions during transmissions, channels are fixedly assigned: Division Multiple Access (FDMA) Each user gets a frequency range assigned which he can use permanently Division Multiple Access (TDMA) Each user becomes all frequencies assigned for certain time intervals Code Division Multiple Access (CDMA) Code All users are using continuously all frequencies, utilization of a code Page 20

CDMA To achieve a more efficient utilization of the bandwidth, all frequency ranges are used at the same time by all participants For the distinction of the transmissions each transmission is

6 CDMA To achieve a more efficient utilization of the bandwidth, all frequency ranges are used at the same time by all participants For the distinction of the transmissions each transmission is characterized by a personal code The code realizes a signal spreading over the entire bandwidth. With the selection of the codes it must be paid attention to the fact that the codes of different transmitters are different enough (orthogonal codes) Advantages: Efficient utilization of the bandwidth No coordination between sending stations with choice of suitable codes (but: synchronization) The existence of transmissions is difficult to recognize (military) Redundancies protect against disturbances Disadvantages: Complex procedure, since the signals must be separated again by the receiver Limited data rate per user, since a code is impressed to the data Code Division Multiplex (CDM) Multiplication of the binary input data with different quasi-orthogonal codes Bit rate input r i << bit rate code r c (for optical systems also, with r i /r c 5000) spread spectrum higher bandwidth need Signal 1 Bit rate r i Signal n Bit rate r i Code A Bit rate rc Code B + Mixing of the different frequencies Code A Bit rate r c Code B Page 21 Page 22 CDMA - Computation Sender A Sends A d = 1, code A k = (set: 0 = - 1, 1 = +1) Resulting signal A s = A d * A k = (- 1, +1, - 1, - 1, +1, +1) Sender B Sends B d = 0, code B k = (set: 0 = - 1, 1 = +1) Resulting signal B s = B d * B k = (- 1, - 1, +1, - 1, +1, - 1) Both signals overlay additive in the air: A s + B s = (- 2, 0, 0, - 2, +2, 0) Remark: the sequences of the form (- 1, +1, - 1, - 1, +1, +1) are called chip sequences Receiver wants to hear sender A: Applies code A k bit by bit (internal product): (- 2, 0, 0, - 2, +2, 0) A k = = 6 Result is larger than 0, therefore the sent bit was a 1 Analogous for B: (- 2, 0, 0, - 2, +2, 0) B k = = - 6, thus 0 Overlay of a code: A Code A Code data A Code A Signal A In practice longer code sequences are being used to obtain distances as large as possible in the code area. Page 23 Page 24

7 Signal A A B Code B Code data B Code B Signal B Integrator Comparator Page 25 Page 26 B Wrong Code K Integrator Comparator Integrator Comparator Page 27 Page 28

8 Principle: Each station is assigned an 8 bit chip sequence *) For transferring a 1 it sends its chip sequence, for transferring a 0 it sends its complement Bipolar notation with 0 as -1 and 1 as +1 All chip sequences are pairwise orthogonal, i.e. if S and T are orthogonal chip sequences (S T), then holds: m 1 S T SiTi = 0, S T = 0, S S = 1 m i= 1 If two or more stations are sending at the same time, their signals are added linear Chip sequences of four stations: A: ( ) B: ( ) C: ( ) D: ( ) Example transmissions: (in each case exactly one bit is transferred) C E 1 = ( ) -11- B + C E 2 = ( ) A + B E 3 = ( ) A + B + C E 4 = ( ) A + B + C + D E 5 = ( ) A + B + C + D E 6 = ( ) *) simplified example, normally at least 10 bits E i = transferred chip sequence for case i Page 29 On the receiver side: For filtering out the bit stream of a certain station, the receiver must know the chip sequence of this station For extracting the bits of the station with chip sequence C from the received sequence E, it computes E C For example: E = A + B + C E C = ( A + B + C) C = A C + B C + C C = = 1 For the six example transmissions one receives: E1 C = (1 + 1) / 8 = 1 E C = ( ) / 8 = 2 1 i.e. station transmits 1 station transmits 1 E C = ( ) / 8 = 3 0 station does not transmit anything E 4 C = ( ) / 8 = 1 station transmits 1 E 5 C = ( ) / 8 = 1 station transmits 1 E 6 C = ( ) / 8 = 1 station transmits 0 Result of orthogonal codes Page 30 Spread Spectrum Technology Principle: Use of a bandwidth which is much larger than the one of the modulated signal Spread of the signal over the complete bandwidth using a pseudo random sequence Advantages: Several signals can be transferred without coordination within the same bandwidth at the same time Small susceptibility for effects of the multipath transmission: due to the high bandwidth in any case only a small part of the frequency spectrum is affected, so that the typical signal weakenings are weaker than with narrow-band systems Small influence of environmental disturbances Existence of transmissions (and as a result their decoding) is difficult to detect (of special relevance for military systems) Procedures: Direct Sequence Spread Spectrum (DSSS) Hopping Spread Spectrum (FHSS) Page 31 Direct Sequence bit 1 code word 1 chip Original signal Code sequence +1 Spread signal -1 Division of the signal into redundant information units (chips), the transmitter sends several (at least 10) bits for one bit of information Both, sender and receiver must know the chip sequence (code) Spreading, i.e. distribution of the chips over a large bandwidth For other users, the transmission appears to be background noise Re-establishment of the original, possibly disturbed signal is possible due to the redundancy Power Power Page 32

9 Direct Sequence Hopping Principle: Doubled modulation: a. Modulation of the data to a spread wide-band signal b. Modulation of this signal to the carrier frequency The receiver processes the inverted procedure with identical chip sequences Integration over the bit period Original signal Hopping signal Power Power 1.Modulation Chip sequence 2.Modulation Carrier frequency Channel Carrier frequency 1.Demodulation 2.Demodulation Chip sequence Integrator Carrier frequency is changed in certain time intervals in accordance to a code sequence (synchronous change of the frequency by sender and receivers) hops of the signal in fixed times of approx ms Collisions are possible, if two or more senders use by coincidence the same frequency. Therefore suitable codes must be used. Interference are limited to short periods, simple implementation Not as robust as DSSS, easier to tap Page 33 Page 34 Hopping Hopping Two variants: Fast change (fast hopping): several frequencies per bit Slow change (slow hopping): several bits per frequency Principle: Vary the carrier frequency in discrete levels: Level sequences are determined by pseudo random sequence Receiver must use identical sequence Two categories regarding the number of transferred bits per level: max. one bit: Fast Hopping Several bits: Slow Hopping Modulator Demodulator generator Pseudo random sequence generator Pseudo random sequence Page 35 Page 36

Structure of the Lecture

Structure of the Lecture Chapter 2 Technical Basics: Layer 1 Methods for Medium Access: Layer 2 Representation of digital signals on an analogous medium Signal propagation Characteristics of antennas Chapter