Wireless & Mobile Communications Chapter 2: Wireless Transmission Frequencies Ø Signals Ø Antennas Ø Signal propagation Ø Multiplexing Ø Spread spectrum Ø Modulation Ø Cellular systems Ø ICS 243 E - Ch. 2 Wireless Transmission Spring 2003
Spectrum Allocation twisted pair coax cable 1 Mm 300 Hz 10 km 30 k. Hz VLF LF optical transmission 100 m 3 MHz MF HF 1 m 300 MHz VHF VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency UHF 10 mm 30 GHz SHF 100 m 3 THz EHF infrared 1 m 300 THz visible light UV UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Relationship between frequency ‘f’ and wave length ‘ ’ : = c/f where c is the speed of light 3 x 108 m/s ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2
Frequencies Allocated for Mobile Communication Ø VHF & UHF ranges for mobile radio allows for simple, small antennas for cars q deterministic propagation characteristics q less subject to weather conditions –> more reliable connections q Ø SHF and higher for directed radio links, satellite communication small antennas with directed transmission q large bandwidths available q Ø Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF q limitations due to absorption by water and oxygen molecules q weather dependent fading, signal loss caused by heavy rainfall, etc. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 3
Allocated Frequencies Ø ITU-R holds auctions for new frequencies, manages frequency bands worldwide for harmonious usage (WRC World Radio Conferences) ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 4
Signals I physical representation of data Ø function of time and location Ø signal parameters: parameters representing the value of data Ø classification Ø continuous time/discrete time q continuous values/discrete values q analog signal = continuous time and continuous values q digital signal = discrete time and discrete values q Ø signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift q sine wave as special periodic signal for a carrier: s(t) = At sin(2 ft t + t) ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 5
Fourier Representation of Periodic Signals 1 1 0 0 t ideal periodic signal ICS 243 E - Ch. 2 Wireless Transmission t real composition (based on harmonics) Spring 2003 6
Signals II Ø Different representations of signals amplitude (amplitude domain) q frequency spectrum (frequency domain) q phase state diagram (amplitude M and phase in polar coordinates) q Q = M sin A [V] t[s] I= M cos f [Hz] Composite signals mapped into frequency domain using Fourier transformation Ø Digital signals need Ø infinite frequencies for perfect representation q modulation with a carrier frequency for transmission (->analog signal!) q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 7
Antennas are used to radiate and receive EM waves (energy) Ø Antennas link this energy between the ether and a device such as a transmission line (e. g. , coaxial cable) Ø Antennas consist of one or several radiating elements through which an electric current circulates Ø Types of antennas: Ø q q q Ø omnidirectional phased arrays adaptive optimal Principal characteristics used to characterize an antenna are: radiation pattern q directivity q gain q efficiency q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 8
Isotropic Antennas Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Ø Real antennas always have directive effects (vertical and/or horizontal) Ø Radiation pattern: measurement of radiation around an antenna Ø y z x ideal isotropic radiator ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 9
Omnidirectional Antennas: simple dipoles Ø Real antennas are not isotropic radiators but, e. g. , dipoles with lengths /4, or Hertzian dipole: /2 (2 dipoles) shape/size of antenna proportional to wavelength /4 Ø /2 Example: Radiation pattern of a simple Hertzian dipole y y x side view (xy-plane) Ø z z side view (yz-plane) x simple dipole top view (xz-plane) Gain: ratio of the maximum power in the direction of the main lobe to the power of an isotropic radiator (with the same average power) ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 10
Directional Antennas Ø Often used for microwave connections (directed point to point transmission) or base stations for mobile phones (e. g. , radio coverage of a valley or sectors for frequency reuse) y y z x z side view (xy-plane) x side view (yz-plane) top view (xz-plane) z z x x top view, 3 sector directed antenna sectorized antenna top view, 6 sector ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 11
Array Antennas Grouping of 2 or more antennas to obtain radiating characteristics that cannot be obtained from a single element Ø Antenna diversity Ø q switched diversity, selection diversity receiver chooses antenna with largest output q diversity combining combine output power to produce gain cophasing needed to avoid cancellation /4 /2 /4 + /2 /2 + ground plane ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 12
Signal Propagation Ranges Ø Transmission range communication possible q low error rate q Ø Detection range detection of the signal possible q no communication possible, high error rate q Ø Interference range signal may not be detected q signal adds to the background noise q ICS 243 E - Ch. 2 Wireless Transmission sender transmission distance detection interference Spring 2003 13
Signal Propagation I Ø Radio wave propagation is affected by the following mechanisms: reflection at large obstacles q scattering at small obstacles q diffraction at edges q reflection ICS 243 E - Ch. 2 Wireless Transmission diffraction scattering Spring 2003 14
Signal Propagation II Ø The signal is also subject to degradation resulting from propagation in the mobile radio environment. The principal phenomena are: pathloss due to distance covered by radio signal (frequency dependent, less at low frequencies) q fading (frequency dependent, related to multipath propagation) q shadowing induced by obstacles in the path between the transmitted and the receiver q shadowing ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 15
Signal Propagation III Ø Interference from other sources and noise will also impact signal behavior: co-channel (mobile users in adjacent cells using same frequency) and adjacent (mobile users using frequencies adjacent to transmission/reception frequency) channel interference q ambient noise from the radio transmitter components or other electronic devices, q Ø Propagation characteristics differ with the environment through and over which radio waves travel. Several types of environments can be identified (dense urban, suburban and rural) and are classified according to the following parameters: q q q terrain morphology vegetation density buildings: density and height open areas water surfaces ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 16
Pathloss I Ø Free-space pathloss: To define free-space propagation, consider an isotropic source consisting of a transmitter with a power Pt W. At a distance ‘d’ from this source, the power transmitted is spread uniformly on the surface of a sphere of radius ‘d’. The power density at the distance ‘d’ is then as follows: Sr = Pt/4 d 2 Ø The power received by an antenna at a distance ‘d’ from the transmitter is then equal to: Pr = Pt. Ae/4 d 2 where A is the effective area of the antenna. e ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 17
Pathloss II Ø Noting that Ae = Gr/(4 / 2) where Gr is the gain of the receiver Ø And if we replace the isotropic source by a transmitting antenna with a gain Gt the power received at a distance ‘d’ of the transmitter by a receiving antenna of gain Gr becomes: Pr = Pt. Gr. Gt/[4 (d/ )]2 Ø In decibels the propagation pathloss (PL) is given by: PL(db) = -10 log 10(Pr/Pt) = -10 log 10(Gr. Gt/[4 (d/ )]2) Ø This is for the ideal case and can only be applied sensibly to satellite systems and short range LOS propagation. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 18
Multipath Propagation I Ø Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction signal at sender signal at receiver Ø Positive effects of multipath: q enables communication even when transmitter and receiver are not in LOS conditions - allows radio waves effectively to go through obstacles by getting around them thereby increasing the radio coverage area ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 19
Multipath Propagation II Ø Negative effects of multipath: Time dispersion or delay spread: signal is dispersed over time due signals coming over different paths of different lengths Causes interference with “neighboring” symbols, this is referred to as Inter Symbol Interference (ISI) q multipath spread (in secs) = (longest 1 – shortest 2)/c For a 5 ms symbol duration a 1 ms delay spread means about a 20% intersymbol overlap. q The signal reaches a receiver directly and phase shifted (due to reflections) Distorted signal depending on the phases of the different parts, this is referred to as Rayleigh fading, due to the distribution of the fades. It creates fast fluctuations of the received signal (fast fading). q Random frequency modulation due to Doppler shifts on the different paths. Doppler shift is caused by the relative velocity of the receiver to the transmitter, leads to a frequency variation of the received signal. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 20
Effects of Mobility Ø Channel characteristics change over time and location signal paths change q different delay variations of different signal parts q different phases of signal parts q quick changes in the power received (short term fading) Ø Additional changes in power distance to sender q obstacles further away q long term fading slow changes in the average power received (long term fading) short term fading ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 t 21
Multiplexing Techniques Ø Multiplexing techniques are used to allow many users to share a common transmission resource. In our case the users are mobile and the transmission resource is the radio spectrum. Sharing a common resource requires an access mechanism that will control the multiplexing mechanism. Ø As in wireline systems, it is desirable to allow the simultaneous transmission of information between two users engaged in a connection. This is called duplexing. Ø Two types of duplexing exist: q Frequency division duplexing (FDD), whereby two frequency channels are assigned to a connection, one channel for each direction of transmission. q Time division duplexing (TDD), whereby two time slots (closely placed in time for duplex effect) are assigned to a connection, one slot for each direction of transmission. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 22
Multiplexing Ø Multiplexing in 3 dimensions time (t) (TDM) q frequency (f) (FDM) q code (c) (CDM) q Ø Goal: multiple use of a shared medium channels ki k 1 k 2 k 3 k 4 k 5 k 6 c t s 1 f s 2 f c t s 3 ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 f 23
Narrowband versus Wideband Ø These multiple access schemes can be grouped into two categories: q Narrowband systems - the total spectrum is divided into a large number of narrow radio bands that are shared. q Wideband systems - the total spectrum is used by each mobile unit for both directions of transmission. Only applicable for TDM and CDM. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 24
Frequency Division Multiplexing (FDM) Separation of the whole spectrum into smaller frequency bands Ø A channel gets a certain band of the spectrum for the whole time – orthogonal system Ø Advantages: Ø no dynamic coordination necessary, i. e. , sync. and framing q works also for analog signals q low bit rates – cheaper, delay spread q Ø k 1 k 2 k 3 k 4 k 5 k 6 c f Disadvantages: waste of bandwidth if the traffic is distributed unevenly q inflexible q guard bands t q narrow filters q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 25
Time Division Multiplexing (TDM) A channel gets the whole spectrum for a certain amount of time – orthogonal system Ø Advantages: Ø only one carrier in the medium at any time q throughput high - supports bursts k 1 q flexible – multiple slots q no guard bands ? ! q Ø Disadvantages: k 2 k 3 k 4 k 5 k 6 c f Framing and precise synchronization necessary q high bit rates at each t Tx/Rx q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 26
Hybrid TDM/FDM Combination of both methods Ø A channel gets a certain frequency band for a certain amount of time (slot). Ø Example: GSM, hops from one band to another each time slot Ø Advantages: Ø better protection against tapping (hopping among frequencies) q protection against frequency selective interference k 1 q Ø k 2 k 3 k 4 k 5 k 6 c f Disadvantages: q Framing and sync. required t ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 27
Code Division Multiplexing (CDM) Each channel has a unique code k 1 (not necessarily orthogonal) Ø All channels use the same spectrum at the same time Ø Advantages: Ø k 2 k 3 k 4 k 5 k 6 c bandwidth efficient q no coordination and synchronization necessary q good protection against interference and tapping q Ø f Disadvantages: lower user data rates due to high gains required to reduce interference q more complex signal regeneration t q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2. 19. 1 28
Issues with CDM Ø CDM has a soft capacity. The more users the more codes that are used. However as more codes are used the signal to interference (S/I) ratio will drop and the bit error rate (BER) will go up for all users. Ø CDM requires tight power control as it suffers from far-near effect. In other words, a user close to the base station transmitting with the same power as a user farther away will drown the latter’s signal. All signals must have more or less equal power at the receiver. Ø Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more delayed) of the signal (multipath signals) can be collected and used to make bit level decisions. Ø Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Two base stations receive the mobile signal and the mobile is receiving from two base stations (one of the rake receivers is used to listen to other signals). Ø Burst transmission - reduces interference ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 29
Types of CDM I Ø Two types exist: q Direct Sequence CDM (DS-CDM) spreads the narrowband user signal (Rbps) over the full spectrum by multiplying it by a very wide bandwidth signal (W). This is done by taking every bit in the user stream and replacing it with a pseudonoise (PN) code (a long bit sequence called the chip rate). The codes are orthogonal (or approx. . orthogonal). This results in a processing gain G = W/R (chips/bit). The higher G the better the system performance as the lower the interference. G 2 indicates the number of possible codes. Not all of the codes are orthogonal. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 30
Types of CDM II q Frequency hopping CDM (FH-CDM) FH-CDM is based on a narrowband FDM system in which an individual user’s transmission is spread out over a number of channels over time (the channel choice is varied in a pseudorandom fashion). If the carrier is changed every symbol then it is referred to as a fast FH system, if it is changed every few symbols it is a slow FH system. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 31
Orthogonality and Codes Ø An m-bit PN generator generates N=2 m - 1 different codes. Ø Out of these codes only ‘m’ codes are orthogonal -> zero cross correlation. Ø For example a 3 bit shift register circuit shown below generates N=7 codes. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 32
Orthogonal Codes Ø A pair of codes is said to be orthogonal if the cross correlation is zero: Rxy(0) = 0. Ø For two m-bit codes: x 1, x 2, x 3, . . . , xm and y 1, y 2, y 3, . . . , ym: For example: x = 0011 and y = 0110. Replace 0 with -1, 1 stays as is. Then: x = -1 -1 1 1 y = -1 1 1 -1 --------Rxy(0) = 1 -1 +1 -1 = 0 ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 33
Example of an Orthogonal Code: Walsh Codes Ø In 1923 J. L. Walsh introduced a complete set of orthogonal codes. To generate a Walsh code the following two steps must be followed: q Step 1: represent a Nx. N matrix as four quadrants (start off with 2 x 2) q Step 2: make the first, second and third quadrants indentical and invert the fourth ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 34
Modulation Ø Digital modulation digital data is translated into an analog signal (baseband) q ASK, FSK, PSK - main focus in this chapter q differences in spectral efficiency, power efficiency, robustness q Ø Analog modulation q Ø shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e. g. , /4) q Frequency Division Multiplexing q medium characteristics q Ø Basic schemes Amplitude Modulation (AM) q Frequency Modulation (FM) q Phase Modulation (PM) q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 35
Modulation and Demodulation digital data 101101001 digital modulation analog baseband signal analog modulation radio transmitter radio carrier analog demodulation analog baseband signal synchronization decision digital data 101101001 radio receiver radio carrier ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 36
Digital Modulation of digital signals known as Shift Keying 1 0 Ø Amplitude Shift Keying (ASK): Ø 1 very simple q low bandwidth requirements q very susceptible to interference q t 1 Ø 0 1 Frequency Shift Keying (FSK): q needs larger bandwidth t Ø Phase Shift Keying (PSK): 1 more complex q robust against interference 0 1 q ICS 243 E - Ch. 2 Wireless Transmission t Spring 2003 37
Advanced Frequency Shift Keying Ø Ø Ø bandwidth needed for FSK depends on the distance between the carrier frequencies special pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying) bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen the frequency of one carrier is twice the frequency of the other even higher bandwidth efficiency using a Gaussian lowpass filter GMSK (Gaussian MSK), used in GSM ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 38
Example of MSK 1 0 1 0 bit data even 0101 even bits odd 0011 odd bits signal value hnnh - - ++ low frequency h: high frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t No phase shifts! ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 39
Advanced Phase Shift Keying Ø BPSK (Binary Phase Shift Keying): q q q Ø Q bit value 0: sine wave bit value 1: inverted sine wave very simple PSK low spectral efficiency robust, used e. g. in satellite systems 1 10 QPSK (Quadrature Phase Shift Keying): 0 Q Often also transmission of relative, not absolute phase shift: DQPSK Differential QPSK (IS-136, PACS, PHS) ICS 243 E - Ch. 2 Wireless Transmission 11 I 2 bits coded as one symbol q symbol determines shift of sine wave q needs less bandwidth compared to BPSK A q more complex q Ø I 00 01 t 11 Spring 2003 10 00 01 40
Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation Ø it is possible to code n bits using one symbol Ø 2 n discrete levels, n=2 identical to QPSK Ø bit error rate increases with n, but less errors compared to comparable PSK schemes Ø Q Ø 0010 0011 0001 Example: 16 -QAM (4 bits = 1 symbol) 0000 I 1000 Ø ICS 243 E - Ch. 2 Wireless Transmission Symbols 0011 and 0001 have the same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude. used in standard 9600 bit/s modems Spring 2003 41
Spread spectrum technology: CDM Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Ø Solution: spread the narrow band signal into a broad band signal using a special code Ø q power protection against narrow band interference spread signal power signal spread interference detection at receiver f protection against narrowband interference Ø f Side effects: coexistence of several signals without dynamic coordination q tap-proof q Ø Alternatives: Direct Sequence, Frequency Hopping ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 42
Effects of spreading and interference P i) P f ii) user signal broadband interference narrowband interference f sender P iii) P P f iv) receiver ICS 243 E - Ch. 2 Wireless Transmission f v) Spring 2003 f 2. 28. 1 43
Spreading and frequency selective fading channel quality 1 2 5 3 6 narrowband channels 4 frequency narrow band signal guard space channel quality 1 2 2 2 spread spectrum ICS 243 E - Ch. 2 Wireless Transmission spread spectrum channels frequency Spring 2003 2. 29. 1 44
DSSS (Direct Sequence Spread Spectrum) I Ø XOR of the signal with pseudo-random number (chipping sequence) q Ø many chips per bit (e. g. , 128) result in higher bandwidth of the signal t b Advantages user data reduces frequency selective fading q in cellular networks q 0 chipping sequence 01101011001010 precise power control necessary ICS 243 E - Ch. 2 Wireless Transmission = resulting signal Disadvantages q XOR tc base stations can use the same frequency range several base stations can detect and recover the signal soft handover Ø 1 tb: bit period tc: chip period Spring 2003 2. 30. 1 45
DSSS (Direct Sequence Spread Spectrum) II spread spectrum signal user data X chipping sequence transmit signal modulator radio carrier transmitter correlator received signal demodulator radio carrier lowpass filtered signal products X integrator sampled sums data decision chipping sequence receiver ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2. 31. 1 46
FHSS (Frequency Hopping Spread Spectrum) I Ø Discrete changes of carrier frequency q Ø sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit q Slow Hopping: several user bits per frequency q Ø Advantages frequency selective fading and interference limited to short period q simplementation q uses only small portion of spectrum at any time q Ø Disadvantages not as robust as DSSS q simpler to detect q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2. 32. 1 47
FHSS (Frequency Hopping Spread Spectrum) II tb user data 0 1 f 0 1 1 t td f 3 slow hopping (3 bits/hop) f 2 f 1 f t td f 3 fast hopping (3 hops/bit) f 2 f 1 t tb: bit period ICS 243 E - Ch. 2 Wireless Transmission td: dwell time Spring 2003 2. 33. 1 48
FHSS (Frequency Hopping Spread Spectrum) III narrowband signal user data modulator frequency synthesizer transmitter received signal hopping sequence spread transmit signal narrowband signal demodulator hopping sequence data demodulator frequency synthesizer ICS 243 E - Ch. 2 Wireless Transmission receiver Spring 2003 2. 34. 1 49
Concept of Cellular Communications Ø In the late 60’s it was proposed to alleviate the problem of spectrum congestion by restructuring the coverage area of mobile radio systems. Ø The cellular concept does not use broadcasting over large areas. Instead smaller areas called cells are handled by less powerful base stations that use less power for transmission. Now the available spectrum can be re-used from one cell to anothereby increasing the capacity of the system. Ø However this did give rise to a new problem, as a mobile unit moved it could potentially leave the coverage area (cell) of a base station in which it established the call. This required complex controls that enabled the handing over of a connection (called handoff) to the new cell that the mobile unit moved into. Ø In summary, the essential elements of a cellular system are: q Low power transmitter and small coverage areas called cells q Spectrum (frequency) re-use q Handoff ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 50
Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Ø Mobile stations communicate only via the base station Ø Ø Advantages of cell structures: higher capacity, higher number of users q less transmission power needed q more robust, decentralized q base station deals with interference, transmission area etc. locally q Ø Problems: fixed network needed for the base stations q handover (changing from one cell to another) necessary q interference with other cells q Ø Cell sizes from some 100 m in cities to, e. g. , 35 km on the country side (GSM) - even less for higher frequencies ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2. 35. 1 51
Cellular Network ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 52
Some Definitions Ø Forward path or down link - from base station down to the mobile Ø Reverse path or up link - from the mobile up to the base station Ø The mobile unit - a portable voice and/or data comm. transceiver. It has a 10 digit telephone number that is represented by a 34 bit mobile identification number -> (215) 684 -3201 is divided into two parts: MIN 1: 215 translated into 10 bits and MIN 2: 684 -3201 translated into 24 bits. In addition each mobile unit is also permanently programmed at the factory with a 32 bit electronic serial number (ESN) which guards against tampering. Ø The cell - a geographical area covered by Radio Frequency (RF) signals. It is essentially a radio communication center comprising radios, antennas and supporting equipment to enable mobile to land to mobile communication. Its shape and size depend on the location, height , gain and directivity of the antenna, the power of the transmitter, the terrain, obstacles such as foliage, buildings, propagation paths, etc. It is a highly irregular shape, its boundaries defined by received signal strength! But for traffic engineering purposes and system planning and design a hexagonal shape is used. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 53
More definitions The base station (BS) - a transmitter and receiver that relays signals (control and information (voice or data)) from the mobile unit to the MSC and vice versa. Ø The mobile switching center (MSC) - a switching center that controls a cluster of cells. Base stations are connected to the MSC via wireline links. The MSC is directly connected to the PSTN and is responsible for all calls related to mobiles located within its domain. MSCs intercommunicate using a link protocol specified by IS (International Standard) 41. This enables roaming of mobile units (i. e. obtaining service outside of the home base). The MSC is also responsible for billing, it keeps track of air time, errors, delays, blocking, call dropping (due to handoff failure), etc. It is also responsible for the handoff process, it keeps track of signal strengths and will initiate a handoff when deemed necessary (note to handoff or not to handoff is not a trivial issue!) Ø ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 54
The Basic Cellular Communication Protocol I Every mobile unit whether at home or roaming, has to register with the MSC controlling the area it is in. If it does not register then the MSC does not know of its existence and will not be able to process any of its calls. Ø The home location register (HLR) is used to keep information regarding a mobile unit/user, it is a database for storing and managing subscriber information. When roaming, a mobile unit registers with a foreign MSC and data from its HRL is relayed to the visitor location register (VLR). The VLR is a dynamic database used to store roaming mobile subscriber information. The HLR and VLR communicate via the MSCs using IS 41. Ø The cellular system uses out of band signalling. Most of the control information is sent over different channels from the user information (voice or data) channels. Inband signalling is used for control during the connection (disconnect, handoff, etc. ) Ø ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 55
The Basic Cellular Communication Protocol II Ø A mobile unit when enabled (power on) scans the control channels and tunes to the one with the strongest signal. The control channels are known and carry signals pertaining to the cell sites, e. g. transmission power to be used by the mobile unit in a particular cell. This process is called initialization. Ø If the mobile wants to initiate a call, it sends in a service request on the reverse path control link. The service request contains the destination phone number and identification information (MIN 1, MIN 2, and ESN) of the source mobile unit to verify the originator. Ø When the base station receives the request, it relays it to the MSC. The MSC then checks to see it is it a number of another mobile or of a fixed user. If the latter the call is forwarded to the PSTN. If the former, it checks to see if the destination mobile unit is a subscriber (local or visitor/roamer). If not it relays the call to the PSTN to forward to the appropriate MSC. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 56
The Basic Cellular Communication Protocol III Ø If the destination is within its cluster it sends out a paging message to all the base stations. Every base station then relays this message by broadcasting it on its control channel. If the destination mobile unit is enabled (power on) it will detect this message and respond to the base station. Ø The base station relays this response to the MSC. The MSC then allocates channels to both the source mobile unit and the destination mobile unit. The corresponding base stations pass this information on to the respective mobile units. The mobile units then tune to the correct channels and the communication link is established. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 57
Spectrum and Capacity Issues Ø Spectrum is limited ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 58
Frequency Re-use I Ø To be able to increase the capacity of the system, frequencies must be re-used in the cellular layout (unless we are using spread spectrum techniques). Ø Frequencies cannot be re-used in adjacent cells because of cochannel interference. The cells using the same frequencies must be dispersed across the cellular layout. The closer the spacing the more efficient the scheme! ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 59
Frequency Re-use II Ø For an omni-directional antenna, with constant signal power, each cell site coverage area would be circular (barring any terrain irregularities or obstacles). Ø To achieve full coverage without dead spots, a series of regular polygons for cell sites are required. Ø The hexagonal was chosen as it comes the closest to the shape of a circle, and a hexagonal layout requires fewer cells (when compared to triangles or rectangles, it has the largest surface area given the same radius R) -> less cells. Ø Goal is to find the minimum distance between cells using same frequencies. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 60
Frequency re-use distance I ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 61
Frequency re-use distance II Ø For two adjacent cells: D=31/2 R Ø The closest we can place the same frequencies is called the first tier around the center cell (minimal re-use distance -> lower -> more capacity!). Ø For simplicity we only take the first tier of cells into account for co-channel interference (i. e. , we ignore 2 nd, 3 rd, etc. tiers, cause much less interference, negligible!). ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 62
Frequency re-use distance III ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 63
Frequency re-use distance III Ø Radius = dist. between two co-channel cells = (3 R 2[i 2+j 2+ij])1/2 = D Ø Since the area of a hexagon is proportional to the square of the distance between its center and a vertex (i. e. , its radius), the area of the large hexagon is: Alarge = k[Radius]2 = k[3 R 2[i 2+j 2+ij]] where k is a constant. Ø Similarly the area of each cell (i. e. , small hexagon) is: Asmall = k[R 2] Ø Comparing these expressions we find that: Alarge/Asmall = 3[i 2+j 2+ij] = D 2/R 2 ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 64
Frequency re-use distance IV Ø From symmetry we can see that the large hexagon encloses the center cluster of N cells plus 1/3 the number of the cells associated with 6 other peripheral hexagons. Thus the total number of cells enclosed by the first tier is: N+6(1/3 N) = 3 N Ø Since the area of a hexagon is proportional to the number of cells contained within it: Alarge/Asmall = 3 N/1 = 3 N Ø Substituting we get: 3 N = 3[i 2+j 2+ij] = D 2/R 2 Ø Or: D/R = q =(3 N)1/2 Ø “q” is referred to as the reuse ratio! ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 65
Co-channel Interference I Ø The co-channel interference ratio S/I is given as: S = desired signal power in a cell (note that many texts use “C” instead of S), Ik = interference signal power from the kth cell, Ni = number of interfering cells. Ø If we only assume the first tier of interfering cells, then Ni=6, and all cells interfere equally (they are all equidistant!). Ø The signal power at any point is inversely proportional to the inverse of the distance from the source raised to the g power. (2<g<5) ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 66
Co-channel Interference II Ø Ik is proportional to Dg , and S is proportional to Rg , where g is the propagation path loss and is dependent upon terrain environment. For cellular systems it is often taken as = 4. Ø Therefore: Ø The relationship between SNR (signal to noise ratio - Eb/No) and S/I for cellular systems with Rayleigh fading channels: SNR = S/I(db) – 9 db. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 67
For a given S/I how to get N Ø Recall that: D/R = q =(3 N)1/2 Ø An S/I = 18 db (decibels=10 log. S/I) = 63. 1, gives an acceptable voice quality. Ø Therefore q = [6 x 63. 1]1/4 = 4. 41 when g = 4 Ø Substituting for N we get N = (4. 41)2/3 equals approx. 7 Ø This means that if we have 49 frequency channels available, each cell gets 49/7 = 7 frequency channels. Ø If we have 82 available then 82/7 = 11. 714 -> which means that 5 cells will have 12 and 2 cells will have 11! Ø How does that translate to “i and j” for a cell layout? N = [i 2+j 2+ij], find i, j that satisfy the equation! ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 68
Calculating i, j, and D from N f 2 f 4 f 3 f 6 ICS 243 E - Ch. 2 Wireless Transmission f 5 f 1 f 2 f 3 f 6 f 7 f 5 f 2 f 4 f 3 f 7 f 5 f 1 f 2 Spring 2003 69
Frequency planning Frequency reuse only with a certain distance between the base stations Ø Standard model using 7 frequencies: Ø f 4 f 3 Ø f 5 f 1 f 2 f 3 f 6 f 7 f 2 f 4 f 5 f 1 Fixed frequency assignment: certain frequencies are assigned to a certain cell q problem: different traffic load in different cells q Ø Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in neighbor cells q more capacity in cells with more traffic q assignment can also be based on interference measurements q ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 2. 36. 1 70
Increasing Capacity Ø We can see that by reducing the area of a cell we can increase capacity as we will have more cells each with its own set of frequencies. Ø What is drawback of shrinking the size of the cells (cell splitting)? Increase in the number of handoffs -> increased load on the system! Also need more infrastrucutre -> base stations (each cell needs a BS). Ø An easier solution exists, sectorization. It does not reduce handoffs, its advantage: it does not require more infrastructure. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 71
Sectorization I Ø We can also increase the capacity by using sectors in cells. Ø Directional antennas instead of being omnidirectional, will only beam over a certain angle. f 3 f 1 f 2 f 3 f 1 f 3 f 1 f 2 f 2 f 1 f h h 3 3 3 h 1 2 g 2 h 3 g 2 g 1 g 1 g 3 g 3 f 3 3 cell cluster ICS 243 E - Ch. 2 Wireless Transmission 3 cell cluster with 3 sectors Spring 2003 72
Sectorization II Ø What does that mean? Ø We can now assign frequency sets to sectors and decrease the re-use distance or improve S/I ratio (i. e. signal quality). Ø Question: By how much? Depends on number of sectors (i. e. , 60% or 120%). ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 73
Other Capacity or Signal Improvement Tech. Ø Dynamic channel allocation (DCA): allows cells to borrow frequencies from other cells within the cluster if not used by them. Can be used to alleviate hotspots. Another implementation basically has all channels available to all cells, they get allocated based upon demand. Ø Power control: by reducing the transmitted power, the battery life of a mobile can be extended. It also helps in reducing -channel and adjacent channel interference. ICS 243 E - Ch. 2 Wireless Transmission Spring 2003 74
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