Mobile Communications

Mobile Communications

Wireless & Mobile Communications Chapter 2: Wireless Transmission Frequencies Signals Antennas Signal propagation Multiplexing Spread spectrum Modulation Cellular systems Spectrum Allocation twisted pair coax cable 1 Mm 300 Hz 10 km 30 kHz VLF LF optical transmission 100 m 3 MHz MF HF 1m 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 3x10x108m/s ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.2 Frequencies Allocated for Mobile Communication

VHF & UHF ranges for mobile radio allows for simple, small antennas for cars deterministic propagation characteristics less subject to weather conditions > more reliable connections SHF and higher for directed radio links, satellite communication small antennas with directed transmission large bandwidths available Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF limitations due to absorption by water and oxygen molecules weather dependent fading, signal loss caused by heavy rainfall, etc. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.3 Allocated Frequencies ITU-R holds auctions for new frequencies, manages frequency bands worldwide for harmonious usage (WRC World Radio Conferences) Mobile phones Cordless telephones

Wireless LANs Europe USA Japan NMT 453-457MHz, 463-467 MHz; GSM 890-915 MHz, 935-960 MHz; 1710-1785 MHz, 1805-1880 MHz CT1+ 885-887 MHz, 930-932 MHz; CT2 864-868 MHz DECT 1880-1900 MHz IEEE 802.11 2400-2483 MHz HIPERLAN 1 5176-5270 MHz AMPS, TDMA, CDMA 824-849 MHz, 869-894 MHz; TDMA, CDMA, GSM 1850-1910 MHz, 1930-1990 MHz; PACS 1850-1910 MHz, 1930-1990 MHz PACS-UB 1910-1930 MHz PDC 810-826 MHz, 940-956 MHz; 1429-1465 MHz,

1477-1513 MHz IEEE 802.11 2400-2483 MHz IEEE 802.11 2471-2497 MHz ICS 243E - Ch.2 Wireless Transmission Spring 2003 PHS 1895-1918 MHz JCT 254-380 MHz 2.4 Signals I physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift

sine wave as special periodic signal for a carrier: s(t) = At sin(2 ft t + t) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.5 Fourier Representation of Periodic Signals 1 g (t ) = c + an sin( 2nft ) + bn cos( 2nft ) 2 n =1 n =1 1 1 0 0 t ideal periodic signal ICS 243E - Ch.2 Wireless Transmission t real composition (based on harmonics) Spring 2003 2.6

Signals II Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in polar coordinates) Q = M sin A [V] A [V] t[s] I= M cos f [Hz] Composite signals mapped into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect representation modulation with a carrier frequency for transmission (->analog signal!) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.7

Antennas Antennas are used to radiate and receive EM waves (energy) Antennas link this energy between the ether and a device such as a transmission line (e.g., coaxial cable) Antennas consist of one or several radiating elements through which an electric current circulates Types of antennas: omnidirectional directional phased arrays adaptive optimal Principal characteristics used to characterize an antenna are: radiation pattern directivity gain efficiency ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.8 Isotropic Antennas

Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertical and/or horizontal) Radiation pattern: measurement of radiation around an antenna y z x ideal isotropic radiator ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.9 Omnidirectional Antennas: simple dipoles 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 /2 Example: Radiation pattern of a simple Hertzian dipole y

y x side view (xy-plane) 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 243E - Ch.2 Wireless Transmission Spring 2003 2.10 Directional Antennas 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 243E - Ch.2 Wireless Transmission Spring 2003 2.11 Array Antennas Grouping of 2 or more antennas to obtain radiating characteristics that cannot be obtained from a single element

Antenna diversity switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain cophasing needed to avoid cancellation /4 /2 /4 + /2 /2 /2 + ground plane ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.12 Signal Propagation Ranges Transmission range communication possible low error rate

Detection range detection of the signal possible no communication possible, high error rate Interference range signal may not be detected signal adds to the background noise ICS 243E - Ch.2 Wireless Transmission sender transmission distance detection interference Spring 2003 2.13 Signal Propagation I Radio wave propagation is affected by the following mechanisms: reflection at large obstacles scattering at small obstacles diffraction at edges

reflection ICS 243E - Ch.2 Wireless Transmission diffraction scattering Spring 2003 2.14 Signal Propagation II 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) fading (frequency dependent, related to multipath propagation) shadowing induced by obstacles in the path between the transmitted and the receiver shadowing ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.15 Signal Propagation III 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 ambient noise from the radio transmitter components or other electronic devices, Propagation characteristics differ with the environment through and over which radio waves travel. Several types of environments can be identified (dense urban, urban, suburban and rural) and are classified according to the following parameters: terrain morphology vegetation density buildings: density and height open areas water surfaces ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.16 Pathloss I 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/4d2

The power received by an antenna at a distance d from the transmitter is then equal to: Pr = PtAe/4d2 where Ae is the effective area of the antenna. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.17 Pathloss II Noting that Ae = Gr/(4 where Gr is the gain of the receiver 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 = PtGrGt/[4d2 In decibels the propagation pathloss (PL) is given by: PL(db) = -10log10(Pr/Pt) = -10log10(GrGt/[4d2) This is for the ideal case and can only be applied sensibly to satellite systems and short range LOS propagation. ICS 243E - Ch.2 Wireless Transmission Spring 2003

2.18 Multipath Propagation I Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction signal at sender signal at receiver Positive effects of multipath: 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 243E - Ch.2 Wireless Transmission Spring 2003 2.19 Multipath Propagation II 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) multipath spread (in secs) = (longest1 shortest2)/c For a 5s symbol duration a 1s delay spread means about a 20% intersymbol overlap. 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). 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 243E - Ch.2 Wireless Transmission Spring 2003 2.20 Effects of Mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in power distance to sender obstacles further away long term fading slow changes in the average power received (long term fading) short term fading

ICS 243E - Ch.2 Wireless Transmission Spring 2003 t 2.21 Multiplexing Techniques 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. 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. Two types of duplexing exist: Frequency division duplexing (FDD), whereby two frequency channels are assigned to a connection, one channel for each direction of transmission. 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 243E - Ch.2 Wireless Transmission Spring 2003

2.22 Multiplexing Multiplexing in 3x10 dimensions time (t) (TDM) frequency (f) (FDM) code (c) (CDM) Goal: multiple use of a shared medium channels ki k1 k2 k3 k4 k5 k6 c t c t s1 f s2

f c t s3 ICS 243E - Ch.2 Wireless Transmission Spring 2003 f 2.23 Narrowband versus Wideband These multiple access schemes can be grouped into two categories: Narrowband systems - the total spectrum is divided into a large number of narrow radio bands that are shared. Wideband systems - the total spectrum is used by each mobile unit for both directions of transmission. Only applicable for TDM and CDM. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.24 Frequency Division Multiplexing (FDM) Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time orthogonal system

Advantages: no dynamic coordination necessary, i.e., sync. and framing works also for analog signals low bit rates cheaper, delay spread k1 k2 k3 k4 k5 k6 c f Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard bands t narrow filters ICS 243E - Ch.2 Wireless Transmission Spring 2003

2.25 Time Division Multiplexing (TDM) A channel gets the whole spectrum for a certain amount of time orthogonal system Advantages: only one carrier in the medium at any time throughput high - supports bursts k1 flexible multiple slots no guard bands ?! Disadvantages: k2 k3 k4 k5 k6 c f Framing and precise synchronization necessary high bit rates at each t

Tx/Rx ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.26 Hybrid TDM/FDM Combination of both methods A channel gets a certain frequency band for a certain amount of time (slot). Example: GSM, hops from one band to another each time slot Advantages: better protection against tapping (hopping among frequencies) protection against frequency selective interference k1 k2 k3 k4 k5

k6 c f Disadvantages: Framing and sync. required t ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.27 Code Division Multiplexing (CDM) Each channel has a unique code k1 (not necessarily orthogonal) All channels use the same spectrum at the same time Advantages: k2 k3 k4 k5 k6

c bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping f Disadvantages: lower user data rates due to high gains required to reduce interference more complex signal regeneration ICS 243E - Ch.2 Wireless Transmission t Spring 2003 2.19.1 2.28 Issues with CDM 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. 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 latters signal. All signals must have more or less equal power at the receiver. 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. 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). Burst transmission - reduces interference ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.29 Types of CDM I Two types exist: 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. G2 indicates the number of possible codes. Not all of the codes are orthogonal. Frequency Code CDMA Time ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.30 Types of CDM II Frequency hopping CDM (FH-CDM) FH-CDM is based on a narrowband FDM system in which an individual users 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. A B B B A A B A A A

ICS 243E - Ch.2 Wireless Transmission A A B B B A B B Spring 2003 2.31 Orthogonality and Codes An m-bit PN generator generates N=2m - 1 different codes. Out of these codes only m codes are orthogonal -> zero cross correlation. For example a 3x10 bit shift register circuit shown below generates N=7 codes. + Mod2 Adder (1+0=1, 0+1=1, 0+0=0, 1+1=0) 1

Initial State: 2 1 0 1 0 0 1 1 ICS 243E - Ch.2 Wireless Transmission 3 1 1 0 1 0 0 1 Spring 2003 1 1 1 0 1 0 0 2.32 Orthogonal Codes A pair of codes is said to be orthogonal if the cross correlation is zero: Rxy(0) = 0 .

For two m-bit codes: x1,x2,x3,...,xm and y1,y2,y3,...,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 243E - Ch.2 Wireless Transmission Spring 2003 2.33 Example of an Orthogonal Code: Walsh Codes In 1923x10 J.L. Walsh introduced a complete set of orthogonal codes. To generate a Walsh code the following two steps must be followed: Step 1: represent a NxN matrix as four quadrants (start off with 2x2) Step 2: make the first, second and third quadrants indentical and invert the fourth b b b b = 1

1 1 0 or 2 codes: 11 and 10 bb b b bb b b bb b b bb b b = 11 10 11 10 ICS 243E - Ch.2 Wireless Transmission 11 10 00 01 or 0 0

Code 1 0 1 Code 2 2 codes: 00 and 01 00 01 00 01 Spring 2003 00 01 11 10 Code 1 Code 2 Code 3 Code 4 2.34 Modulation Digital modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness Analog modulation

shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., /4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.35 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 243E - Ch.2 Wireless Transmission Spring 2003 2.36 Digital Modulation Modulation of digital signals known as Shift Keying 1 0 Amplitude Shift Keying (ASK):

1 very simple low bandwidth requirements very susceptible to interference t 1 0 1 Frequency Shift Keying (FSK): needs larger bandwidth t Phase Shift Keying (PSK): 1 more complex robust against interference 0 1 ICS 243E - Ch.2 Wireless Transmission t

Spring 2003 2.37 Advanced Frequency Shift Keying 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 243E - Ch.2 Wireless Transmission Spring 2003 2.38 Example of MSK 1 0 1 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 243E - Ch.2 Wireless Transmission Spring 2003 2.39 Advanced Phase Shift Keying BPSK (Binary Phase Shift Keying): 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-13x106, PACS, PHS) ICS 243E - Ch.2 Wireless Transmission

11 I 2 bits coded as one symbol symbol determines shift of sine wave needs less bandwidth compared to BPSK A more complex I 00 01 t 11 Spring 2003 10 00 01 2.40 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation it is possible to code n bits using one symbol

2n discrete levels, n=2 identical to QPSK bit error rate increases with n, but less errors compared to comparable PSK schemes Q 0010 0011 0001 Example: 16-QAM (4 bits = 1 symbol) 0000 I 1000 ICS 243E - 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 2.41 Spread spectrum technology: CDM Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band

signal using a special code power protection against narrow band interference interference spread signal power signal spread interference detection at receiver f protection against narrowband interference f Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.42

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 243E - Ch.2 Wireless Transmission f v) Spring 2003 f

2.28.1 2.43 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 2 2 spread

spectrum ICS 243E - Ch.2 Wireless Transmission spread spectrum channels frequency Spring 2003 2.29.1 2.44 DSSS (Direct Sequence Spread Spectrum) I XOR of the signal with pseudo-random number (chipping sequence) many chips per bit (e.g., 128) result in higher bandwidth of the signal t b Advantages user data reduces frequency selective fading in cellular networks 0 chipping sequence 01101010110101

01101011001010 precise power control necessary ICS 243E - Ch.2 Wireless Transmission = resulting signal Disadvantages XOR tc base stations can use the same frequency range several base stations can detect and recover the signal soft handover 1 tb: bit period tc: chip period Spring 2003 2.30.1 2.45 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 243E - Ch.2 Wireless Transmission

Spring 2003 2.31.1 2.46 FHSS (Frequency Hopping Spread Spectrum) I Discrete changes of carrier frequency sequence of frequency changes determined via pseudo random number sequence Two versions Fast Hopping: several frequencies per user bit Slow Hopping: several user bits per frequency Advantages frequency selective fading and interference limited to short period simple implementation uses only small portion of spectrum at any time Disadvantages not as robust as DSSS simpler to detect ICS 243E - Ch.2 Wireless Transmission

Spring 2003 2.32.1 2.47 FHSS (Frequency Hopping Spread Spectrum) II tb user data 0 1 f 0 1 1 t td f3 slow hopping (3 bits/hop) f2 f1 f t td f3

fast hopping (3 hops/bit) f2 f1 t tb: bit period ICS 243E - Ch.2 Wireless Transmission td: dwell time Spring 2003 2.33.1 2.48 FHSS (Frequency Hopping Spread Spectrum) III narrowband signal user data modulator modulator frequency synthesizer transmitter received signal hopping sequenc e spread transmit signal

narrowband signal demodulator hopping sequenc e data demodulator frequency synthesizer ICS 243E - Ch.2 Wireless Transmission receiver Spring 2003 2.34.1 2.49 Concept of Cellular Communications In the late 60s it was proposed to alleviate the problem of spectrum congestion by restructuring the coverage area of mobile radio systems. 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 another thereby increasing the capacity of the system.

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. In summary, the essential elements of a cellular system are: Low power transmitter and small coverage areas called cells Spectrum (frequency) re-use Handoff ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.50 Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures:

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes from some 100 m in cities to, e.g., 3x105 km on the country side (GSM) - even less for higher frequencies ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.35.1 2.51 Cellular Network Other MSCs F1,F2,..,F6 (IS 41) F7,F8,..,F12 PSTN F7,F8,..,F12 MSC F1,F2,..,F6

MSC: Mobile Switching Center PSTN: Public Switched Telephone Network Base Station Handoff Cell (Theoretical) Practical Cell - coverage depends on antenna location and height, transmitter power, terrain, foliage, buildings, etc. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.52 Some Definitions Forward path or down link - from base station down to the mobile Reverse path or up link - from the mobile up to the base station The mobile unit - a portable voice and/or data comm. transceiver. It has a 10 digit telephone number that is represented by a 3x104 bit mobile identification number -> (215) 684-3x10201 is divided into two parts: MIN1: 215 translated into 10bits and MIN2: 684-3x10201 translated into 24bits. In addition each mobile unit is also permanently programmed at the factory with a 3x102 bit electronic serial number (ESN) which guards against tampering.

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 and 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 243E - Ch.2 Wireless Transmission Spring 2003 2.53 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. 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!)

ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.54 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. 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. 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.) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.55 The Basic Cellular Communication Protocol II 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. 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 (MIN1, MIN2, and ESN) of the source mobile unit to verify the originator. 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 243E - Ch.2 Wireless Transmission Spring 2003 2.56 The Basic Cellular Communication Protocol III 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. 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 243E - Ch.2 Wireless Transmission Spring 2003 2.57 Spectrum and Capacity Issues Spectrum is limited Allocated Spectrum F1 F2 F3 F4 F5 F6 F7 F8 F9 FDM F1,F2,...F9: frequency channels ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.58 Frequency Re-use I 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). 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! Fx:subset of

frequencies used in a cell Cochannel Interference F2 F1 F1 F2 Minimum Re-use distance ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.59 Frequency Re-use II For an omni-directional antenna, with constant signal power, each cell site coverage area would be circular (barring any terrain irregularities or obstacles). To achieve full coverage without dead spots, a series of regular polygons for cell sites are required. 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.

Goal is to find the minimum distance between cells using same frequencies. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.60 Frequency re-use distance I 60% i,j - integers -> intercell distance along cell centers i A i,j: multiples of 3 1/2 R j D A R D - min. dist. 1/2 2 2 1/2 D=3 R[i +j +ij] R = radius of hexagonal

R: cell radius i,j are integers v 1/2 R 1/2 3 R R u 3 1 3 R (u,v) D 300 2 1 (0,0) u2-u1=3 1/2 1/2

v2-v1=3 ICS 243E - Ch.2 Wireless Transmission Spring 2003 Ri Rj 2.61 Frequency re-use distance II For two adjacent cells: D=3x101/2R The closest we can place the same frequencies is called the first tier around the center cell (minimal re-use distance -> lower -> more capacity!). For simplicity we only take the first tier of cells into account for co-channel interference (i.e., we ignore 2nd, 3x10rd, etc. tiers, cause much less interference, negligible!). Original cell First tier of interferers Second tier of interferers Cluster of N cells with different frequencies They are all equidistant away from each other (D) Each cell has exactly six equidistant interfering cells ICS 243E - Ch.2 Wireless Transmission

Spring 2003 2.62 Frequency re-use distance III Radius = D D First Tier (all use same frequencies as center cell) Radius R Cluster of N cells with frequencies different from center cell (large hexagon) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.63 Frequency re-use distance III Radius = dist. between two co-channel cells = (3x10R2[i2+j2+ij])1/2 = D 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[3x10R2[i2+j2+ij]] where k is a constant.

Similarly the area of each cell (i.e., small hexagon) is: Asmall = k[R2] Comparing these expressions we find that: Alarge/Asmall = 3x10[i2+j2+ij] = D2/R2 ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.64 Frequency re-use distance IV From symmetry we can see that the large hexagon encloses the center cluster of N cells plus 1/3x10 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(1/3x10N) = 3x10N Since the area of a hexagon is proportional to the number of cells contained within it: Alarge/Asmall = 3x10N/1 = 3x10N Substituting we get: 3x10N = 3x10[i2+j2+ij] = D2/R2 Or: D/R = q =(3x10N)1/2

q is referred to as the reuse ratio! ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.65 Co-channel Interference I The co-channel interference ratio S/I is given as: S S --- = -----------------------N I i I k k1= 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. If we only assume the first tier of interfering cells, then Ni=6,and all cells interfere equally (they are all equidistant!). The signal power at any point is inversely proportional to the inverse of the distance from the source raised to the power. (2<<5)) ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.66 Co-channel Interference II Ik is proportional to D , and S is proportional to R , where is the propagation path loss and is dependent upon terrain environment. For cellular systems it is often taken as = 4. Therefore: S--- === R 1 q-----------------------------------I 6 6D 6q The relationship between SNR (signal to noise ratio - Eb/No) and S/I for cellular systems with Rayleigh fading channels: SNR = S/I(db) 9db. ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.67 For a given S/I how to get N

Recall that: D/R = q =(3x10N)1/2 An S/I = 18db (decibels=10logS/I) = 63x10.1, gives an acceptable voice quality. Therefore q = [6x63x10.1]1/4 = 4.41 when = 4 Substituting for N we get N = (4.41)2/3x10 equals approx. 7 This means that if we have 49 frequency channels available, each cell gets 49/7 = 7 frequency channels. If we have 82 available then 82/7 = 11.714 -> which means that 5 cells will have 12 and 2 cells will have 11! How does that translate to i and j for a cell layout? N = [i2+j2+ij], find i,j that satisfy the equation! ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.68 Calculating i, j, and D from N 7 2 1

7 j i 2 6 1 7 1 2 D 5) 3 4 N=7 -> i=2, j=1 f2 f4 f3 f6 ICS 243E - Ch.2 Wireless Transmission f5 f1 f2 f3 f6 f7 f5 D = 4.41R

f2 f4 f3 f7 f5 f1 f2 Spring 2003 2.69 Frequency planning Frequency reuse only with a certain distance between the base stations Standard model using 7 frequencies: f4 f3 f5 f1 f2 f3 f6 f7 f2 f4 f5 f1

Fixed frequency assignment: certain frequencies are assigned to a certain cell problem: different traffic load in different cells Dynamic frequency assignment: base station chooses frequencies depending on the frequencies already used in neighbor cells more capacity in cells with more traffic assignment can also be based on interference measurements ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.36.1 2.70 Increasing Capacity 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. 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). An easier solution exists, sectorization. It does not reduce handoffs, its advantage: it does not require more infrastructure.

ICS 243E - Ch.2 Wireless Transmission Spring 2003 2.71 Sectorization I We can also increase the capacity by using sectors in cells. Directional antennas instead of being omnidirectional, will only beam over a certain angle. F1+F2+F3=Fa 120% F1+F2+F3+F4+F5+F6=Fa F1 F3 F2 3 sectors f3 f1 f2 f3 f2 f3 f1 f3 f1 f2 f3 F1 F6

Fa: A cells set of frequencies f2 f3 f1 F2 60% F3 F4 F5 6 sectors f3 f1 f2 f2 f2 f2 f1 f f1 f f1 f h h 3 3 3 h1 2 h1 2 g 2 h3 g2 h3 g2 g1 g1 g1 g3

g3 g3 f3 3 cell cluster ICS 243E - Ch.2 Wireless Transmission 3 cell cluster with 3 sectors Spring 2003 2.72 Sectorization II What does that mean? We can now assign frequency sets to sectors and decrease the re-use distance or improve S/I ratio (i.e. signal quality). Question: By how much? Depends on number of sectors (i.e., 60% or 120%). A: set of frequencies in a sector A A First Tier (all use same frequencies in sectors as A center cell) A:Do not interfere with Asector of center cell A

AA A A:Cause Cell site to mobile interference ICS 243E - Ch.2 Wireless Transmission A A:Cause Mobile to cell site interference Spring 2003 2.73 Other Capacity or Signal Improvement Tech. 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. 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 243E - Ch.2 Wireless Transmission Spring 2003 2.74

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