TLCM 411: GSM and Wireless Communication PDF

TLCM 411: GSM and Wireless Communication Lec: Mourice Ojijo Department of Computer Science and Information Technology, Faculty of Telecommunication Engineering,Kabarak University May 13, 2024  Course content 1 Fundamental...

TLCM 411: GSM and Wireless Communication Lec: Mourice Ojijo Department of Computer Science and Information Technology, Faculty of Telecommunication Engineering,Kabarak University May 13, 2024  Course content 1 Fundamentals of wireless propagation Propagation Modes Noise The Expression Eb/N0 The Expression Eb/N0 Fading in the mobile environment The Effects of Multipath Propagation Types of fading The fading channel Error Compensation mechanisms Empirical Path Loss Models The Okumura Model The Hata Model Path loss model for multiple rays Simplified Path Loss Model Other definitions 2 Cellular Communication 3 Cellular Communication Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 2 / 160  Fundamentals of wireless propagation A wireless network refers to any type of network that is note connected by cables It is a method by which organizations, homes, and businesses are connected In wireless transmission, a single transmitter radiates radio waves which are then received by a single receiver or multiple receivers. Consider an antenna setup in Figure1. radiating outward towards a receiving antenna. The antennas are marked as TX and TR for transmitter and receiver respectively Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 3 / 160  d TX RX Figure 1: Transmitter Receiver setup where d = distance between the two antennas TX = Transmitting antenna RX = Receiving antenna Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 4 / 160  Consider and power PT assumed to be radiating equally in all directions (isotropically) from TX sin the power spreads out spherically, then at a distance d, the power density in the wave which is the power per unit area and is given by PT PDi = 2 w /m2 (1) 4πd where PDi = Isotropic power density PT = Transmit power Equation 2 is so because 4πd 2 is the surface area of a sphere with radius d Since practical antennas radiate more power in one direction at the expense of other directions. We can define the directivity gain as the ratio of the actual power density along the main axis of radiation of the antenna to that which would be radiated isotropically Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 5 / 160  Let GT be the maximum directivity gain of the transmitting antenna, then the power density along the direction of propagation is given by PT GT PD = PDi GT = (2) 4πd 2 When RX is place in such a way the it receives maximum power such that the effective area of the receiving antenna being Aeff then the power received PR given by PT GT PR = (3) 4πd 2 It can be shown (not in this) that the maximum directivity gain to effective area is given by Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 6 / 160  Aeff λ2 = (4) GR 4π where GR = is the RX gain λ = wavelength of transmitter G R λ2 ∴ Aeff (5) 4π We now show that GR λ2 PR = PT GT ( ) (6) 42 π 2 d 2 PR λ2 = GT GR (7) PT 4πd Since c = λf λ = cf Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 7 / 160  !2 PR λ = GT GR (8) PT 4πd putting d = km(x 1000) f = MHz(x 106 ) c = 3x 108 m/s PR (0.57x 10−3 ) = GT GR (9) PT (df )2 PR ( )dB = (GT )dB + (GR )dB − (32.5 + 20 log 10d + 20 log 10f ) (10) PT The term L = (32.5 + 20 log 10d + 20 log 10f ) is the loss resulting from spreading of the waves. It is known as a transmission path loss. Therefore PR ( )dB = (GT )dB + (GR )dB − LdB (11) PT Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 8 / 160  Propagation Modes Ground wave propagation: These waves follow the contour of the earth and can propagate considerable distance well over the visual horizon. This effect is found at frequencies of about 2MHz Figure 2: Ground wave propagation Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 9 / 160  Propagation Modes Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused by refraction. Figure 3: Sky wave propagation Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 10 / 160  Propagation Modes Above 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must be by line of sight (Figure 4) This mode of propagation is mostly used in satellite and microwave communication links Figure 4: Line of sight propagation Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 11 / 160  Noise In transmission the received signal will be modified by the various distortions imposed by the transmission system plus addition unwanted signals that are inserted. The unwanted signals are referred to as noise. Noise may be divided into four categories: ▶ Thermal noise ▶ Intermodulation noise ▶ Crosstalk ▶ Impulse noise Thermal noise: Thermal noise is due to thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise. Thermal noise cannot be eliminated and therefore places an upper bound on communications system performance. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 12 / 160  Noise The amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor is N0 = kT (W /Hz) (12) where N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann’s constant = 1.38X 10−23 J/K T = temperature, in kelvins (absolute temperature) The noise is assumed to be independent of frequency. Thus the thermal noise in watts present in a bandwidth of B Hertz can be expressed as N = kTB (13) N = 10logk +10logT +10logB = −228.6dBW +10logT +10logB (14) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 13 / 160  Noise Example Given a receiver with an effective noise temperature of 294K and a 10-MHz bandwidth, determine the thermal noise level at the receiver output Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 14 / 160  Noise Intermodulation noise produces signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies. For example, the mixing of signals at frequencies f1 and f2 might produce energy at the frequency fl + f2. This derived signal could interfere with an intended signal at the frequency f1 + f2. Crosstalk has been experienced by anyone who, while using the telephone, has been able to hear another conversation; it is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. Impulse noise, however, is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 15 / 160  Noise The Expression Eb/N0 The parameter is the ratio of signal energy per bit to noise power density per Hertz, Eb /N0 Consider a signal, digital or analog, that contains binary digital data transmitted at a certain bit rate R. Recalling that 1 watt = 1J/s, the energy per bit in a signal is given by Eb = STb , where Sis the signal power and Tb is the time required to send one bit. The data rate R is just R = 1/Tb. Thus Eb S/R S = = (15) N0 N0 kTR or, in decibel notation Eb ( )d B = SdBW − 10logR − 10logk − 10logT (16) N0 = SdBW − 10logR + 228.6dBW − 10logT (17) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 16 / 160  Noise The Expression Eb/N0 We can relate Eb /N0 to SNR as follows. We have Eb S = (18) N0 N0 R The parameter N0 is the noise power density in watts/hertz. Hence, the noise in a signal with bandwidth BT is N = N0 BT. Substituting, we have Eb S BT = (19) N0 N R Another formulation of interest relates to Eb /N0 spectral efficiency. Shannon’s result that the maximum channel capacity, in bits per second, obeys the equation C = B log2 (1 + S/N) (20) where C is the capacity of the channel in bits per second and B is the bandwidth ofScience the Lec: Mourice Ojijo (Department of ComputerTLCM channel 411: andGSM Information in Hertz and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 17 / 160  Noise The Expression Eb/N0 Equation 20 can be rewritten as S = 2C /B − 1 (21) N Using Equation 19, and equating B T with Band R with C, we have Eb B = (2C /B − 1) (22) N0 C This is a useful formula that relates the achievable spectral efficiency C /B to Eb /N0 Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 18 / 160  Noise Example Find the minimum Eb /N0 required to achieve a spectral efficiency of 6pbs/Hz Solution Eb /N0 = (1/6)(26 − 1) = 10.5 = 10.21dB Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 19 / 160  Fading in the mobile environment The term fading refers to the time variation of received signal power caused by changes in the transmission medium or path(s) Multipath:The signal can be reflected by such obstacles so that multiple copies of the signal with varying delays can be received. Figure 5: Examples of multipath fading Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 20 / 160  Fading in the mobile environment Refraction: Radio waves are refracted (or bent) when they propagate through the atmosphere. The refraction is caused by changes in the speed of the signal with altitude or by other spatial changes in the atmospheric conditions. Multipath Propagation:Reflection occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal. Diffraction occurs at the edge of an impenetrable body that is large compared to the wavelength of the radio wave. When a radio wave encounters such an edge, waves propagate in different directions with the edge as the source Scattering: If the size of an obstacle is in the order of the wavelength of the signal or less, scattering occurs. An incoming signal is scattered into several weaker outgoing signals. At typical cellular microwave frequencies, there are numerous objects, such as lamp posts and traffic signs, that can cause Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 21 / 160  Fading in the mobile environment Figure 6: Examples of multipath signals Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 22 / 160  The Effects of Multipath Propagation As just noted, one unwanted effect of multipath propagation is that multiple copies of a signal may arrive at different phases. If these phases add destructively, the signal level relative to noise declines, making signal detection at the receiver more difficult A second phenomenon, of particular importance for digital transmission, is intersymbol interference (lSI). Figure 7: Two Pulses in Time-Variant Multipath Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 23 / 160  Types of fading Fast fading: When the waveform is rapidly changing in amplitude between 20 to 30dB over a very short distance Slow fading: When the waveform amplitude changes over longer distances Flat fading: or nonselective fading, is that type of fading in which all frequency components of the received signal fluctuate in the same proportions simultaneously Selective fading affects unequally the different spectral components of a radio signal. Figure 8: Typical Slow and Fast Fading in an Urban Mobile Environment Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 24 / 160  The fading channel In designing a communications system, the communications engineer needs to estimate the effects of multipath fading and noise on the mobile channel. The simplest channel model, from the point of view of analysis, is the additive white Gaussian noise (AWGN) channel. In this channel, the desired signal is degraded by thermal noise associated with the physical channel itself as well as electronics at the transmitter and receiver (and any intermediate amplifiers or repeaters). This model is fairly accurate in some cases, such as space communications and some wire transmissions, such as coaxial cable. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 25 / 160  The fading channel Rayleigh fading occurs when there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path. This represents a worst case scenario. Rician fading best characterizes a situation where there is a direct LOS path in addition to a number of indirect multipath signals. The Rician model is often applicable in an indoor environment whereas the Rayleigh model characterizes outdoor settings. The Rician model also becomes more applicable in smaller cells or in more open outdoor environments. The channels can be characterized by a parameter K, defined as follows: power in the dominant path K= (23) power in the scattered paths When K = 0 the channel is Rayleigh (i.e., numerator is zero) and when K = ∞, the channel is AWGN (i.e., denominator is zero). Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 26 / 160  Error Compensation mechanisms The efforts to compensate for the errors and distortions introduced by multipath fading fall into three general categories ▶ Forward Error Correction: is applicable in digital transmission applications: those in which the transmitted signal carries digital data or digitized voice or video data The term forward refers to procedures whereby a receiver, using only information contained in the incoming digital transmission, to correct bit errors in the data. ▶ Adaptive equalization can be applied to transmissions that carry analog information (e.g., analog voice or video) or digital information (e.g., digital data, digitized voice or video) and is used to combat intersymbol interference Fig. 9. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 27 / 160  Error Compensation mechanisms ▶ Diversity is based on the fact that individual channels experience independent fading events. ▶ We can therefore compensate for error effects by providing multiple logical channels in some sense between transmitter and receiver Figure 9: Linear Equalizer Circuit Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 28 / 160  Empirical Path Loss Models Most mobile communication systems operate in complex propagation environments that cannot be accurately modeled by free-space path loss or ray tracing A number of path loss models have been developed over the years to predict path loss in typical wireless environments such as large urban macrocells, urban microcells, and, more recently, inside buildings These models are mainly based on empirical measurements over a given distance in a given frequency range and a particular geographical area or building However, applications of these 36models are not always restricted to environments in which the empirical measurements were made, which makes the accuracy of such empirically-based models applied to more general environments somewhat questionable. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 29 / 160  Empirical Path Loss Models Analytical models characterizePr /Pt as a function of distance, so path loss is well defined. In contrast, empirical measurements of Pr /Pt as a function of distance include the effects of path loss, shadowing, and multipath. In order to remove multipath effects, empirical measurements for path loss typically average their received power measurements and the corresponding path loss at a given distance over several wavelengths This average path loss is called the local mean attenuation (LMA) at distance d, and generally decreases with d due to free space path loss and signal obstructions Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 30 / 160  Empirical Path Loss Models The LMA in a given environment, like a city, depends on the specific location of the transmitter and receiver corresponding to the LMA measurement To characterize LMA more generally, measurements are typically taken throughout the environment, and possibly in multiple environments with similar characteristics Thus, the empirical path loss PL (d) for a given environment (e.g. a city, suburban area, or office building) is defined as the average of the LMA measurements at distance d, averaged over all available measurements in the given environment Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 31 / 160  The Okumura Model One of the most common models for signal prediction in large urban macrocells is the Okumura model This model is applicable over distances of 1-100 Km and frequency ranges of 150-1500 MHz. Okumura used extensive measurements of base station-to-mobile signal attenuation throughout Tokyo to develop a set of curves giving median attenuation relative to free space of signal propagation in irregular terrain The base station heights for these measurements were 30-100 m, the upper end of which is higher than typical base stations today The empirical path loss formula of Okumura at distance d parameterized by the carrier frequency fc is given by PL (d)dB = L(fc , d) + Amu (fc, d) − G(ht ) − G(hr ) − GAREA (24) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 32 / 160  The Okumura Model whereL(fc , d) is free space path loss at distance d and carrier frequency fc , Amu (fc , d) is the median attenuation in addition to free space path loss across all environments, G(ht ) is the base station antenna height gain factor, G(hr ) is the mobile antenna height gain factor, and GAREA is the gain due to the type of environment. The values of Amu (fc , d) and GAREA are obtained from Okumura’s empirical plots Okumura derived empirical formulas for G(ht) and G(hr) as G(ht) = 20log10 (ht/200), 30m < ht < 1000m (25)  10log 10 (hr /3), hr ≤ 3m G(hr ) = 20log10 (hr /3) 3m < hr < 10m Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 33 / 160  The Hata Model The Hata model is an empirical formulation of the graphical path loss data provided by Okumura and is valid over roughly the same range of frequencies, 150 -1500 MHz This empirical model simplifies calculation of path loss since it is a closed-form formula and is not based on empirical curves for the different parameters. The standard formula for empirical path loss in urban areas under the Hata model is PL,urban (d)dB = 69.55 + 26.16log10(fc ) − 13.82 log10 (ht) − a(hr ) + (44.9 − 6.55 (26) log10 (ht ))log10 (d) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 34 / 160  The Hata Model The parameters in this model are the same as under the Okumura model, and a(hr ) is a correction factor for the mobile antenna height based on the size of the coverage area. For small to medium sized cities, this factor is given by a(hr ) = (1.1 log10 (fc) − 0.7)hr − (1.56log10 (fc) − 0.8)dB (27) And for larger cities at frequencies fc > 300 MHz by a(hr ) = 3.2[log10 (11.75hr )]2 − 4.97dB (28) Corrections to the urban model are made for suburban and rural propagation, so that these models are, respectively PL,suburban (d) = PL,urban (d) − 2[log10 (fc/28)]2 − 5.4 (29) and PL,rural (d) = PL, urban(d)−4.78[log10(fc)]2 +18.33log10 (fc)−K (30) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 35 / 160  The Hata Model Remarks K ranges from 35.94 (countryside) to 40.94 (desert) The Hata model does not provide for any path specific correction factors, as is available in the Okumura model The Hata model well-approximates the Okumura model for distances d > 1 Km It is a good model for first generation cellular systems, but does not model propagation well in current cellular systems with smaller cell sizes and higher frequencies. Indoor environments are also not captured with the Hata model. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 36 / 160  Worked example Figure 10 Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 37 / 160  Exercise Describe the COST 231 Extension to Hata Model providing the equation for the pathloss and expressing all the available constraints Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 38 / 160  Path loss model for multiple rays When considering the reflection from ground, we can use the two-ray ground reflection model as follows Figure 11: Two-ray ground reflection model !2 ht hr Pr = Pt Gr Gt (31) d2 where ht and hr denote the transmitter antenna height and receiver antenna height, respectively. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 39 / 160  Simplified Path Loss Model Figure 12: Two-ray ground reflection model Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 40 / 160  Simplified Path Loss Model In practical systems, we use empirical path loss models. The received power of the empirical models is expressed as follows: !γ d Pr = Pt P0 (32) d0 where P0 is the power at the reference distance, d0 , and γ is the path loss exponent which typically has 2–8 according to carrier frequency, environment, and so on The path loss can be defined as ! d PL (d) = PL0 + 10γlog10 (33) d0 where PL0 is the path loss at the reference distance d0 (dB) Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 41 / 160  Simplified Path Loss Model When a propagating electromagnetic wave encounters some obstruction such as wall, building, and tree, reflection, diffraction, and scattering happen and the electromagnetic wave is distorted. We call this effect shadowing or slow fading and use the modified path loss model as follows: ! d PL (d) = PL0 + 10γlog10 +X (34) d0 where X denotes a Gaussian distributed random variable with standard deviation, σ and represents shadowing effect. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 42 / 160  Other definitions Coherence bandwidth The coherence bandwidth is a statistical measurement of the bandwidth where the channel is regarded as the flat channel, which means two signals passing through the channel experience similar gain and phase rotation. Coherence time The coherence time is the time interval over which a channel impulse response is regarded as not varying. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 43 / 160  Cellular Communication Cellular telephone system provides wireless connection to the PSTN for any user location within the radio coverage region. Cellular systems accommodate a large number of users over a larger geographical area. High capacity is achieved by limiting the coverage area of a base station to geographical area called a cell The cell system allow frequencies to be reused by another base station located at a distance A sophisticated switching system called handoff allow a call to proceed uninterrupted when a user moves from the coverage area from one base station to another. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 44 / 160  Figure 13: An Illustration of a cellular system Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 45 / 160  Cellular Communication Fig 13 Shows a basic cellular system consisting of mobile stations, base stations and mobile switching center (MSC) Each of the mobile station communicates via radio withe one of the base stations and may be handed off to any number of base stations throughout the duration of a call. The mobile station contains a transceiver , an antenna and control circuitry, The base stations consist of several transmitters and receivers which simultaneously handle full duplex communications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC. The MSC coordinates the activities of all of the base stations and connects the entire cellular system to the PSTN Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 46 / 160  Cellular Communication Communication between the base station and the mobile is defined by a standard common air interface that specify different channels. The channels used for voice communications from the base station to mobiles are called forward voice channels The channels used for voice transmission from mobile to the base station is called reverse voice channels The channels responsible for initiating movile calls are the forward control channels and reverse control channels. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 47 / 160  Cellular Communication How a cellular Telephone call is made When a cellular phone is switched on it scans the group of forward control channels to determine the one with the strongest signal and the monitors that control signal channels until the signal drops below a usable level. Since the control channels are standardized and are identical, every phone scans the same channels while idle. When a phone call is placed to a mobile user, the MSC dispatches the request to all base stations in the cellular systems. The mobile identification number which is the subscriber’s telephone number is then broadcast as a paging message over all of the forward control channels throughout the cellular system. The mobile receives the paging message sent by the base station which it monitors, and responds by identifying itself over reverse control channel. The base station relays the acknowledgment sent by the mobile Lec: Mouriceand informsofthe Ojijo (Department MSC Computer TLCM ofandGSM Science 411: the handshake. Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 48 / 160  Cellular Communication How a cellular Telephone call is made The MSC then instructs the base station to move the call to an unused voice channel within the cell The base station signals the mobile to change frequencies to an unused voice channel pair at which point a call alert is transmitted over the forward voice channel. Once a call is in progress the MSC adjust the transmit power of the mobile and changes the channel of the mobile unit and base stations in order to maintain and call quality a the subscriber moves in and out of range of each base station. This is called handoff Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 49 / 160  Course content 1 Access Network Architectures 1 GSM Base Station Subsystem (BSS) 2 3G Radio Network System (RNS) 3 4G Long Term Evolution (LTE) 4 Wimax Access Network 2 Core Network Architectures ▶ Evolved Packet Core (EPC) 3 Multimedia Subsystems ▶ IP Voice ▶ Voice over LTE ▶ SIP Servers Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 50 / 160  Cellular Technology A cellular radio system provides standard telephone service by two-way radio at remote locations. The basic concept behind the cellular radio system is that rather than serving a given geographic area with a single transmitter and receiver, the system divides the service area into many smaller areas known as cells Each cell is connected by telephone lines or a microwave radio relay link to a master control center known as the mobile telephone switching ofi ce (MTSO) The MTSO controls all the cells and provides the interface between each cell and the main telephone office As the person with the cell phone passes through a cell, it is served by the cell transceiver Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 51 / 160  Cellular Technology Figure 14 Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 52 / 160  Cellular Technology Frequency Allocation Cellular radio systems operate in the UHF and microwave bands as assigned by the Federal Communications Commission (FCC) The original frequency assignments were in the 800- to 900-MHz range previously occupied by the mostly unused UHF TV channels 68 through 83 Figure 15 Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 53 / 160  Cellular Technology Multiple Access Multiple access refers to how the subscribers are allocated to the assigned frequency spectrum. Access methods are the ways in which many users share a limited amount of spectrum. The techniques include frequency reuse, frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and spatial-division multiple access (SDMA). Frequency Reuse. In frequency reuse, individual frequency bands are shared by multiple base stations and users. This is possible by ensuring that one subscriber or base station does not interfere with any others. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 54 / 160  Cellular Technology Frequency reuse and base station Frequency reuse is achieved by controlling such factors as transmission power, base station spacing, and antenna height and radiation patterns With low-power and lower-height antennas, the range of a signal is restricted to only a mile or so most base stations use sectorized antennas with 1200 radiation patterns that transmit and receive over only a portion of the area they cover Figure 16: Horizontal antenna radiation pattern of a common cell site 0 Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 55 / 160  Cellular Technology Frequency-Division Multiple Access FDMA systems are like frequency- division multiplexing in that they allow many users to share a block of spectrum by simply dividing it up into many smaller channels. Each channel of a band is given an assigned number or is designated by the center frequency of the channel. One subscriber is assigned to each channel. Typical channel widths are 30 kHz, 200 kHz, 1.25 MHz, and 5 MHz. There are usually two similar bands, one for uplink and the other for downlink. Figure 17: Frequency-division multiple-access (FDMA) spectrum Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 56 / 160  Cellular Technology Time-Division Multiple Access TDMA relies on digital signals and operates on a single channel. Multiple users use different time slots. Because the audio signal is sampled at a rapid rate, the data words can be interleaved into different time slots Of the two common TDMA systems in use, one allows three users per frequency channel and the other allows eight users per channel.. Figure 18: Time division multiple access (TDMA). Different callers use different time slots on the same channel. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 57 / 160  Cellular Technology Code-Division Multiple Access CDMA is just another name for spread spectrum. A high percentage of cell phone systems use direct sequence spread spectrum (DSSS). Here the digital audio signals are encoded in a circuit called a vocoder to produce a 13-kbps serial digital compressed voice signal It is then combined with a higher-frequency chipping signal. One system uses a 1.288-Mbps chipping signal to encode the audio, spreading the signal over a 1.25-MHz channel. With unique coding, up to 64 subscribers can share a 1.25-MHz channel. A similar technique is used with the wideband CDMA system of third-generation cellphones. A 3.84-Mbps chipping rate is used in a 5-MHz channel to accommodate multiple users Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 58 / 160  Cellular Technology Code-Division Multiple Access Figure 19: Code-division multiple access (CDMA). (a) Spreading the signal.(b) Resulting bandwidth Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 59 / 160  Cellular Technology Orthogonal Frequency Division Multiplexing Access OFDMA is the access method used with OFDM. OFDM uses hundreds, even thousands, of subcarriers in a wideband channel. This large number of subcarriers can be subdivided into smaller groups, and each group can be assigned to an individual user. In this way, many users can use the wideband channel assigned to the OFDM signal. Groups of subcarriers are formed to create a subchannel assigned to one user. In some systems, the subcarriers do not have to be contiguous, but instead might be spread around inside the total OFDM signal bandwidth. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 60 / 160  Cellular Technology Orthogonal Frequency Division Multiplexing Access Figure 20: The Concept of OFDMA. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 61 / 160  Cellular Technology Spatial-Division Multiple Access This form of access is actually an extension of frequency reuse. It uses highly directional antennas to pinpoint users and reject others on the same frequency In Fig. 21, very narrow antenna beams at the cell site base station are able to lock in on one subscriber but block another while both subscribers are using the same frequency Modern antenna technology using adaptive phased arrays is making this possible. Such antennas allow cell phone carriers to expand the number of subscribers by more aggressive frequency reuse because iner discrimination can be achieved with the antennas Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 62 / 160  Cellular Technology Spatial-Division Multiple Access Figure 21: The concept of spatial-division multiple access (SDMA) using highly directional antennas. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 63 / 160  Global System for Mobile Communication (GSM) When the acronym GSM was used for the first time in 1982, it stood for Groupe Spéciale Mobile, a committee under the umbrella of Conférence Européenne des Postes et Télécommunications (CEPT), the European standardization organization. The task of GSM was to define a new standard for mobile communications in the 900 MHz range. The goal of GSM was to replace the purely national, already overloaded, and thus expensive technologies of the member countries with an international standard. Lec: Mourice Ojijo (Department of ComputerTLCM Science 411: andGSM Information and Wireless Technology, Communication Faculty of Telecommunication May 13, 2024Engineering,Kabarak 64 / 160  The System Architecture of GSM: A Network of Cells The basic idea of a cellular network is to partition the available frequency range, to assign only parts of that frequency spectrum to any base transceiver station, and to reduce the range of a base station in order to reuse the scarce frequencies as often as possible. One of the major goals of network planning is to reduce interference between different base stations. Besides the advantage of reusing frequencies, a cellular network also comes with the following disadvantages: ▶ An increasing number of base stations increases the cost of infrastructure and access lines. ▶ All cellular networks require that, as the mobile station moves, an active call is handed over from one cell to another, a process known as handover. ▶ The network has to be kept informed of the approximate locationof Computer Lec: Mourice Ojijo (Department of theTLCMmobile Science 411: station, andGSM Information even and Wireless Technology, without Communication a call in2024 progress, Faculty of Telecommunication