CHAPTER 5 PRACTICAL COMMUNICATION SYSTEMS Fiber Optic Communication System Introduction Fiber optic system : a communication system where the information is put on a light beam and transmitted through a guided cable. Light frequencies used in fiber optic systems are between 4.3x1014 and 7.5x1014 Hz. Thus the higher the carrier frequency, the wider the bandwidth, and consequently, the greater the information-carrying capacity. Block diagram of a fiber optic communication link, is illustrated in Figure 5.1. Transmitter Input Driving Light Light Coupler signal circuit Electrical Electrical Coupler VIC Light source Fiber cable Repeater Repeater Light detector Receiver Output IVC Light Coupler signal Electrical VIC - Voltage-to-Current converter IVC - Current-to-Voltage converter Figure 5.1 Basic elements of a fiber optic communication system The main elements in an optical fiber communication link are:- 1. Driving circuitry: Driving circuitry serves as an electrical interface between the input circuitry and the light source. It converts voltage signal to current signal (voltage to current converter) to drive the light source. 2. Light Source: Light source can be either a light-emitting diode (LED) or Laser (Light Amplification by Stimulated Emission of Radiation). Light source is to convert electrical energy to optical energy, where the amount of light emitted is proportional to the amount of drive current. Another words, the light intensity depends on the amplitude variations of the input signal. 3. Light Source-to-fiber coupler: This is an interface to couple the light emitted by the source into the optical fiber cable. 4. Fiber Optics Cable: It is a long thin strand of glass or plastic fiber used to transfer signal in a form of light from a point to another point. Most fiber cables have a circular cross section with a diameter of only a fraction of an inch (Its size is almost the same as the size of human hair). The characteristics of light transmission through a glass fiber depend on many factors, for examples:- The composition of the fiber. The amount & type of light introduced into the fiber. The diameter and length of the fiber. The fiber optic consists of three parts:- The core, where the light is passing through. The cladding, which surrounds the core with a lower refractive index. It is to ensure that the light waves remain within the core, to protect the fiber core from scratches and to strengthen the fiber core. Protective jacket/coating. This is the outer coating, made of specially formulated plastic coating that provides a first level shock and resistance to damage and moisture, for the fiber. Typically it is a clear protective coating or a material made of stranded steel or a special yarn known as Kevlar. Kevlar is strong and preferred over steel as it is an insulator. Kevlar may forms a protective sleeve or jacket over the cladding. Fiber optic cable are also available in a flat ribbon form, which are easier to use and more space-efficient, especially for multiple fibers. 5. Fiber-to-detector coupler: This is an interface between fiber and light-detector to couple as much light as possible from the fiber cable into the light detector. 6. Light detector: The commonly used light detector or photo sensor is either a PIN (p-type-intrinsic-n-type) diode or an APD (avalanche photodiode). Both the APD and PIN diode convert light energy to electrical energy or current. Consequently, a current-to-voltage converter (IVC) is required, which transforms variations of detector current to output signal voltage variations. Plastic Kevlar Protective Cladding Outer Sleeve Core Coating Jacket Figure 5.2 Typical layers in a fiber optic cable Types of fiber optic cable There are three commonly types of fiber optic cable: 1. Single mode fiber: The single mode or mono-mode has a very small core diameter, typical core sizes are 2 μm to 15 μm diameter. So, only a single path that light may take as it propagates down the cable. All light rays follow approximately the same path down the cable and take approximately the same amount of time to travel the length of the cable. Here, modal dispersion is zero. Modal dispersion is where the pulse at the end of the cable is lower in amplitude due to the attenuation of the light in the cable and increase in duration due to the different arrival times of light rays. Single mode fiber 2. Multimode step index fiber: The centre core is larger of about from 50 μm to 100 μm diameter. It has a large light-to-fiber aperture and allows more light to enter the cable. The light rays are propagated down the cable in a zig-zag pattern, continuously reflecting off the interface boundary, resulting many paths. As a result, all light rays do not follow the same path, with different propagation time. Modal dispersion exists. Multimode step index fiber 3. Multimode graded index fiber: The centre core is about from 50 μm to 100 μm diameter. Multimode graded-index fiber is characterized by a central core that has a refractive index that is non-uniform; it is maximum at the centre and decreases gradually toward the side of the cable. Light is propagated down the fiber by refraction, which results in a continuous bending of the light rays, at many different angles. As they propagate down the fiber, the light rays that travel in the outermost area travel a greater distance, but with a higher velocity, than the rays traveling near the centre (because velocity is inversely proportional to the refractive index). Finally, all the light rays arrive at the end point at almost the same time, resulting less modal dispersion. Multimode graded index fiber Advantages fiber optic cables over conventional electrical cables 1. Wider bandwidth: Fiber optic cables have higher information-carry capability. 2 Lower loss/attenuation: With fiber-optic cables, there is less signal attenuation over long distances. The loss is low as 0.2 dB/km, and the repeater spacing’s are longer, thus reducing both system cost and complexity. 3. Light weight: Glass or plastic cables are much lighter than copper cables and offer benefits in those areas where low weight is critical. Advantages – cnt’d 4. Small size: Practically, fiber-optic cables are much smaller in diameter than electrical cable. Therefore, more can be contained in a smaller space. 5. Strength: Fiber-optic cables are stronger than electrical cable and can support more weight. They are manufactured with very high tensile strengths, can be bent or twisted without damage. So, they are superior in terms of storage, transportation, handlings and installation than corresponding copper cables. 6. Security: Fiber-optic cables cannot be “tapped” as easily as electrical cables, and they do not radiate signals that can be picked up for eavesdropping purposes. There is less need for complex and expensive encryption techniques. 7. Interference immunity: Fiber-optic cables do not radiate signals as some electrical cables and cause interference to other cables. They are also immune to pickup of interference from other sources. As they are dielectric waveguides, they are free from electromagnetic interference (EMI), radio frequency interference (RFI) or switching transients giving electromagnetic pulses (EMP). They have no optical interference between them and no crosstalk. 8. Greater safety: Fiber -optic cables do not carry electricity. Therefore, there is no shock hazard, no arching, no sparks and no short circuits. They are also insulators so are not susceptible to lightning strikes as electrical cables. 9. System reliability and easy maintenance: The lifetime is about 20-30 years, it yields good reliability, reduces maintenance time, manpower and maintenance costs. 10. Potential low cost as the total costs are continuing to decline. 11. No problems of corrosions. Attenuation/losses in fiber optics Cable attenuation Cable attenuation of a fiber optic cable is expressed in dB per unit length, dB/km. The total cable attenuation depends on the cable length. For a fiber optic cable of a length, L (in km), the cable attenuation is, [10 log10 (Po/Pi)]/L , in dB/km. where Po is the output power and Pi is the input power. The possible causes of cable attenuation are: 1. Scattering losses: i.e. the radiation of propagated light due to the microscopic imperfection of the fiber. When light rays that are propagating strike these impurities, they are diffracted. Diffraction causes the light to disperse or spread out in many directions, which represent a loss in light power. Scattering also refers to the light loss because of light waves entering at the wrong incident angle and being lost in the cladding due to refraction. This is called Rayleigh scattering loss. 2. Absorption losses: Absorption refers to how the light waves are actually “soaked-up” in the fiber core due to the impurity of the glass or plastic or due to any imperfections. Bending losses: occur because the light rays on the outside of a sharp bend cannot travel fast enough to keep up with the other rays and are lost. Table 5.1 shows losses in a typical in a typical multimode fiber as a function of wavelength with Rayleigh and absorption components shown for comparison, showing three windows. Window is a short range of wavelength where the attenuation is at minimum. Table 5.1 Loss spectrum of typical optical fiber Optical signal attenuation: Optical signal attenuation is expressed in dB, Attenuation (dB) = 10 log10 (Po/Pi) where Po is the output power and Pi is the input power. The possible causes of cable attenuation are: 1. Splicing loss: Splicing is a permanent jointing of two fibers. 2. Coupling losses: These represent a large source of loss in commercial fiber optic system. They occur at either; light source-to-fiber connections, fiber-to-fiber, or, fiber-to-photo detector connections, mainly due to misalignments. Applications of Fiber-Optic Cable: Long-haul, backbone public and private networks Local loop networks Fiber backbone networks (LAN connectivity) High-resolution image and digital video TV studio-to- transmitter interconnection, elimination microwave radio link. Closed-circuit TV systems used in building for security. Secure communications systems at military bases. Computer networks, wide area and local area. Shipboard communications. Aircraft communications and controls. Interconnection of measuring and monitoring instruments in plants and laboratories. Data acquisition and control signal communications in industrial process control systems. Public Switching Telephone Networks (PSTN) Introduction The public telephone network accommodates two types of subscribers: 1. Private-line circuits or dedicated circuits – the customers lease the equipments, transmission media/facilities and services from the telephone companies or service providers, on a permanent basis, eg large banks. 2. Public subscribers – the customers share the equipments and facilities that are available to all the public subscribers to the network, includes transmission facilities and telephone switches. Since subscribers to the public network are interconnected only temporarily through switches, the network is called Public Switched Telephone Network (PSTN). PSTN’s primary characteristics They include: i. Analog access, 300 to 3,400 kHz ii. Circuits-switched duplex connection iii. Switched bandwidth 64 kbps, or 0.3 – 3.4 kHz for analog exchanges iv. Immobility or very limited mobility v. Many functions in common with another bearer network. PSTN Teleservices The terminals that can be connected to PSTN are as shown in Figure 5.3. Telephone Cordless Telephone PSTN Modem Computer Figure 5.3 Terminals that can be connected to PSTN Fixed telephone: For fixed telephone, the generated information (voice) is analog. Fax service: For fax service, the generated information is digital, but has a built-in modem providing an analog signal. It is a distributive service involves unidirectional communication to one recipient or broadcast to many recipients. Data communication: For data communication, computer generates digital signal and connected to PSTN through a modem, i.e. data communication via modem. Video, multimedia and teleworking: The limited bandwidth of the PSTN is the bottleneck for the video and multimedia services into PSTN, eg video telephony service. The most tcommonly used teleworking is a telephone, a fax and a computer with a modem – all are connected to the PSTN. Cordless telephone: Cordless telephone has an analog to digital converter ADC in the terminal. Since the PSTN interface is analog, fax and data traffic must be converted to analog signals in the frequency band 0.3 to 3.4 kHz with the help of built-in modem or stand-alone modem. The modem adapts the signal to the PSTN by converting information from digital form to analog form and vice versa. Integrated Services Digital Network (ISDN) Introduction ISDN is to support a wide range of voice (telephone) and non-voice (digital data) applications in the same network using a limited number of standardized facilities includes both switched and non- switched connections. Services introduced into an ISDN should be compatible with 64-kbps switched digital connections, which is the basic building block of ISDN. ISDN provides service features, maintenance and network management function. Primary Rate Interface Basic Rate Interface (PRI) (BRI) Network 2B+D 30B+D Terminal N-ISDN PBX (NT) Figure 5.4 Basic ISDN concepts Services in ISDN Telephony Telefax (FAX) Videoconferencing and video-telephony High quality sound Data communication including internet access Combinations of teleservices Mobile and Cellular Communication Introduction to Cellular Telephone The key principle of Cellular Telephone or Cellular Radio is determined by sub-dividing a relatively large geographic market area (called coverage zone) into small sections (called cells). It uses the concept of frequency re-use to increase the capacity of a mobile telephone channel Cellular telephone system allows a large number of users to share the limited number of common usage of radio channels Fundamental Concept of Cellular Telephone Each coverage zone is further sub-divided into hexagonal- shaped cells that fit together to form a honey-comb pattern, as shown in Figure 5.5. Each geographical area is allocated a fixed number of cellular voice channels. cell 1 6 2 7 5 3 4 Figure 5.5 Hexagonal honey-comb patterns The concept of a cellular are: 1. A cellular system allows frequency reused in the same area by splitting the entire region into many smaller cells. 2. Each cell has a base station. 3. Because of low power of base station, the same frequency can be used for another cell, which is not adjacent (i.e. adjacent cells have different frequency groups). 4. Each mobile user is initially assigned a frequency and communicates with the closest base stations. 5. When the mobile user crosses to some other cell, it is given a new base station (RBS), i.e. the cellular phone changes from one channel to another as it crosses cell boundaries, even while the conversation is in progress. 6. The RBSs are linked together so that a conversation can pass from one cell to another. This will link the users in any two cells regardless of their channel assignments. Physical size of a cell varies, depends on user density and calling pattern: 1. Large Cell - Macrocells: Radius: Between 1.8 km - 28 km Base station transmit power:1 W - 6 W 2. Small Cells - Microcells: Radius:Less than 450 m Base station transmit power:0.1 W - 1 W Applications:High density areas eg large cities and inside buildings. Limitations :Low effective working areas, reflections and signal delays 3. Very small cells - Picocells: To provide reliable communication indoors, well-shielded areas or ares with high level of interference, eg underground malls. Advantage: Can use the same frequency as regular cells in the same area. SMART ANTENNA Figure 5.6 illustrates the base station location in a hexagonal-shaped cells. Centre-excited cell Edge-excited cell Corner-excited cell (omni-directional (sectored (sectored antenna) Directional Directional antenna) antenna) Figure 5.6: Locations of base stations Frequency Reuse Frequency reuse is the process in which the same set of frequencies or channels can be allocated to more than one cell, provided that the cells are separated by sufficient distance. Each cellular base station is allocated a group of channel frequencies that are different from those of neighboring cells, and base station antennas are chosen to achieve a desired coverage pattern within a cell. The main objectives of frequency re-use are: (i) To keep the transmitted power from each base station to a minimum. (ii) To keep the position of the antenna of the base station just high enough to provide for the area coverage of the respective cells. Concept of frequency reuse: A coverage area G Cluster 1 F B A E C G D F B G A F B E C A D E C D Cluster 3 Cluster 2 Figure 5.7 Cellular frequency reuse concept Cluster is a group of cells. From Figure 5.7 above: The coverage area consists of 3 clusters. Each cluster has 7 cells Each cell is assigned the same number of full-duplex telephone channels Cell with the same letter use the same set of channel frequencies Total number of channels in a cluster: F =G N (channels/cluster) where G = Number of channels in a cell N = Number of cells in a cluster (Typical values: N = 3, 7, 12) When a cluster is duplicated m times within a given service area, the total number of full-duplex channel in a service area becomes, C= mGN (channels/service area) = mF where C = Number of full-duplex channels in an area m = number of clusters in a service area (clusters/service area) G = Number of full-duplex channels in a cell N = Number of cells in a cluster i.e C m Cellular System Topology Radio Network is defined by a set of radio-frequency transceivers (base stations) located within each of the cells. Figure 5.8 illustrates a simplified cellular telephone system that includes the basic components: Mobile unit, Base station and Mobile Telephone Switching Office (MTSO). RBS RBS RBS RBS MTSO MTSO RBS PSTN PSTN Trunk Circuit Wired Telephone Wired Telephone Figure 5.8 Simplified cellular telephone system topology Mobile Unit : Mobile Unit communicates directly with the base stations (over dedicated data links - metallic & non-metallic) and the base station communicates directly with a MTSO (over free- space/airwaves). Base Station : 1. Serves as central control for all users within a cell. 2. Consists of a low-power radio transceiver, power amplifiers and cell-site controller 3. Provides an interface between mobile telephone sets and the MTSO. Mobile Telephone Switching Office (MTSO) 1. Controls channel assignments, call processing, call setup and call termination, includes signaling, switching, supervision, and allocating radio-frequency channels. 2. Provides a centralized administration and maintenance point for the entire network 3. Interfaces with the public telephone network over wire line voice trunks for the conventional wired telephones and interfaces with data links. 4. Most MTSO are connected to the SS7 signaling network, which allows cellular telephone to operate outside their service area. Satellite Communication System Introduction to Satellite Communication System A satellite system consists of: (i)a transponder (a radio repeater in the sky), (ii) a ground-based station to control its operation (iii) a user network of earth stations that provide the facilities for transmission and reception of communication traffics through the satellite system. Satellites received a signal from the ground station, do the amplification and frequency translation, and broadcast it to earth stations that are able to receive transmissions. A satellite begins at a single earth station, passes through the satellite, and ends at one or more earth stations. Satellite communication systems utilize microwave terminals both on satellites and in earth stations for high reliable and high-capacity system. Figure 5.9 below shows the basic elements of a communications satellite. Satellite in geosynchronous orbit Transponder Uplink Uplink Downlink Downlink Earth station Earth station Satellite transponder Satellite transponder acts like a repeater, consists of a receiver and a transmitter. The main functions of a satellite transponder are: (i) to pick up the transmitted signal from the transmitter on the earth, (ii) to amplify the signal, (iii ) to translate the carrier frequency to another frequency (iv) to retransmit the amplified signal to the receiver on the earth Figure 5.10 below shows a block diagram of a satellite transponder. Band pass Low-noise Mixer BPF Low-power filter (BPF) amplifier (LNA) amplifier Local oscillator Frequency converter From To earth earth station station The BPF limits the total noise. The LNA amplifies the received signal and fed it to the frequency converter which converts the high-band uplink frequency to the low- band downlink frequency. This is an RF-to-RF repeater. Each RF satellite channel requires a separate transponder. Satellite system links 1. Up link: It is the path of the satellite signal from the earth transmitter to the receiver of the satellite. The frequency signal being transmitted from the earth station to the satellite is called the uplink frequency. For example: the uplink frequency for a C-band is 6 GHz. 2. Down link: It is the path of the satellite signal from the satellite transmitter to the receiver on the earth. The retransmitted signal from the satellite to the receiving stations is called the down-link. For example: the downlink frequency for a C-band is 4 GHz. Note: The uplink and downlink use different carrier frequencies to avoid interference, and the frequency conversion is done in the transponder. Earth stations Figure 5.11 shows an earth station transmitter. From Figure 5.11, the intermediate frequency (IF) modulator converts the input baseband signals to an FM, a PSK, or a QAM modulated intermediate frequency. The up- converter (mixer and bandpass filter) converts the IF to an appropriate RF carrier frequency. The high power amplifier (HPA) provides adequate input sensitivity and output power to propagate the signal to the satellite transponder. To satellite Tel Baseband in Modulator High Power Data Mixer BPF High power FDM or (FM, PSK Amplifier Amplifier Video PCM/TDM or QAM) (HPA) r Generator Up-Converter Figure 5.11 An earth station transmitter Figure 5.12 shows an earth station receiver. From Figure 5.12, LNA which is a highly sensitive and low- noise device amplifies the received signal. The RF-to-IF down-converter is a mixer and bandpass filter combination, which converts the received RF signal to an intermediate frequency (IF). From satellite Low noise Demodulator Baseband out Tel Mixer BPF Amplifier (FM, PSK (FDM or Data (LNA) or QAM) PCM/TDM) video Generator Figure 5.12 An earth station receiver Down-converter Cross-Links Cross-link sat1 sat2 Uplink/Downlink Uplink/Downlink station1 Earth station2 Intersatellite link Figure 5.13 Intersatellite link Frequency allocations Table 5.1 shows the frequency bands for the satellite frequency range. The most common carrier frequencies used for satellite communications are the 6/4 GHz (C-band) and 14/12 GHz (Ku-band) bands, especially for voice and data telecommunications. At these bands, the signal attenuation is minimum. The first number is the uplink frequency, and the second number is the downlink frequency, that is 6 GHz is the uplink frequency and 4 GHz is the downlink frequency for a C-band. Table 5.1 Satellite frequency bands Satellite Orbit Three basic types of orbits are: 1. Polar Orbit: It is a north-south orbit, normally used for navigation, weather satellites, meteorological and land resource satellite systems. It is not used for telecommunication purposes. 2. Elliptically inclined orbit: It is used for Russian domestic systems, with inclination of 63 degrees and a 12-hour orbit period, but visible for 8 hours only. So, 3 satellites are needed for continuous coverage. 3. Circular Equatorial Orbit: It is called geosynchronous orbit. This satellite in a geosynchronous orbit is at a height of about 35,800 km, has 24-hour orbit period, and its angular speed is equal to the rotational speed of the earth. So, it appears stationary or motionless over a fixed point on the earth’s surface. The satellite is visible from 1/3 of the earth’s surface, so 3 satellites are needed for full coverage of the earth. The above basic orbits are as shown in Figure 5.14. Figure 5.14 Three satellite orbits Advantages of geosynchronous satellites: 1. The satellite remains almost stationary relative to the earth station, so, the computer-controlled tracking of the satellite is minimized. 2. A geosynchronous satellite is permanently in view, so, there are no breaks in transmission. There is no need to switch from one satellite to another. 3. Due to high altitude, it can cover a large area on earth (about 1/3 of the earth), and a large number of earth stations may intercommunicate. Three satellites can give global coverage except in the polar regions. 4. The effects of Doppler shift are negligible. Doppler shift is the change in the apparent frequency of the radiation to and from the satellite caused by the motion of the satellite to and from the earth station. Disadvantages of geosynchronous satellites: 1. Latitudes greater than 81.250 North and South are not covered. 2. Due to the high latitudes of geosynchronous satellite, the received signal power, which is inversely proportional to the square of the distance, is very weak, and the signal propagation delay is about 300 msec. System performance Figure 5.15 shows a simplified block diagram of a satellite system which is showing the various gains and losses that may affect the system performance. When evaluating the performance, the uplink and downlink parameters are considered separately, and then the overall performance is determined by combining them in the appropriate manner. satellite Pin(s) Pcap G/Te(s) Gtx(s) Lfb Prad(s) Grx(s) Lp(up) Lp(down) Pin Prad Pcap HPA Gtx Grx(e) Pt G/Te(e) Lfb Lfb Transmitter Receiver Figure 5.15 A simplified satellite links Satellite System Parameters 1. Effective Isotropic Radiated Power (EIRP): EIRP is defined as an equivalent transmit power. EIRP = Pin x Gtx Where, Pin = antenna input power Gtx = transmit antenna gain Or, EIRP (dBW) = 10 log10 Pin (dBW) + Gtx (dB) In respect to the transmitter output: Pin = Pt - Lbo - Lfb (dBW) (dB) (dB) (dB) Where, Pt = saturated output power Lbo = back-off loss Lfb = feeder & branching loss Therefore, EIRP = Pt - Lbo - Lfb + Gtx (dBW) (dBW) (dB) (dB) (dB) Example 1: A satellite earth station transmitter operates with an antenna input power of 10 kW, a back-off loss of 3 dB and a total feeder and branching loss of 4 dB. The transmit antenna gain is 40 dB. Determine the station EIRP. Solution: EIRP = Pt - Lbo - Lfb + Gtx (dBW) (dBW) (dB) (dB) (dB) = 10 log10 (10x103) - 3 - 4 +40 = 40 -3 - 4 + 40 = 73 dBW Example 2 A satellite transponder has a gain of 50 dB. Its receiving and transmitting antenna have equal gain of 20 dB. If the receiving antenna receives a signal power of 10 uW from the earth transmitter, determine the signal power at the output of the satellite transmitting antenna. Solution: Satellite Transponder Gain = Gs = 50 dB Receiving Transmitting antenna antenna Gr=20 dB Gt=20 dB Pin=10 uW Po= ? Uplink Downlink Received power, Pin = 10 uW = 10 log10 10 x 10-6 W 1 x 10-3 W = -20 dBm or Pin = 10 log10 10 x 10-6 W 1 W = -50 dBW Total gain GT = Gr + Gs + Gt = 20 + 50 + 20 = 90 dB But, Po = Pin + GT = (-50) dBW + 90 dB = 40 dBW = 104 W or 10 kW Applications of satellite communications Some of the applications of satellite communications are: (i) Digital audio broadcasting (ii) Television broadcasting (iii) Serving remote areas (iv) Weather satellites (v) Remote monitoring and control (vi) Vehicle tracking (vii) Mobile communications (viii)Maritime and air navigation (ix) Video teleconferencing (x) Defence communications. Comparisons between satellite system and terrestrial microwave system Advantages of a satellite system include: 1. It can access to wide geographical area. 2. Wide bandwidth 3. High reliability 4. Distance insensitive cost. 5. Independent of terrestrial infrastructure 6. Rapid installation, and low cost per added site 7. Uniform service characteristics 8. Single provider Disadvantages of a satellite system include: 1. High initial cost 2. It has propagation delay 3. Regulatory licensing requirement and limited orbital parking slots.
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