Wireless Communication (EC43107FP) Communication Systems Chapter 1 PDF

Wireless Communication (EC43107FP) Chapter Communication Systems 1 Learning Objectives 1. Describe the ‘Communication System Model’...

Wireless Communication (EC43107FP) Chapter Communication Systems 1 Learning Objectives 1. Describe the ‘Communication System Model’ Transmitter/Receiver/Medium/Noise 2. Explain the types of ‘Communication Links’ Simplex/Half-Duplex/Full-Duplex 3. Compare the features of ‘Analogue’ & ‘Digital’ transmission 4. Explain the various Data Transmission techniques Transmission Mode/Synchronization/Multiplexing/Multiple-Access Chapter (1) Communication System - Page 1  Wireless Communication (EC43107FP) 1.0 ELECTRONIC COMMUNICATION SYSTEMS 1.1 All electronic communication systems have a ‘Transmitter’, a ‘Communication Channel’ (Medium), and a ‘Receiver’. FIG 1 – Electronic Communication System 1.2 The electronic communication model can be drawn as show in FIG 2. Transmitter (Tx) Communication Channel Receiver (Rx) Noise Noise FIG 2 - Electronic Communication Model 1.3 A ‘Transmitter’ is a collection of electronic components and circuits designed to convert information signals (audio, video or data) to a signal (e.g. radio waves) suitable for transmission over a given Communication Channel (e.g. air). 1.4 A ‘Receiver’ is a collection of electronic components and circuits that accept transmitted signals (e.g. radio waves) from the Communication Channel (air) and convert it back to a form understandable by humans (e.g. Sound & Images). 1.5 A ‘Communication Channel’ is the medium by which an electronic signal is sent from one point to another. 1.6 Many different types of media are used in communication systems: Electrical Conductors Optical Media Free Space (Air) 1.7 In actual situations, ‘Noise’ is always unintentionally added in the ‘Communication Channel’ and in the ‘Receiver’ which degrades or interferes with the transmitted information. Chapter (1) Communication System - Page 2 Wireless Communication (EC43107FP) 2.0 TYPES OF COMMUNICATION LINKS 2.1 ‘Communications Links’ are classified according to whether they are ‘1-way’ (Simplex) or ‘2-way’ (Duplex). 2.2 Simplex Link 2.2.1 In ‘Simplex’ communications, information travels in one direction only. (Example: Transmission is from ‘A’ to ‘B’ but not from ‘B’ to ‘A’) One way only A Tx Rx B f1 FIG 3- Simplex Link 2.2.2 Common examples of ‘Simplex’ communications are Radio and TV Broadcasting. (TV station is transmitting to your TV receiver at home, but not the other way!) 2.2.3 In wireless transmission, only one transmission frequency is needed. 2.3 Duplex Link 2.3.1 In ‘Duplex’ communications, information travel in two directions. (Example: Transmission is from ‘A’ to ‘B’ and from ‘B’ to ‘A’) 2.3.2 To support two-way communications, both ends need to have a ‘Transmitter’ & ‘Receiver’. 2.4 Full-Duplex Link 2.4.1 A ‘Full-Duplex’ link features simultaneous transmission from ‘A’ to ‘B’ and ‘B’ to ‘A’. 2.4.2 An example of ‘Duplex’ communication is a Telephony System where one can talk and listen. 2.4.3 However, two different transmission frequencies are needed to support simultaneous transmission or else one transmission will jam the other. f1 Tx Rx A 2-Way Simultaneously B Rx Tx f2 FIG 4- Full Duplex Link Chapter (1) Communication System - Page 3 Wireless Communication (EC43107FP) 2.5 Half-Duplex Link 2.5.1 In a ‘Half-Duplex’ link, transmission is from ‘A’ to ‘B’ during one interval and from ‘B’ to ‘A’ during another interval. 2.5.2 An example is the Walkie-Talkie where you can talk and listen, but not together at the same time f1 Tx Rx A 2-Way, but 1-Way at a time B Rx Tx f1 FIG 5 - Half-Duplex 2.5.3 In wireless transmission, only one transmission frequency is needed. 3.0 ANALOGUE vs DIGITAL TRANSMISSION 3.1 ‘Analogue’ transmission conveys voice, data, image or video information using an information signal that is continuously varying. 3.2 ‘Digital’ transmission transfers data over a transmission medium discretely. Rather than being a continuously variable wave form, it is a series of discrete pulses, representing one bits and zero bits. FIG 6- Analogue & Digital Transmission 3.3 In real life, most sources of information are analogue in nature. Such analogue or continuously varying signal must be converted to digital form through an ‘Analog-to-Digital Converter’ (ADC) before ‘Digital’ transmission. At the receiver, the digital information is converted back into analogue form through a ‘Digital-to-Analogue Converter’ (DAC). Chapter (1) Communication System - Page 4 Wireless Communication (EC43107FP) 3.4 Nowadays, ‘Digital transmission’ is widely used because: Maintain Data Integrity (Less Sensitive to Noise & Impairments) Transmit Longer Distance (Repeater can function as Re-Generators) Easier to Multiplex (Integrate Voice/Video/Data) Easy to Apply Encryption (Digitally Processed) 3.5 However, the disadvantages of digital transmission are: Costly to convert from Analogue system. Higher Bandwidth & System requirements 4.0 DIGITAL TRANSMISSION MODES 4.1 There are two basic methods of transmitting data from one place to another: 4.2 Parallel Transmission 4.2.1 In ‘Parallel Transmission’, all bits in a single character are transmitted simultaneously. 4.2.2 It sends data bits in parallel (typically 8 bits at a time) using separate wires or circuits for each bit. 4.2.3 Using a ‘byte-wide’ transmission results in a faster transmission rate than ‘Serial Transmission’. FIG 7: Parallel Transmission 4.2.4 However, each wire has its own unique characteristics and so each channel is delaying each pulse by a different amount. The difference is more significant over long distance transmission. 4.2.5 Due to the above-mentioned ‘Skew’ problems, parallel transmission is limited to shorter distance. Chapter (1) Communication System - Page 5 Wireless Communication (EC43107FP) FIG 8: Skew Problems 4.3 Serial Transmission 4.3.1 In ‘Serial Transmission’, data bits are sent serially or sequentially bit-by-bit over a single- wire or communication circuit. 4.3.2 It is slower than ‘Parallel Transmission’. 4.3.3 It is used for longer distance transmission as there are no ‘skew’ problems. 4.3.4 It is widely used and is the predominant method of transferring information in data communications. FIG 9: Serial Transmission Chapter (1) Communication System - Page 6 Wireless Communication (EC43107FP) 5.0 DIGITAL TRANSMISSION SYNCHRONIZATION 5.1 There are two types of transmission technique used in data communication networks: 5.2 Asynchronous Transmission 5.2.1 Asynchronous transmission uses a simple interface. 5.2.2 It enables a series of bytes (or ASCII characters) to be sent along a single wire. 5.2.3 Each character is transmitted independently of all other characters. A ‘Start’ and ‘Stop’ signal separate characters from each other. 5.2.4 Data Rate is low at typically < 64 kbps. 5.2.5 It is called ‘Asynchronous Communication’ because Transmitter & Receiver use independent clocks. There is no need to transmit ‘CLOCK’ signal. 5.2.6 Receiver senses the ‘Start Bit’ (1-to-0 transition), and counts about 10 bits lengths. From there it can derive the value of each bit. 5.2.7 Due to the need to transmit ‘Start’ and ‘Stop’ Bits for byte timing, this increases Overhead and so results in Low Efficiency. 5.2.8 Most widely used by Personal Computers to provide connectivity to printers, modems, fax machines, etc. 5.2.9 Due to its low efficiency, it is ideal for short-bit sequence data. FIG 10 - Asynchronous Transmission of a Series of Characters 5.3 Synchronous Transmission 5.3.1 ‘Synchronous’ transmission uses a more complex interface. 5.3.2 Here, the ‘Start’ and ‘Stop’ bits are eliminated and the data is transmitted in a continuous block. 5.3.3 A ‘Clock’ is used to synchronize all parts of the system. It has the advantage that the timing information is accurately aligned to the received data, allowing operation at much higher data rates. Chapter (1) Communication System - Page 7  Wireless Communication (EC43107FP) FIG 11 – Additional ‘Clock’ Line 5.3.4 At the Receiver, the ‘Clock’ is retrieved from the incoming data stream and used to ensure accurate sampling of data stream. 5.3.5 Since Transmitter and Receiver clocks are synchronized it is able to support high data rates up to 10 Gbps. 5.3.6 The Transmitter will send characters in a group, called a ’Block’ or ‘Frame’, and ‘Special Synchronization’ characters are transmitted at the beginning of each block. 5.3.7 At the Receiver, the detection of these ‘Special Synchronization’ characters will signal the beginning of each block. 5.3.8 Although the ‘Special Synchronization’ characters add some overhead, the overhead incurred is still much lesser than the overhead incurred with Asynchronous transmission. 5.3.9 Due to its higher efficiency, it is used for longer bit sequence data. 5.3.10 Widely used for connections between computer and telephony networks. FIG 12 – ‘Data’ Transmission in Blocks 6.0 MULTIPLEXING & MULTIPLE ACCESS 6.1 Multiplexing 6.1.1 ‘Multiplexing’ refers to the simultaneous transmission of several separate information channels over the same transmission medium without interference. FIG 13 - Multiplexing 6.1.2 The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire. Chapter (1) Communication System - Page 8  Wireless Communication (EC43107FP) 6.1.3 A device that performs the multiplexing is called a ‘Multiplexer’ (MUX), and a device that performs the reverse process is called a ‘De-Multiplexer’ (DEMUX or DMX). 6.2 Multiple-Access 6.2.1 ‘Multiple-Access’ is the accessing of a common communication channel from different locations. 6.2.2 An analogy to the problem of ‘Multiple Access’ is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (Time Division), speak at different pitches (Frequency Division), or speak in different languages (Code Division). 6.2.3 Various types of ‘Multiple Access’ schemes are: Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Space Division Multiple Access (SDMA) SDMA FIG 14 – Multiple Access Schemes 6.2.4 In ‘FDMA’, the whole frequency band is divided into small frequency bands called channels. Since each channel can accommodate single user at time, then the whole frequency bandwidth can be accessed by many users. Hence, if a total bandwidth of 200 kHz is available, and the allocated channel bandwidth is 50 kHz, then the system accommodate 4 users at a time by giving 50 kHz channel to each user. 6.2.5 ‘TMDA’ allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission bandwidth. 6.2.6 ‘CDMA’ is another example of multiple access to permit sharing of communication channel without undue interference amongst users. CDMA employs spread-spectrum technology and a special coding scheme where each user (transmitter) uses a different code to modulate their signal. The spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. 6.2.7 ‘SDMA’ divides the geographical space where the users are located, into smaller spaces. The key element of the design is a one-to-one map between the space divisions and the bandwidth divisions of time slots, frequency divisions and etc. ‘SDMA’ is compatible with any multiple access scheme such as TDMA, FDMA and CDMA. ‘Space Division Multiple Access’ or ‘Spatial Division Multiple Access’ is a technique which is ‘MIMO’ (Multiple-Input Multiple-Output) architecture and used mostly in wireless and satellite communication. Chapter (1) Communication System - Page 9 Wireless Communication (EC43107FP) Tutorial #1 1 List the three basic components of a communication system. 2 List three different types of media are used in communication systems. f1 Tx Rx A B 3 The figure above shows a type of communication link (a) State the type of communication link (b) Explain the operation of this communication link 4 Explain the difference between a ‘Simplex’ and ‘Full-Duplex’ link. 5 Explain the difference between ‘Half-Duplex’ and ‘Full-Duplex’ communication links. 6 State 2 advantages of ‘Digital Transmission’. 7 State an advantage of ‘Serial’ digital transmission mode. 8 Describe ‘Asynchronous’ transmission in digital communications. 9 State the various types of ‘Multiple Access’ scheme. 10 What does ‘TDMA’ stand for? 11. State the name of the device that enables simultaneous transmission of several separate information channels over the same transmission medium without interference. 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