ECEN 90 Communications 2 Lecture Notes PDF

ECEN 90 - Communications 2 Lemuel G. Tatad College of Engineering and Information Technology Name  Learning Objectives After the completion of the chapter, students will be able to: 1. Di...

ECEN 90 - Communications 2 Lemuel G. Tatad College of Engineering and Information Technology Name  Learning Objectives After the completion of the chapter, students will be able to: 1. Differentiate coherent and non-coherent detection schemes. 2. Comprehend the distinct features of QPSK, OQPSK and MSK College of Engineering and Information Technology  Passband Digital Transmission Optimum Receiver Principle The optimum receiver principle is fundamental in digital communications, particularly for passband transmission systems. It involves designing a receiver that maximizes the probability of correctly detecting the transmitted symbols. The receiver's goal is to minimize the probability of error, which can be achieved through various techniques, including matched filtering and decision-making algorithms that consider the noise characteristics of the channel. College of Engineering and Information Technology  Passband Digital Transmission Passband Transmission In passband transmission, the amplitude, phase or frequency of the carrier signal is regulated to transmit the bits. The incoming data stream is modulated onto a carrier and then transmitted over a band-pass channel. College of Engineering and Information Technology  Passband Digital Transmission System Model Block Diagram of a Passband digital communication system College of Engineering and Information Technology  Passband Digital Transmission Binary Digital Modulation Scheme Binary digital modulation schemes are techniques used to transmit binary data (0s and 1s) by modifying a carrier signal. The primary types include: 1. Binary Amplitude Shift Keying (BASK): Modulates the amplitude of the carrier signal based on the input bits. The simplest version is on–off keying (OOK). In OOK, either bursts of a carrier wave are transmitted or nothing is transmitted depending whether the input message signal is 1 or 0. College of Engineering and Information Technology  Passband Digital Transmission College Figure 1of Engineering ASK: (a) data; (b) and unmodulated carrier; (c) on–off keying (OOK); (d) binary amplitude-shift keying (BASK) Information Technology  Passband Digital Transmission In OOK and BASK, the modulated carrier can take one of two different states: one state representing a 0, the other a 1. These different carrier states Figure 2 shows ASK with four possible amplitude levels, or are what are known as four symbols. Two-bit binary numbers: 11, 10, 01 and 00. symbols. If there are more than two possible carrier states – that is, more than two symbols available – then it is possible for each symbol to College of Engineering and represent more than one bit. Information Technology  Passband Digital Transmission If there were eight symbols, each could represent three data bits. The relationship between the number of available symbols, M, and the number of bits that can be represented by a symbol, n, is: M = 2n The term baud refers to the number of symbols per second, where one baud is one symbol per second. College of Engineering and Information Technology  Passband Digital Transmission Example: a. If a communications system uses 16 symbols, how many bits does each symbol represent? b. If the same system has a symbol rate of 10 000 baud, what is the data rate? College of Engineering and Information Technology  Passband Digital Transmission Example: a. If a communications system uses 16 symbols, how many bits does each symbol represent? Answer: If there are 16 symbols, then each of these can represent 4 bits, because 16 = 2^4. b. If the same system has a symbol rate of 10 000 baud, what is the data rate? Answer: There are 10 000 symbols per second, and each symbol represents 4 bits, so the number of bits per second is 4 × 10 000 = 40 000. So the data rate (or bit rate) is 40 000 bit s−1, also written 40 kbit/s. College of Engineering and Information Technology  Passband Digital Transmission Binary Frequency Shift Keying (BFSK): Changes the frequency of the carrier signal to represent different bits. An illustration of binary FSK, or BFSK, is given in below. Here, bursts of a carrier wave at one frequency or bursts of a carrier wave at a second frequency are transmitted according to whether the input data is 1 or 0. College of Engineering and Information Technology Figure 3: Binary FSK  Passband Digital Transmission Binary Phase Shift Keying (BPSK): Alters the phase of the carrier signal to convey the binary information. In BPSK, 0 and 1 are represented by segments of sinusoids that differ in their phase. At the receiver, distinguishing between the two segments is easier if their phases differ by as much as possible. In BPSK the phases are separated by half a cycle (equivalent to π radians or 180°). See Figure below. College of Engineering and Information Technology Figure 4: Binary PSK  Passband Digital Transmission Example The figure on the right shows three examples of digitally modulated waveforms. For each example, decide which modulation scheme has been used and, based on the figures you saw earlier, work out what binary data each of these represents. College of Engineering and Information Technology  Passband Digital Transmission Answers: Waveform (a) is an example of a BPSK-modulated waveform representing the data: 0 0 0 1 0 1 1 1. Waveform (b) is an example of a BASK-modulated waveform representing the data: 0 0 0 0 1 1 0 1. Waveform (c) is an example of a BFSK-modulated waveform representing the data: 1 1 1 0 0 1 0 1. College of Engineering and Information Technology  Passband Digital Transmission It is possible to combine ASK, FSK and PSK. One benefit of combining different modulation methods is to increase the number of symbols available. Increasing the number of available symbols is a standard way to increase the bit rate, because increasing the number of symbols increases the number of bits per symbol. It is rare for all three methods to be combined, but very common for ASK and PSK to be combined to create Quadrature amplitude modulation (QAM). College of Engineering and Information Technology  Passband Digital Transmission QAM is based on the application of ASK and PSK to two sinusoidal waves of the same frequency but with a phase difference of 90°. Sinusoidal waves 90° apart are said to be in a quadrature phase relationship. It is customary to refer to one of these waves as the I wave, or in-phase Figure 5: (a) I (in-phase or sine) wave or component, and the other as wave and (b) Q (quadrature or cosine) wave the Q wave, or quadrature wave or component College of Engineering and Information Technology  Passband Digital Transmission The set of symbols in QAM can be conveniently represented on a signal constellation diagram (Figure 6). This is a plot of the I and Q amplitudes with I on the horizontal axis and Q on the vertical axis. Each dot in Figure 5 is a symbol, as it represents a unique combination of amplitude and phase of the I and Q waves. Figure 6: Constellation diagram for 16-QAM College of Engineering and Information Technology  Passband Digital Transmission Example How many bits are represented by each symbol in 64-QAM? Sketch a constellation diagram for 64-QAM. College of Engineering and Information Technology  Passband Digital Transmission How many bits are represented by each symbol in 64-QAM? Sketch a constellation diagram for 64- QAM. Answer: For 64-QAM, the number of symbols M = 64. There are six bits per symbol, as M = 2^n and 64 = 2^6. ACollege constellation diagram for 64- of Engineering and QAM might Information look like this: Technology  Passband Digital Transmission Coherent Detection An estimate of the channel phase and attenuation is recovered. It is then possible to reproduce the transmitted signal and demodulate. Requires a replica carrier wave of the same frequency and phase at the receiver. The received signal and replica carrier are cross-correlated using information contained in their amplitudes and phases. Also known as synchronous detection. Applicable to PSK, FSK and ASK College of Engineering and Information Technology  Passband Digital Transmission Non-Coherent Detection Requires no reference wave; does not exploit phase reference information(Envelope detection). Applicable to DPSK,FSK,ASK. Non-coherent detection less complex than coherent detection but has worse performance. College of Engineering and Information Technology  Passband Digital Transmission Coherent Quaternary Signaling Scheme Quaternary signaling schemes use four distinct states to represent data, allowing for more efficient use of bandwidth. Quaternary signaling schemes enhance spectral efficiency by allowing more data to be transmitted in the same bandwidth. For example, QPSK encodes two bits into one symbol, effectively doubling the data rate compared to binary schemes. College of Engineering and Information Technology  Passband Digital Transmission Quadrature Phase Shift Keying (QPSK) is a form of Phase Shift Keying in which two bits are modulated at once, selecting one of four possible carrier phase shifts (0, 90, 180, or 270 degrees). Encodes two bits per symbol by using four phases. QPSK allows the signal to carry twice as much information as ordinary PSK using the same bandwidth. QPSK is used for satellite transmission of MPEG2 video, cable modems, videoconferencing, cellular phone systems, and other forms of digital communication over an RF carrier. College of Engineering and Information Technology  Passband Digital Transmission QPSK The QPSK Modulator uses a bit-splitter, two multipliers with local oscillator, a 2-bit serial to parallel converter, and a summer circuit. Following is the block diagram for the same. College of Engineering and Information Technology  Passband Digital Transmission QPSK The QPSK waveform for two-bits input is as follows, which shows the modulated result for different instances of binary inputs. College of Engineering and Information Technology  Passband Digital Transmission QPSK The QPSK Demodulator uses two product demodulator circuits with local oscillator, two band pass filters, two integrator circuits, and a 2- bit parallel to serial converter. Following is the diagram for the same. College of Engineering and Information Technology  Passband Digital Transmission Offset QPSK (OQPSK): A variant of QPSK that reduces the risk of signal distortion. is a variant of the QPSK modulation scheme where the phase or timing of either the in-phase or Quadrature component is shifted relative to each other by a one bit-period or half a symbol-period Ts as compared to QPSK. As a result, this limits the phase of the total OQPSK signal to no more than 90˚ at a time. College of Engineering and Information Technology  Passband Digital Transmission Offset QPSK (OQPSK): Figure 7: QPSK Signal College of Engineering and Information Technology  Passband Digital Transmission Minimum-Shift Keying (MSK): A continuous phase modulation technique that minimizes bandwidth. is a form frequency modulation based on a system called continuous-phase frequency-shift keying. Minimum shift keying, MSK offers advantages in terms of spectral efficiency when compared to other similar modes, and it also enables power amplifiers to operate in saturation enabling them to provide high levels of efficiency. College of Engineering and Information Technology  Passband Digital Transmission Minimum-Shift Keying (MSK): Figure 9: Concept of a minimum shift keying, MSK signal College of Engineering and Information Technology  Passband Digital Transmission M-ary Coherent Modulation Technique M-ary modulation techniques generalize binary and quaternary schemes to M symbols, allowing for even greater data rates. In general, Multi-level (M-ary) modulation techniques are used in digital communications as the digital inputs with more than two modulation levels are allowed on the transmitter’s input. Hence, these techniques are bandwidth efficient. College of Engineering and Information Technology  Passband Digital Transmission M-ary Coherent Modulation Technique 1. M-ary ASK This is called M-ary Amplitude Shift Keying (M-ASK) or M-ary Pulse Amplitude Modulation (PAM). The amplitude of the carrier signal, takes on M different levels. This method is also used in PAM. Its implementation is simple. M-ary ASK is susceptible to noise and distortion. College of Engineering and Information Technology  Passband Digital Transmission M-ary Coherent Modulation Technique 2. M-ary FSK This is called as M-ary Frequency Shift Keying (M-ary FSK). The frequency of the carrier signal, takes on M different levels. Not susceptible to noise as much as ASK. The transmitted M number of signals are equal in energy and duration. The signals are separated by 1/2Ts Hz making the signals orthogonal to each other. Since M signals are orthogonal, there is no crowding in the signal space. The bandwidth efficiency of M-ary FSK decreases and the power efficiency increases with the increase in M. College of Engineering and Information Technology  Passband Digital Transmission M-ary Coherent Modulation Technique 3. M-ary PSK This is called as M-ary Phase Shift Keying (M-ary PSK). The phase of the carrier signal, takes on M different levels. The envelope is constant with more phase possibilities. This method was used during the early days of space communication. Better performance than ASK and FSK. Minimal phase estimation error at the receiver. The bandwidth efficiency of M-ary PSK decreases and the power efficiency increases with the increase in M. College of Engineering and Information Technology  Passband Digital Transmission Orthogonal Frequency-Division Multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies, allowing for efficient use of bandwidth. is a digital communication technique initially developed for use in cable television systems. OFDM is similar to the broadcasting technique known as frequency division multiplexing (also known as FDM), which uses a multitude of transmitters and receivers to send information on different frequencies over a single wire, such as an electrical power cable. The first use of OFDM was by Bell Labs in 1984, and it has since become widely used in wireless applications such as mobile telephony and broadband communications. College of Engineering and Information Technology  Passband Digital Transmission Orthogonal Frequency-Division Multiplexing (OFDM) Figure 10: OFDM College of Engineering and Information Technology  Passband Digital Transmission Orthogonal Frequency-Division Multiplexing (OFDM) Uses of OFDM: OFDM is used in Digital radio, Digital Radio Mondiale, digital audio broadcasting, and satellite radio. OFDM is used in Wired data transmission, Asymmetric Digital Subscriber Line (ADSL), Institute of Electrical and Electronics Engineers (IEEE) 1901 powerline networking, and cable internet providers. College of Engineering and Information Technology  References: GeeksforGeeks. (2024b, April 29). Digital Modulation Techniques. GeeksforGeeks. https://www.geeksforgeeks.org/digital-modulation- techniques/ Exploring communications technology. (n.d.). Open Learning. https://www.open.edu/openlearn/digital-computing/exploring- communications-technology/content-section-1.7 Quadrature phase shift keying. (n.d.). https://www.tutorialspoint.com/digital_communication/digital_communic ation_quadrature_phase_shift_keying.htm What is OQPSK Modulation? - everything RF. (n.d.). https://www.everythingrf.com/community/what-is-oqpsk-modulation College of Engineering and Information Technology  References: Notes, E. (n.d.). What is MSK: Minimum Shift Keying Modulation » Electronics Notes. https://www.electronics- notes.com/articles/radio/modulation/what-is-msk-minimum-shift- keying.php Digital Communication - Information Theory. (n.d.). https://www.tutorialspoint.com/digital_communication/digital_communic ation_information_theory.htm GeeksforGeeks. (2022, June 30). Orthogonal FrequencyDivision Multiplexing (OFDM). GeeksforGeeks. https://www.geeksforgeeks.org/orthogonal-frequency-division- multiplexing-ofdm/ College of Engineering and Information Technology  End of Presentation College of Engineering and Information Technology