Outlines. LECTURE 3: Wireless Transmission Technologies. Wireless Transmission on Unguided Media

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1 LECTURE 3: Wireless Transmission Technologies CIS 472 Wireless Communications and Networks Winter 2016 Instructor: Dr. Song Xing Outlines Wireless Data Transmission Modulation Spread Spectrum Department of Information Systems California State University, Los Angeles 2 Transmitting Information using RF Waves To transmit any type of information using RF (radio frequency) waves such as radio wave and microwave, the electrical current and its electromagnetic field must be converted (modulated) for transmission through the air. An un-modulated wave is called a carrier signal. Carrier signal Sent by radio transmitters. Continuous analog wave of constant amplitude and frequency. Modulation Carrier signals are modulated to represent analog or digital data. 3 Wireless Transmission on Unguided Media Transmission and reception are achieved by means of an antenna. For transmission, the antenna radiates electromagnetic energy into the air. For reception, the antenna picks up electromagnetic waves from the surrounding air. Antenna configurations for wireless transmission Directional: transmitted signals at higher frequencies Omnidirectional: transmitted signals at lower frequencies 4 1

2 Wireless Transmission And Reception using Antenna Antennas Directional antenna Issues wireless signals along single direction E.g., dish antennas for satellite downlink. Omni-directional antenna Issues, receives wireless signals Equal strength, clarity All directions 5 6 Omni-directional and Dish Antennas Omni-directional and Dish Antennas (Cont.) Omni-directional antennas are best for short distance communications between two devices: such as those found in a wireless LAN (WLAN) or a cellular wireless network. Dish antennas are good for longer distance communications between two devices. One common application of a dish antenna is to receive satellite signals

3 Smart Antennas Used primarily in mobile or cellular telephony Use the strength of the signal coming from a mobile device to learn where the mobile receiver is located, track it, and focus the RF energy in the device s direction to avoid wasting energy and to prevent interference with other antennas. Instead of sending signals with wide beams, smart antennas send narrow beams of energy toward the receiver. Classes of Smart Antennas A switched beam antenna Uses several narrow beam antennas pointing in different directions and turns each one on or off as the receiver moves across the path of the beams. Adaptive or phased array antennas It is divided into a matrix of radiating elements. A computer-based signal processor controls circuits in the antenna system, turning elements of the matrix on or off as well as adjusting the phase of transmission signal supplied to each one as the mobile user moves across the front of the antenna. Has the effect of sending the energy beam in a particular direction (generally called beam forming ) The signal processor is used to detect which elements receive a stronger signal; this determines the position of the mobile user. Directional Antenna (left) vs. Smart Antenna (right) Digital Transmission Speed: Bit Rate vs. Baud Rate Bit rate (or called data rate) Number of bits to be transmitted per second, or bits per second (bps) Baud rate Number of carrier signal elements per second that are required to represent the bits transmitted Or, the number of distinct symbol changes (signaling events) made to the transmission medium per second in a digitally modulated signal. Hence, commonly used to refer to the symbol rate. Baud is a change in the carrier signal. 12 3

4 Bit Representation of Signal Changes It is possible to have a change in signal (a baud) represent more than 1 bit. For example, one signal carries 2 bits (It is called dibit.) which is shown in the below table. Four distinct signals are needed since 2 # of bits = 2 2 = 4 Bit Rate vs. Baud Rate Generally, baud rate (or called symbol rate) is distinct from the bit rate. One symbol (signal) may carry more than one bit of information. For example, in modems, where bandwidth efficiency is important, it is commonly arranged for one symbol to carry 3 or more bits. So a 3000 bit per second modem, which is transmitting symbols that each carry 3 bits, should be described as operating at 1000 baud. The digital modulation QPSK, QAM are such the examples. We will discuss them shortly. 14 Bit Rate vs. Baud Rate (cont.) Conversely, Direct Sequence Spread Spectrum (DSSS) operation requires many symbols (signals) to carry only one bit of the original data. Note that one bit of the original data is encoded by multiple bits of the barker code (chips) to transmit with DSSS. Still, one signal (symbol) will carry on one or more bits of transmitted data using digital modulation techniques. We will discuss DSSS shortly. 15 Example 1 An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate. Solution In this case, the baud rate is 1000 baud, and one signal carries 4 bits. Hence, the bit rate will be 1000x4 or 4000 bps. 16 4

5 Example 2 A transmitted radio signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many bits are carried by each signal element? How many signal elements do we need? Solution In this example, the number of bits carried by each signal is 8000/1000 or 8 bits per signal. The number of distinct signals needed is 2 #ofbits =2 8 = 256. Outlines Wireless Data Transmission Modulation Analog Modulation Digital Modulation Spread Spectrum Modulation In order for an electromagnetic wave to transmit information it must be modified Modification is called modulation or keying An electromagnetic wave that has been modified in order to carry information is a carrier Also called carrier wave or carrier signal Modulation is the process of varying one or more properties of a periodic carrier signal, with a modulating data signal that typically contains information to be transmitted. Modulations can be performed on either analog or digital transmissions. Analog Carrier for Wireless Transmission Analog signals are continuously varying electromagnetic waves that may be propagated over a variety of media, depending on frequency Examples of media for analog signal transmission: Copper wire media (twisted pair and coaxial cable) Fiber optic cable Atmosphere or space propagation Wireless communications use periodic analog carrier signals to propagate analog or digital data. 20 5

6 Analog Modulation: Transmit Analog Data using Analog Carrier Signal For unguided transmission, it is virtually impossible to directly transmit original analog baseband signals of the data such as voice, audio or video; The required antennas would be many kilometers in diameter for wireless communication. A baseband signal is a signal that can include frequencies that are very near zero, by comparison with its highest frequency (e.g., a sound waveform can be considered as a baseband signal, whereas a radio signal or any other modulated signal is not). Typically, an original analog data signal must be modulated onto higher-frequency carrier for transmission using analog modulation. Analog modulation: AM, FM, PM Analog modulation permits frequency division multiplexing (FDM). 21 Digital Modulation: Transmit Digital Data using Analog Carrier Signals It is common to digitize voice signals prior to transmission over either guided or unguided media to improve quality and to take advantage of TDM (time division multiplexing) schemes. For wireless transmission, the resulting digital data must be encoded (modulated) onto to an analog carrier using digital modulation techniques. Digital modulation: ASK, FSK, PSK 22 Outlines Wireless Data Transmission Modulation Analog Modulation Digital Modulation Spread Spectrum Multiple Access Analog Modulation Representation of analog data by an periodic analog carrier signal Analog modulation types Amplitude modulation (AM) Frequency modulation (FM) Phase modulation (PM)

7 Amplitude Modulation (AM) Frequency Modulation (FM) Amplitude modulation (AM) varies with the amplitude or height of the wave. Height of the carrier wave is changed in accordance with the height of the modulating signal. 25 Frequency modulation (FM) modulates the vibration rate or frequency of a signal. In FM the number of carrier waves that occur in one second changes based on the amplitude of the modulating signal. 26 AM and FM Phase Modulation Amplitude modulation (AM) Used by broadcast radio stations 540 khz khz in Americas with 10 khz spacing Very susceptible to interference from outside sources Frequency modulation (FM) Broadcast radio and television are very common examples of FM. 88 MHz MHz in US with 200 khz spacing Not as susceptible to interference from outside sources FM carrier has a wider bandwidth Allows it to carry Hi-Fi as well as stereophonic signals 27 Phase modulation (PM) Changes the starting point of the cycle of the carrier signal. A signal composed of sine waves has a phase associated with it. A phase change is always measured with reference to some other signal. PM systems almost always use the previous wave cycle as the reference signal. However, PM is not generally used to represent analog data or signals in applications. 28 7

8 Outlines Wireless Data Transmission Modulation Analog Modulation Digital Modulation Spread Spectrum Digital Modulation Digital modulation is the process of changing one of the characteristics of a periodic analog carrier signal based on the information in digital data. Method of encoding a digital data onto an analog carrier wave for transmission over an unguided medium that does not support digital signals Digital Modulation Example Types of Digital Modulation There are three basic types of digital modulations: Amplitude (ASK), frequency (FSK), and phase (PSK) A modulator is a device that performs modulation. A demodulator is a device that performs demodulation, the inverse of modulation. A modem (from modulator demodulator) can perform both operations

9 Amplitude Shift Keying (ASK) ASK: Baud Rate = Bit Rate Binary modulation technique similar to amplitude modulation (AM) The amplitude (or height) of the carrier signal varies to transmit the ones and zeros. One binary digit 1 represented by presence of carrier, at constant amplitude Other binary digit 0 represented by absence of carrier Frequency Shift Keying (FSK, or BFSK) Binary modulation technique similar to frequency modulation (FM). Frequency of the carrier wave varies in accordance with the digital data to be sent. One frequency f1 encodes binary digit 0 while another frequency f2 encodes binary digit 1. Signal transmitted at constant amplitude. FSK: Baud Rate = Bit Rate 36 9

10 Phase Shift Keying (PSK, or BPSK) Common Used Phases for PSK Binary modulation technique similar to phase modulation (PM). Receivers can detect phase changes much more reliably than a frequency or amplitude change in the presence of noise. Transmitter varies the starting point (i.e., phase change) of the carrier wave to send one(1) or zero(0). Uses two signals to represent two binary digits of the sending data. One signal with phase 1 encodes a 0 while another signal with phase 2 encodes a 1. For example, a phase shift of 180. I.e., = 0 and =180, or - = PSK (BPSK) PSK: Baud Rate = Bit Rate In the example, a phase shift is 180. I.e., 2 - =

11 Digital Modulation Methods QPSK (Quadrature PSK) One signal can carry more bits. QPSK is one PSK-based technology and is also called Four-level PSK. Four distinct carrier signals with different phases (0, 90, 180, or 270 ) are used, each representing two bits (00, 01, 10, or 11) QAM (Quadrature Amplitude Modulation) PSK-based systems are more attractive for highspeed wireless communications. One signal represents one status. The greater the number of status, the higher the data rate that is possible with a given bandwidth. QAM is a combination of ASK and PSK. For example, 16-QAM has 16 distinct carrier signals with each sending 4 bits. 2 # of bits = 2 4 = 16 QAM is the most efficient one of ASK, FSK, and PSKbased digital modulation techniques and is commonly used for wireless communications QAM for Transmitting Quadbits 16 signals (status) with the combination of twolevel amplitude (Amplitude1, Amplitude2) and 8 different phases (0, 45, 90, 135, 180, 225, 270, 315 ) are used. 2x8 =16 Each represents four bits (0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011, 1100, 1101, 1110, or 1111)

12 Outlines Wireless Data Transmission Modulation Spread Spectrum FHSS DSSS Bandwidth Utilization: Multiplexing and Spreading Bandwidth utilization is the wise use of available bandwidth to achieve specific goals. Efficiency can be achieved by multiplexing. Privacy and anti-jamming can be achieved by spreading the spectrum Narrow-band Transmissions and Spread Spectrum Narrow-band transmissions Each signal transmits on one radio frequency Or a very narrow range of frequencies. Vulnerable to outside interference from another signal. AM and FM broadcasting radio signal transmissions are narrow-band. Spread spectrum transmission Transmitter takes a narrow band signal and spreads it over a broader portion of the radio frequency band. Receiver recollects the original narrow-band frequency back from the received spread signal. 47 Spread Spectrum vs. Narrow-Band Transmission 48 12

13 Purpose of Spread Spectrum In spread spectrum, we combine signals from different frequency sources to fit into a larger bandwidth. But our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. 49 Concept of Spread Spectrum On the transmitting end, The transmitted digital data is fed into a channel encoder to produce the narrow band signal. The produced narrow bandwidth signal is further modulated using sequence of digits known as spreading code or spreading sequence (Hopping code in FHSS, Barker code in DSSS). The spreading code is generated by pseudo-random number generator. Effect of this modulation is to increase the bandwidth (spread the spectrum) of original data signal to be transmitted. Then the resulting spread spectrum signal is sent out over the wireless channel. 50 Concept of Spread Spectrum (Cont.) On the receiving end, The same digit sequence (spreading code) is used to demodulate the spread spectrum signal. Finally, the signal is fed into a channel decoder to recover the original digital data sent from the transmitting end. General Model of Spread Spectrum Digital Communication System

14 Spread Spectrum Transmission Advantages Advantages Replaces narrowband transmitters. Several users with different spreading code can independently use the same higher bandwidth with very little interference and fewer errors. Outlines Wireless Data Transmission Modulation Spread Spectrum E.g., Bluetooth and coreless phone transmitting on the same 2.4 GHz band; CDMA FHSS Relatively secure Uses a spreading code to produce a noise-like signal DSSS that is hard to detect and intercept. Can be used for hiding and encrypting signals. Immunity from various kinds of noise and multipath distortion. Two common techniques: FHSS, DSSS Frequency Hopping Spread Spectrum (FHSS) Use a range of frequency channels (not a single one) for transmission. Use the hopping code to change frequency channels several times during transmission. Hopping code (spreading code or spreading sequence) the sequence of changing frequencies The receiving station must also know the hopping code. Advantage: Multiple radio devices can each use a different sequence of frequencies within the same area and never interfere with each other. For example, Bluetooth devices and cordless phone use the same 2.4 GHz frequency bands, but can be used in the same area. If interference is encountered on a frequency, only a small part of the message is lost. 55 A Channel Hopping Example Send a burst of data on the 2.44 GHz channel for 1 microsecond, then switch to 2.41GHz channel and transmit for the next microsecond, Channel switching: 2.44GHz -> 2.41GHz -> 2.42GHz -> 2.40GHz -> 2.43GHz 56 14

15 FHSS Performance Considerations Large number of frequencies are used. Results in a system that is quite resistant to jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency band Used in cordless phone, Bluetooth Outlines Wireless Data Transmission Modulation Spread Spectrum FHSS DSSS Direct Sequence Spread Spectrum (DSSS) Uses an expanded redundant code to transmit each data bit (called the first modulation) And then follows a digital modulation (second modulation) technique such as QPSK Hence, a DSSS signal is effectively modulated twice Barker code (or chipping code) A particular sequence of 1s and 0s Ideal for modulating radio waves As well as for being detected correctly by the receiver It is also called a pseudo-random code, spreading code Before transmission, add the original data bit to the chipping code First Modulation of DSSS Transmission In the example, each bit of transmitted data is added by the same 11-bit Barker Code (11 chips)

16 Chipping Code Added Using Boolean Operation of XOR Transmitter and Receiver Use the Same Barker Codes in DSSS EXCLUSIVE-OR (XOR) A and B are two inputs. C is the result. Result TRUE if either A or B is TRUE but not both C = A B A B C Transmitter and Receiver Use the Same Barker Codes in DSSS (cont.) Note that in the previous example, each bit of transmitted data is added by the same 11-bit Barker Code The receiver should know the same Barker Code(s) to recover the original data. DSSS Transmission Characteristics DSSS system transmits combinations of multiple chips. Multiple chips are transmitted at a higher rate than the original data rate; it is called the chip rate. The original data rate does not change. Characteristics Frequency of the digital component of the DSSS signal is much higher than that of the original data. Thus DSSS signal is a spread spectrum signal. A plot of the frequency spectrum of this DSSS signal would look similar to random noise. Hard to be detected by the unintended receiver. All of the information contained in the original signal (a 0 or a 1 bit) is still there!

17 DSSS Advantages Advantages To an unintended narrow-band receiver, DSSS signal appears to be low-powered noise. The intended receiver can recover the original data bit by using statistical techniques and mathematical algorithms if there is the interference to cause bit loss or change during transmission, thus avoiding the need for retransmission. In Use: ZigBee and higher-end products (WLAN, DSSS-based systems such as CDMA cellular phone) Because they are more expensive to manufacture 65 than FHSS systems 17