A Wireless Communication System Antenna
The Transmitter Let’s start from voice. How can we send one’s voice thousands of miles away? We first need a transmitter. • Voice (the “goods”) is transformed into an electrical signal (the “package”). • This signal is carried by a high frequency electrical current (the “rocket”). • The antenna (the “launcher”) sends the high frequency current out in the form of radio wave.
How Does Transmitter work? • The microphone transforms the voice into an electrical signal. • The modulator “loads” the voice signal onto a high frequency electrical current. • The amplifier magnifies the high frequency current and sends it to the antenna. • The antenna, driven by the current, emits radio wave to the space.
Diagram of A Transmitter Amplifier Modulator Amplifier Carrier Freq Generator
The Receiver • At the receiver end, the antenna receives the “rocket”—the high frequency radio wave. • The receiver separates (“unloads”) the electrical voice signal from the “rocket.” • The receiver transforms (“unpacks”) the electrical signal into voice.
How does the receiver work? • The antenna receives the radio wave sent by the transmitter. • The demodulator “unload” the voice signal. • The speaker turns the voice signal back to voice.
Diagram of A Receiver Amplifier Demodulator Amplifier
Antenna • Antenna size is closely related to the wavelength λ, which is equal to the speed of light (a constant value) divided by the radio frequency being used: • λ=speed of light (3x108 m/s)/frequency • 300 kHz (AM radio), λ= 3x108 / 300,000 = 1,000 m • 3 GHz (3x109/s, Wireless LAN), λ=0.1m=10 cm • Quarter-wave antenna: ¼ λ • Half-wave dipole: ½ λ • Parabolic reflective antenna
Antenna Gains • Omnidirectional (isotropic) antennas and directional antennas • Antenna gain is defined as the power output in a particular direction compared to that produced in any direction by an isotropic antenna. For example, antenna gain of 3 dB in a particular direction means an improvement over an isotropic antenna by 3 dB, or a factor of 2. • The increased power radiated in a given direction is at the expense of other directions. Antenna gain does not mean obtaining more output power.
Parabolic Antenna Gain η: Antenna efficiency, 45%-75% for parabolic D: diameter λ: wave length
Example: antenna gain Assume η=50%, D=0.6m, frequency=12GHz. Therefore, λ=3x108/12x109=0.025m
The Modulator The modulator “loads” the voice signal onto the high frequency current. There are several ways to load the voice signal: • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM)
Why modulation? • While voice signal can be sent out through wire in a wired communication system, its frequency (300 Hz to 4kHz) is too low to be sent out by antennas in a wireless communication system. • Only certain frequencies assigned by FCC, say, a frequency band around 1.8 GHz, can be used. Therefore you must use modulation to bring the frequency inside that frequency band. • Often some sort of frequency division has to be used to separate users (phones). Thus each phone may use a specific carrier frequency in its modulator.
Amplitude Modulation • Voice signal controls the amplitude of the high frequency current (called the carrier)—the amplitude of the carrier changes proportionally to the strength of the voice signal. • As a result, the voice signal becomes the “envelope” of the carrier.
Frequency Modulation • The voice signal controls the frequency of the carrier. That is, the frequency of the latter changes proportionally to the strength of the voice signal. • The amplitude is always constant.
Example of frequency modulation • Digital FM (FSK, frequency shift keying), see pp. 97. • The difference between the two frequencies used is an important parameter. If it is too small, it will be difficult to differentiate them. If it is too large, the bandwidth will be too wide. • The minimum is 1/2T where T is the duration of the transmitted data symbols. This is called the Minimum Shift Keying (MSK). • The most popular one is Gaussian MSK (GMSK). • The bandwidth efficiency
Phase Modulation • The voice signal controls the phase of the carrier. That is, its phase changes as the voice signal varies (often proportionally).
Example of phase modulation • Binary digital phase modulation (BPSK), see pp 100. • QPSK (pp. 101)
Analog and digital signals • Voice signal from a microphone is an analog signal, which changes continuously. • A digital signal only has two states, say, low voltage, and high voltage, representing zero and one. • An analog signal can be converted into a digital signal, or vice versa.
How to convert an analog signal to a digital one? • Measure the amplitude of the signal at regular intervals (it’s called sampling). • Convert the measurements into binary form. For example, 2->010, 3->011, 5->101, and so forth.
Two representations of a signal • A signal can be viewed in time domain or in frequency domain. The two views are the two representations of the same signal. • Time domain: a signal’s amplitude (strength) changes over time, therefore a time series graph can be used to characterize a signal • Frequency domain: a signal occupies a frequency band • The faster a signal changes (higher data rate) the wider its frequency band. The magnitudes of the two are similar. For example, if the data speed is several Mbps (Mega-Bits Per Second), then it will occupy a frequency band of several MHz (Mega-Hertz) wide.
View from the Frequency domain • As an example, human voice occupies a frequency band roughly from 300 hertz to 3400 hertz. (A music piece has much wider frequency band.) • The carrier frequency is always much higher, say, 100kHz. • After modulation, the carrier carrying the voice signal may (depending on the modulation method) occupy a band of 100,000 Hz to 103,400 Hz.
How can many people simultaneously use their phones? • A home telephone has a line. Different homes use different lines. They don’t interfere with each other. • Wireless phones share the same medium—the air. A phone can receive all the signals send to other phones which are located close enough. Therefore there has to be a way to separate the signals.
Ways to separate signals • Frequency division • Time division • Code division
Frequency Division • Each phone uses a specific frequency that is different from the frequencies used by other phones. • The transmitter of a base station sends signals to mobile phones using different frequencies. Each phone has a pre-assigned frequency. Only the signal sent at that particular frequency can be received by that phone. • A phone also is assigned a unique frequency for sending signal back to the base station.
Frequency Division (contd.) • Specifically, voice signal is modulated onto a specific carrier frequency for a mobile phone. Carrier frequencies of phones in a small area (a cell, as we will explain later) are different and sufficiently separated. • In fact, each phone needs not just a single frequency but a small frequency band. For voice signals, that band (called a channel) is about 4 kHz wide. Therefore a 48 kHz wide frequency band can accommodate 12 channels of voice. • Frequency Division Duplexing (FDD): forward (from a base station to phones) and backward (from phones to a base station) links in cell phone system use different frequencies.
Time Division • Different time slots are assigned to different MSs (mobile stations, which can be phones, PDAs, or computers). They may use the same carrier frequency but because they use different time slots, they don’t communicate at the same time. Therefore they don’t interfere with each other. • Typically 3 to 8 mobile stations will be given different time slots but the same carrier frequency. • An example: If three mobile stations share the same frequency, then each will be given a time slot and they take turns to transmit or receive signals. The sequence will be 1, 2, 3, 1, 2, 3, 1, …. • Time division and frequency division can be used simultaneously. • Time Division Duplexing (TDD): downlink and uplink use different time slots so they don’t interfere with each other
Code Division • Each mobile station is given a unique code. • Signals sent by a transmitter are coded. • An MS can only receive the signal that is coded with its unique code. • An MS sends back signal using its assigned code.
Difference between voice and data • Voice (conversation) has to be continuous. You don’t want to get cut off in the middle of conversation. • Data transmission often doesn’t have to be continuous. You can send data several times.