11
Radio System
Performance of a whole radio system, such as a cellular network, navigation system, or radar, depends on the characteristics of the transmitters, receivers, and antennas as well as of propagation of radio waves between the transmitting and receiving antennas. If the transmitted power and the gains and attenuations in different parts of the system are known, the received power can be calculated. However, in addition to the received power, there are other factors affecting the signal detection: modulation of the signal, frequency stability, interference from other radio systems, noise, dispersion due to the radio channel, and so on.
In this chapter we first briefly discuss transmitters and receivers. Then we study noise in more detail as it decreases the performance of any radio system. We also study different modulation techniques, that is, how information can be attached to the carrier. Finally we consider the link budget. In Chapter 12 some radio systems are studied in more detail.
11.1 Transmitters and Receivers
A radio transmitter must produce a signal that has enough power, has generally a very accurate frequency, and has a clean enough spectrum so that the transmitter does not disturb users of other radio systems. Information to be transmitted, the baseband signal, is attached to a sinusoidal carrier signal by modulating the carrier amplitude, frequency, or phase either analogically or digitally (see Section 11.3).
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Low-power transmitters are usually based on a semiconductor device, a transistor or diode oscillator. When a transmitter power of hundreds of watts is needed, power is generated with electron tubes or so-called microwave tubes. Tetrodes are used from LF to VHF, and klystrons at UHF and SHF. In radar transmitters, a magnetron oscillator is the most common. In klystrons, magnetrons, and other microwave tubes, microwave oscillation is generated by an electron beam interacting with a resonance cavity or a slowwave structure [1].
In order to have a sufficiently accurate and clean signal, the oscillator frequency must be stabilized and the signal must be bandpass filtered before transmitting. Oscillators may be stabilized using a resonator with a sufficiently high quality factor. Often the accurate frequency is based on a quartz crystal oscillator at a frequency of 1 to 40 MHz and with a frequency stability of 10−9 to 10−10 per day. The signal of the quartz oscillator is frequency multiplied to the transmission frequency and then amplified, or it is used to injection lock or phase lock another oscillator to the correct frequency.
Figure 11.1 presents a direct-conversion transmitter. A digital baseband signal modulates the carrier in an IQ-modulator (see Section 11.3.2). The modulated signal is then filtered and amplified. In a superheterodyne transmitter the signal is further upconverted to the final frequency. The carrier frequency is stabilized by phase locking [2]. A basic phase-locked loop (PLL) is a feedback system consisting of a VCO, a phase detector (for example, a double-balanced mixer), and a low-pass filter. In the loop of Figure 11.1 there is also in the feedback branch a digital circuit that divides the frequency of the VCO by N. The output of the divider is compared with the signal from the reference oscillator in the phase detector. A possible difference in frequency (in phase) is transformed into a voltage proportional to the phase difference, and after lowpass filtering this voltage is used to control the VCO frequency until the frequency difference is zero. The output frequency of the locked loop is Nf ref . The loop also stabilizes within its bandwidth the random phase variations of the VCO and, thus, the reference oscillator determines the phase-noise characteristics.
Figure 11.1 Direct-conversion transmitter with a phase-locked oscillator.
If the system must operate at several nearby frequencies (channels), the transmitter and receiver signals are generated in a frequency synthesizer [3]. The synthesis may be based on a PLL, on a direct synthesis, or on a direct digital synthesis (DDS). If a programmable divider is employed in Figure 11.1, this PLL can produce several frequencies spaced by f ref . Smaller frequency steps are obtained by combining several PLLs or by using a fractional-N loop, where the division ratio is a fraction and is not limited to integer values. The direct synthesis uses mixers, frequency multipliers, dividers, filters, and switches to produce accurate frequencies. This method may not be practical if a large number of closely spaced frequencies are needed, because the synthesizer would become very complex. In the DDS a digital-to-analog converter (DAC) generates the waveform using a look-up table. Accurately modulated signals with very good frequency resolution may be produced using DDS, but the upper frequency is limited by the available DACs to a few hundred megahertz.
The receiver must be sensitive and selective. It must be able to detect even a weak signal among many other, possibly stronger signals. Therefore, a good receiver must have good filters, an accurate local oscillator frequency, and low-noise components. It should have also a large, spurious-free dynamic range.
Receivers are usually superheterodyne receivers (here ‘‘super’’ comes originally from ‘‘supersonic,’’ meaning that there is an intermediate frequency higher than the audio frequency, that is, the baseband signal frequency in a voice radio). Figure 11.2 presents such a receiver, where the signal from the antenna is first filtered and then amplified by an LNA by 10 to 20 dB. In front of a mixer an image-rejection filter blocks the image band. A frequency synthesizer generates an accurate local oscillator (LO) signal. The mixer downconverts the signal to an IF, where it is again filtered with a narrowband filter (e.g., a SAW filter) and amplified by an IF amplifier (e.g. by 70 to 100 dB) before demodulation. The demodulator recovers the original analog or digital baseband signal. If the signal power level may vary
Figure 11.2 Block diagram of a typical superheterodyne receiver used in radio communication.
274 Radio Engineering for Wireless Communication and Sensor Applications
significantly, the IF gain should be controllable with an automatic gain control (AGC) circuit in order to avoid saturation in the back end.
There are many other receiver architectures. Often in a superheterodyne receiver the signal is downconverted twice. In such a dual-conversion receiver the first IF is high and the second IF is low. This facilitates the realization of filters. Distribution of the gain to several frequencies also reduces the tendency of the receiver to oscillate.
Figure 11.3 shows a direct-conversion receiver, which is called also a homodyne or zero-IF receiver. The signal is now downconverted directly to the baseband. There is no IF stage and, thus, this structure is simpler and better suited for integration than the superheterodyne architecture. Lower power consumption and the possibility of making the channel filter at the baseband are other advantages. However, as the LO frequency equals the carrier frequency, leaking of the LO power may cause severe problems. In a dual-conversion receiver the channel selection can be realized with a synthesizer operating at the IF, but in a direct-conversion receiver the synthesizer must operate at the higher carrier frequency.
In a direct IF sampling receiver, which is also called a software radio, the whole band at the IF is sampled with an analog-to-digital converter (ADC). Channel filtering and demodulation is then realized with digital signal processing. This architecture is flexible and suited multimode operation, because the system standard may easily be changed. However, the ADC is now a critical component; it has to operate very linearly over a large dynamic range.
A transceiver is a combination of a transmitter and receiver sharing the same antenna. Now the isolation of the transmitter and receiver has to be very large, for example, 120 dB. If the transmitter and receiver operate
Figure 11.3 Direct-conversion receiver.
