Wireless Communications (4)

Wireless Communications (4)

Wireless Communications

Block diagrams of a wireless communication system

Block diagram of a receiver:

System block diagram of a direct conversion receiver and transmitter. An antenna is followed by a band pass filter, used as a band select filter. This eliminates out-of-band noise and presents the signal to the low-noise amplifier (LNA). The LNA then amplifies the desired signal, adding a minimum amount of inherent noise. The signal processed by the LNA is then down-converted to the desired IF frequency by a set of mixers operating in quadrature. These mixers are often image-reject mixers and have some gain as well. After down-conversion, the low-frequency IF signal is low pass -filtered to remove aliasing components and converted to digital samples by the analog-to-digital converter. In the digital domain, more filtering is applied for channel selection, and plenty of signal processing is performed to remove any channel effect before the detection stage.

Block diagram of transmitter:

On the transmitter side, the digital I and Q data – which are already processed by a digital-to-analog converter, filtered and amplified – are up-converted by quadrature mixers to the carrier frequency of interest. After combining, the signal is again filtered to contain the spectral content of the signal in the required bandwidth stipulated by the emission mask. After that, it is applied to the power amplifier and transmitted over the air with an antenna.

Details for each of the blocks in the transmitter and receiver:

ü Antennas

Antennas are coupling circuits to space that radiate or receive information-bearing electromagnetic waves.

 In a receiving antenna, the EM wave impinging on the surface produces currents, which in a 50-ohm system are applied to an LNA for amplification and subsequent processing.

On the transmitter side, the surface current density on the antenna produces a magnetic field around the antenna.

ü Filters

Filters remove the effect of broadband noise and thereby increase the SNR of a desired signal. They are also used to select channels in multiple transmission environments and to remove image frequencies in broadband services and other out-of-band interference.

 In the transmitter, digital pulse-shaping filters are used for efficient utilization of the RF spectrum and externally to suppress RF splatter in adjacent channels.

ü Amplifiers

The RF signal at a receiver’s antenna is very small in magnitude. The IEEE 802.15.4 standard defines a minimum signal of -85 dBm = 3.16 pW, whose voltage in a 50-ohm system is 12.6 µV. At the detector, the typical signal requirement is at 1 mVp-p for detection and decoding of digital waveforms.

On the transmitter side, power amplifiers (PAs) are used to transmit the EM wave. PAs come in various classes and can be linear and nonlinear. They usually employ matching circuits between the output and the load.

ü Mixers

Mixers are fundamental building blocks that translate frequencies from one band to another for further processing without changing the information content.

On the transmitter side, they up-convert a baseband signal for efficient transmission over a channel.

At the receiver, they down-convert to a suitable intermediate frequency for the extraction of information.

ü Oscillator

Oscillators produce sinusoidal signals that up-convert or down-convert an RF signal to the required frequency, where subsequent processing might begin. They are designed to operate at a specified frequency. Generally, there is an amplifier and feedback circuit that returns a portion of the amplified signal back to the input. When feedback is aligned in phase, sustained oscillators occur. In practice, they are not perfect, and drift in frequency from time to time. They are also susceptible to phase noise. Due to this, many transceivers operate them in a phase-locked loop (PLL) that can provide frequency stability and lower phase noise.

ü Analog-to-digital converter

Analog-to-digital converters are required to convert analog signals to digital signals for baseband processing. After digitizing, signal channel selection can occur in the digital domain, as can equalization.

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