ANEKA INFO TEKNIK: DIGITAL MODULATION ; HOW DIGITAL TRANSMITTER AND RECEIVER WORK

7. How digital transmitters and receivers work

7.1 A digital communications transmitter
Here is a simplified block diagram of a digital communications transmitter. It begins and ends with an analog signal. The first step is to convert a continuous analog signal to a discrete digital bit stream. This is called
digitization.

Figure 33. A Digital Transmitter

The next step is to add voice coding for data compression. Then some channel coding is added. Channel coding encodes the data in such a way as to minimize the effects of noise and interference in the communications channel. Channel coding adds extra bits to the input data stream and removes redundant ones. Those extra bits are used for error correction or sometimes to send training sequences for identification or equalization.
This can make synchronization (or finding the symbol clock) easier for the receiver. The symbol clock represents the frequency and exact timing of the transmission of the individual symbols. At the symbol clock transitions, the transmitted carrier is at the correct I/Q (or magnitude/phase) value to represent a specific symbol (a specific point in the constellation). Then the values (I/Q or magnitude/ phase) of the transmitted carrier are changed to represent another symbol. The interval between these two times is the symbol clock period. The reciprocal of this is the symbol clock frequency.
The symbol clock phase is correct when the symbol clock is aligned with the optimum instant(s) to detect the symbols.
The next step in the transmitter is filtering. Filtering is essential for good bandwidth efficiency. Without filtering, signals would have very fast transitions between states and therefore very wide frequency spectra —
much wider than is needed for the purpose of sending information. A single filter is shown for simplicity, but in reality there are two filters; one each for the I and Q channels. This creates a compact and spectrally efficient signal that can be placed on a carrier.
The output from the channel coder is then fed into the modulator. Since there are independent I and Q components in the radio, half of the information can be sent on I and the other half on Q. This is one reason
digital radios work well with this type of digital signal. The I and Q components are separate.
The rest of the transmitter looks similar to a typical RF transmitter or microwave transmitter/receiver pair. The signal is converted up to a higher intermediate frequency (IF), and then further upconverted to a higher
radio frequency (RF). Any undesirable signals that were produced by the upconversion are then filtered out.

7.2 A digital communications receiver
The receiver is similar to the transmitter but in reverse. It is more complex to design. The incoming (RF) signal is first downconverted to (IF) and demodulated. The ability to demodulate the signal is hampered by factors including atmospheric noise, competing signals, and multipath or fading.

Figure 34. A Digital Receiver

Generally, demodulation involves the following stages:

1. carrier frequency recovery (carrier lock)

2. symbol clock recovery (symbol lock)

3. signal decomposition to I and Q components

4. determining I and Q values for each symbol (“slicing”)

5. decoding and de-interleaving

6. expansion to original bit stream

7. digital-to-analog conversion, if required

In more and more systems, however, the signal starts out digital and stays digital. It is never analog in the sense of a continuous analog signal like audio. The main difference between the transmitter and receiver is the

issue of carrier and clock (or symbol) recovery.

Both the symbol-clock frequency and phase (or timing) must be correct in the receiver in order to demodulate the bits successfully and recover the transmitted information. A symbol clock could be at the right frequency but at the wrong phase. If the symbol clock was aligned with the transitions between symbols rather than the symbols themselves, demodulation would be unsuccessful.

Symbol clocks are usually fixed in frequency and this frequency is accurately known by both the transmitter and receiver. The difficulty is to get them both aligned in phase or timing. There are a variety of techniques and most systems employ two or more. If the signal amplitude varies during modulation, a receiver can measure the variations. The transmitter can send a specific synchronization signal or a predetermined bit sequence such as 10101010101010 to “train” the receiver’s clock. In systems with a pulsed carrier, the symbol clock can be aligned with the power turn-on of the carrier.

In the transmitter, it is known where the RF carrier and digital data clock are because they are being generated inside the transmitter itself. In the receiver there is not this luxury. The receiver can approximate where the carrier is but has no phase or timing symbol clock information. A difficult task in receiver design is to create carrier and symbol-clock recovery algorithms. That task can be made easier by the channel coding performed in the transmitter.

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Rabu, 08 Mei 2013

DIGITAL MODULATION ; HOW DIGITAL TRANSMITTER AND RECEIVER WORK

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