Digital modulation in ENG microwave

Earlier we discussed AM and FM, and as we saw, both are methods of transmitting information on a carrier wave radio signal. Both AM and FM are used in broadcasting radio and analogue TV. In the context of digital, a new method of modulation joins the fray – coded orthogonal frequency division multiplex (COFDM).

Like many of the subjects in this book, it is too complex a subject to cover in detail, so we will just look at the bare essentials.

COFDM

For COFDM to work, it is necessary for the information signal to undergo a process known as compression, but as it is common to satellite communications, we will look at how compression works in more detail later. So for the time being, assume our digital signal has been compressed.

Instead of using just a single information carrier, COFDM uses a large number of narrow information carriers – typically at least 2000 – each carrying a slice of the total compressed data.

The term ‘orthogonality’ comes from the property where the phase of one carrier is different from that of the carrier next to it. The demodulator for one carrier does not ‘see’ the modulation of the others, hence there is no crosstalk or interference between carriers, even though the signals overlap in frequency.

As each carrier has a low data rate in itself, i.e. the data is being transmitted at a low speed, the susceptibility to interference from multipath (ghosting) is greatly reduced, as any reflected signal will be detected at the same time as the direct signal – and in fact that additional reflected signal can enhance the direct signal.

This is because the period in which the data is received and decoded is lengthy compared with the time in which any delayed signals will arrive. It is also possible to enhance the multipath immunity by the addition of a ‘guard interval’, which is a space between each transmitted carrier.

So data is transmitted for a period that is longer than the time during which the receiver is detecting it. Also, the use of coding (the ‘C’ in COFDM) is another significant factor in coping with both interference between channels and the frequency-dependent fading caused by multipath.

To sum up, COFDM thrives in an environment where there are a lot of reflections, as these can actually enhance certain aspects of the received signal. However, although it seems to be an ideal step forward from analogue FM, there are certain limitations to its use, and there can be situations where analogue FM performs better.

At the time of writing, many are still exploring the use of COFDM for ENG/OB applications, and further development and refining will result.

Compression

If digital signals were used in direct replacement for the same analogue information, the resultant bit-rate at which the information would have to be transmitted would be very high.

This would use much more bandwidth (frequency spectrum) than an equivalent analogue transmission, and therefore is not very efficient.

There is a need to strip out the redundant and insignificant information in the signal, and this process of data rate reduction, or compression as it is more commonly referred to, is dealt with in more detail later.

The important point to grasp is that digital signals are now the norm particularly for SNG transmissions. In ENG microwave, analogue transmission is still the preferred method, but that is slowly changing with the new generation of digital ENG microwave equipment that has come onto the market.

Advantages of digital microwave

The spread in use of COFDM includes:

Many broadcast operations have turned to DSNG trucks for covering events outside their primary market. Unfortunately, such trucks are often under utilized because they do not include terrestrial microwave systems for in-market applications. Often, DSNG trucks also cannot get a clean line of sight in urban concrete ‘canyons’, limiting their usefulness. By adding a low-profile omnidirectional antenna, and a COFDM modulator and transmitter, such vehicles can become cost-effective dual-mode (hybrid) trucks without the added weight, mass and cost of a hydraulic mast. We will examine hybrid trucks in more detail later. For new DSNG trucks, COFDM provides a low-cost way of increasing versatility and vehicle utilization.

Some of the pioneering COFDM applications include news cars or sports utility vehicles (SUV). In a typical configuration, a 2 m pop-up mast with a directional 30 cm antenna is combined with a roof-level omnidirectional antenna. The digital RF transmitter is placed on the roof adjacent to the antennas and can be automatically switched from directional to omnidirectional antennas as desired. (For example, using the pop-up mast and directional antenna for stationary shots requiring more system gain, and the omnidirectional antenna for mobile shots.) Inside the vehicle is a small rack housing the digital compression encoder and COFDM modulator.

For helicopter and fixed-wing applications, the RF transmitter is mounted on the lower side of the aircraft near the antenna pod, while the digital compression encoder and COFDM modulator are fitted inside the aircraft.

Digital vs analogue

You need to appreciate the fundamental difference between analogue and digital transmissions to see the advantages and disadvantages of each type of system.

The essential difference between an analogue signal and a digital signal is that the analogue signal amplitude can be any value between pre-defined limits. Analogue transmissions require significantly greater power and bandwidth than compressed digital transmissions, but the quality of the signal recovered can be very high.

In terms of microwave transmissions, this greater power can only be delivered by a larger transmit amplifier and/or a larger antenna than is required for a digital transmission.

Digital transmissions are always compressed for SNG and DENG, so as to acheive savings in power and bandwidth. The effect of the compression is that although at first glance the quality of the signal can look very good, there are particular picture degradations (termed artefacts) that can be objectionable on certain types of picture content – particularly fast-moving material and pictures with large areas of gradual colour change in the background.

However, as the cost of satellite capacity and the size of the SNG equipment are often primary considerations, these artefacts have to be tolerated as the compromise for the financial savings.

The compression process also increases the overall delay of the signal, as the processing involved in compressing and decompressing the signal takes an appreciable time. This is in addition to the delay that the signal suffers in travelling up from the earth to the satellite and back down to the earth, as we shall see later. Any delay can be problematical in conducting ‘live’ interviews.