OutdoorUnit (ODU) orRadio Transceiver

The Outdoor Unit (ODU) comprises the Radio Frequency (RF) (also called High Frequency (HF)) transmitter and receiver. In simple terms,the ODU function is simply to

`translate' the (low frequency) digital signal received from the indoor unit into a radio signal in the transmit direction,and to convert the received radio signal into a digital signal to pass to the indoor unit. The content of the digital signal,its bitrate and synchronisation, as received from or passed to the modem is usually unaltered by the outdoor unit (radio).

90 Basic Radio System Design and Functionality

Table 6.1 Waveguide speci®cations

Figure 6.6 illustrates a functional block diagram of a modern digital radio (outdoor unit).

The incoming radio signal from the partner radio terminal at the other end of the link is picked up by the antenna and passed to the outdoor unit (radio unit) by means of the waveguide. A diplexor in the outdoor unit splits the incoming signal from the outgoing signal,which is being carried to the antenna for transmission. Optionally,a Low Noise Ampli®er (LNA) is sometimes also used in the antenna or front end of the outdoor unit to amplify weak incoming signals. A low noise ampli®er strengthens very weak signals without adding extraneous `noise' --- this helps to preserve the clarity and quality of reception. Low noise ampli®ers are relatively expensive components.

The Automatic Gain Control (AGC) is a signal ampli®er of variable strength which is designed to ensure that the signal strength on entry to the radio receiver is constant, independent of the signal strength received by the antenna. In other words,the AGC is designed to compensate for unpredictable radio signal losses during transmission caused by radio path attenuation (e.g. caused by bad weather).

The received radio frequency signal is demodulated to either an Intermediate Frequency (IF) or a baseband (BB) signal for onward transmission to the indoor unit over the indoor-to-outdoor connection cable. In the case of Figure 6.6,this is achieved in two stages,by mixing with demodulation signals. The ®rst stage of demodulation is carried out by mixing a frequency to shift the RF signal to a given channel frequency within the radio band (e.g.

channel 1 of the 26 GHz band). The second stage of mixing then demodulates the signal from this standard frequency to the standard IF frequency. By using two stages of demodulation like this,it is possible to adjust the frequency of the ®rst demodulation stage by using software to control the synthesiser frequency used for the demodulation. This allows the receiver to be tuned to various different channel frequencies within the band.

The second stage of demodulation can then be a `standard' frequency shift corresponding

Outdoor Unit (ODU) or Radio Transceiver 91

Figure 6.6 Functional block diagram of typical outdoor unit

to the radio band frequency --- so allowing the use of a ®xed frequency oscillator. Such a receiver design allows both accuracy and ¯exibility. The frequency `shifting' is achieved simply by mixing and subsequent ®ltering of the resultant signal,as we discussed in Chapter 2.

In the case of Figure 6.6,the indoor-to-outdoor connection cable is by means of a single coaxial cable. This requires the use of two separate intermediate frequencies for the transfer of the incoming (down from `outdoor' to `indoor') and outgoing (up from

`indoor' to `outdoor') signals. An intermediate frequency,as the name suggests,is a frequency intermediate between the original frequency of the user signal (comparatively low frequency) and the high frequency radio signal. A multiplexor is required to combine (in the downward direction) and separate (in the upward direction) the two intermediate frequency signals signals (see Figure 6.6). Alternatively,where baseband signals (i.e. the original low frequency user signals) are used on the indoor-to-outdoor connection,two separate cables may be required.

In the design of the outdoor unit shown in Figure 6.6,a telemetry signal is also added by the multiplexor,so that the outdoor unit can communicate with its respective indoor unit.

This allows the control and monitoring of the outdoor unit from the indoor unit (or by means of a remote connection to the indoor unit) from a remote network management centre location.

Not shown in the receiver part of the outdoor unit of Figure 6.6,but sometimes necessary,is an equaliser. In cases where the incoming radio signal is carrying a relatively large amount of information (e.g. is a high ®delity signal,or a high bitrate signal coded with higher order modulation (as we discussed in Chapter 4),it may be necessary to add an equaliser. During transmission of a radio channel with a wide frequency bandwidth,it often happens that the frequencies at the lower end of the radio channel are attenuated relatively

92 Basic Radio System Design and Functionality

Figure 6.7 The principle of equalisation

more or less than the frequencies at the highest extreme range of the channel. An equaliser is designed to compensate this effect.

An equaliser has a function similar to the AGC already discussed. In particular,the equaliser is designed to compensate with different ampli®cation the different frequency ranges within the overall incoming signal (Figure 6.7).

The transmitter part of the outdoor unit works like a receiver in reverse. The incoming signal from the indoor unit (received by means of the indoor-to-outdoor connection cable) is recived by the multiplexor (Figure 6.6). The direct current (power component) of this signal is removed to power the outdoor unit electronics. The remaining (user) signal is shifted from the baseband or intermediate frequency by means of the two stages of mixing. The ®rst stage mixes an RF carrier signal to shift the signal into the appropriate radio band (e.g. 18 GHz, 23 GHz or 26 GHz,etc.). The second stage of modulation mixes a secondary RF signal component (controlled by a synthesisor) to shift the resultant signal to the correct radio channel within the band for transmission. The resultant signal is then applied to a High Power (radio frequency) Ampli®er (HPA),and then conveyed by means of the diplexor and waveguide to the antenna for transmission.

The inside of an outdoor unit (high frequency radio unit) is not like the inside of a PC or other domestic electronic device. Rather than rows of digital electronic components, resistors and integrated circuits,there are bulky components made by high precision machining,for the diplexor,high power ampli®er and mixers are all waveguide-like in appearance --- accurately machined chambers and channels within a series of precision-manufactured metal blocks. The electronic components (if present) are most likely devices for controlling and monitoring the outdoor unit.

Once enclosed in its casing,the outdoor (radio) unit is a plain-looking article (Figure 6.8). However,its design is not without considerable challenge. The most stringent international speci®cations de®ne `extreme' ambient temperature range of operation (i.e the temperature of the atmosphere outside the casing) from 338C to ‡ 558C. Inside the casing the temperature could be even higher (i.e. ‡ 708C or more). These temperatures must be withstood without any form of cooling or other ventilation. The casing itself must be water and weather-proof.

Outdoor Unit (ODU) or Radio Transceiver 93

Figure 6.8 Typical radio outdoor unit --- inside and outside views (Reproduced by permission of Netro Corporation)

If you look closely at Figure 6.8,you will notice that there are three external connectors to the outdoor unit. Two of these are for the waveguide (to the antenna) and for the coaxial cable connection (to the indoor unit). The third connector is for the attachment of a measurement device. This port allows a technician,during installation of the antenna and outdoor unit (ODU),to monitor the Automatic Gain Control (AGC) or the Received Signal Level (RSL). This value is used for the correct and exact alignment of the antenna.

The automatic gain control,as discussed earlier in the chapter,is an ampli®er applied to the incoming radio signal to adjust its strength to a set given value. The more the signal has to be ampli®ed to reach the set level,so the weaker the received signal must have been.

Conversely,the less gain we have to apply,so the stronger the signal must have been. The amount of gain which needs to be applied by the AGC is thus a direct measure of the incoming signal (correctly,received signal level) strength.

The received signal strength is dependent upon the transmitted signal strength,the path loss (i.e. the signal attenuation incurred during radio transmission through the atmosphere across the link) and upon the alignment of the transmitting and receiving antennas. Since the transmitted signal strength can be set to a known value,and since it is normal also to install the antennas and radios during good weather (for which the path loss can be accurately calculated according to the length of the link,as we discuss in Chapter 7),then the AGC or RSL measurement can be used as a direct measure of the accuracy of the alignment of the antennas at either end of the radio link.

The third connector (or `port') visible in Figure 6.8 is typically designed for the connection of a simple voltmeter. The voltage value presented at this connector is either a direct measure of the AGC or a calibrated value directly proportional to the RSL. Knowing the transmitter output power of the radio at the other end of the radio link; the length of the link and the gain of the two antennas (at each end of the link),it is possible to calculate (using the method presented in Chapter 7) the exact voltage to be expected as AGC or RSL.

Only when both antennas are exactly aligned will it be possible to measure this value. If either or both antennas are misaligned,then the received signal strength will be considerably degraded.