IndoorUnit (IDU) orRadio Modem

The Indoor Unit (IDU) of a radio terminal comprises the radio modem,the user port interfaces for connecting external user communications equipment,the control circuitry and the power supply circuitry. In simple terms,the IDU function is to convert the digital input signal received from the end users communication equipment into the modulated form necessary for radio transmission. First the signal is converted to an intermediate frequency format for transmission to the Outdoor Unit (ODU). The content of the digital signal carried by the radio (outdoor unit),its bitrate and synchronisation,are all controlled by the indoor unit.

Figure 6.12 illustrates the functional block diagram of a typical indoor unit of a digital radio terminal. The unit comprises seven basic functions,which we shall discuss in turn:

. User data multiplexing,conversion and/or protocol preparation for radio transmission;

. Forward Error Correction (FEC) function;

. Modulation/demodulation (modem function);

. Terminal control unit;

. Telemetry signalling for intercommunication of indoor and outdoor units;

. Power supply unit;

. Cable multiplexor.

User Data Multiplexing and Transmission Protocol Preparation

The indoor unit receives data from end user communications equipment (e.g. a telephone,a computer,a data network,a Local Area Network (LAN)) in one of a number of possible standardised interface formats. The number of ports and the type of ports provided on the indoor unit varies from one radio equipment to another. Typical point-to-point (PTP)

Indoor Unit (IDU) or Radio Modem 105

microwave radio systems offer multiple 2 Mbit/s called E1-interface) or 1.5 Mbit/s (so-called T1-interface),for example 2  E1,4  E1,8  E1,etc. The PTP radio link thereby provides the equivalent of a direct wire connection between the two corresponding E1 ports at either end of the link. Point-to-multipoint (PMP) systems,meanwhile,seek to consolidate and switch the connections within the radio network. Thus in PMP networks, the interface type and the con®guration of the individual end-user connections may be changed from one end of the link to the other,as a result of the switching undertaken within the network.

The user input/output (I/O) ports are presented to a multiplexing and/or switching device within the indoor unit. This device may perform any number or combination of three basic functions:

. Multiplexing of the individual connection data streams into a single highspeed digital bitstream;

. Switching and/or concentration of the individual data streams into a single,formatted digital bitstream;

. Signalling of control information (i.e. protocol information) to enable the two radio terminals at either end of the link to optimise the use of the radio channels and other resources within the particular PTP or PMP radio system.

A simple multiplexing function is included in most PTP radio systems in cases where multiple connections are carried simultaneously from one end of the link to the other (e.g.

2  E1 PTP system or 4  E1 PTP system). This is shown as the `User port MUX' in Figure 6.12.

106 Basic Radio System Design and Functionality

Figure 6.12 Functional block diagram of typical radio terminal indoor unit

In contrast to PTP,the `User port MUX' of Figure 6.12 is replaced in most PMP systems by a switching and/or concentration function and a communication function for sending control and coordination messages (so-called protocol control information) between base stations and remote end-user terminals. Typically the base station indoor unit of a PMP system comprises a real `switch' function (so that individual remote end users can be connected to one another in any combination). Meanwhile the individual remote end-user terminals comprise a `concentration' function which enables them to `activate' or

`deactivate' their usage of the radio channel (i.e. the `air interface'),restricting their usage to the times when they have a need to support an active connection. So that this switching process (and radio channel allocation process) runs smoothly and in an coordinated manner between base station and remote end-user terminals,it is usually necessary to add Protocol Control Information (PCI ) (i.e. system control messages).

The end-user input/output (I/O) ports may take any number of different forms. PTP systems typically offer serial port interfaces conforming to one of the common leaseline interface types,either ITU-T G.703 (e.g. E1 [2 Mbit/s],T1 [1.5 Mbit/s],E3 [34 Mbit/s],T3 [45 Mbit/s],STM-1 or OC-3 [155 Mbit/s],X.21 [typically n  64 kbit/s to 2 Mbit/s] or V.35 (typically n  64 kbit/s to 2 Mbit/s]. Such interfaces are carried by PTP systems

`transparently' (i.e. without alteration). The signals retain the same bitrate and synchronisation. The jitter of the signal (slight variation of the pulse lengths and timing over time) is also largely unaffected by the radio system.

Point-to-multipoint (PMP) systems also offer leaseline-like interfaces like PTP systems, but in addition,many systems also interpret both data protocol and voice signalling information to determine the desired destination of switched connections. PMP systems therefore typically also support one or more of the following interface types:

. Plain Old Telephone Service (POTS),also called a/b-interface (analog telephone signalling);

. Basic Rate ISDN (BRI);

. Primary Rate ISDN (PRI);

. Channel Associated Signalling (CAS) (Analogue telephone signalling via digital line plant);

. Frame relay;

. Ethernet (10baseT or 10/100baseT);

. Internet Protocol (IP);

. ATM (Asynchronous Transfer Mode).

We return in later chapters to the `network integration' of these interface types (i.e. how to build a public switched data or telephone network incorporating PMP radio and offering these interfaces to end customers).

Before we leave the subject of the end-user input/output (I/O) ports,we should also brie¯y mention the wayside channels, engineering order wire and external alarm relay channels,which are typically also available (these are particularly common in the case of PTP systems). One or two wayside channels are typically made available from one end of a PTP system to another. A wayside channel is a low bitrate additional data connection between the two ends (thus a 2  E1 or 4  E1 PTP system might carry 2  9.6 kbit/s or 2  64 kbit/s wayside channels in addition to the main payload).

Wayside channels are intended for the interconnection or networking of `peripheral' equipment in the remote operations location. Thus,for example,over the wayside channel

Indoor Unit (IDU) or Radio Modem 107

of a PTP link interconnecting a main switch location to a remote operations room,it would be possible to connect directly to the control interface of remote switching equipment for network management purposes.

The ability to relay of external alarms,like the ability to carry of wayside channels is often built-in to radio systems to allow remote monitoring of remote operational locations which are connected to the main switch or network backbone site only by radio. Thus,for example,by relaying the `door open' signal or the `room temperature alarm',the network operations staff can be kept abreast of problems at the remote site.

An engineering order wire is another form of wayside channel,and one also intended for network operations staff usage. Speci®cally,the engineering order wire is a telephone connection between the two ends of the link which allows technicians during installation and maintenance work to talk with one another. With the increasing use by ®eld technicians of mobile telephones,the need for engineering order wires has somewhat receded.

Forward Error Correction (FEC)

Because radio systems are susceptible to fading and,in particular,to burst fading (i.e. large runs of corrupted bits),it is usual in digital radio systems to employ Forward Error Correction (FEC) to detect and correct errors in the received bitstream at the receiver end.

The `forward' nature of the error correction allows the detection and correction of the errors without the need for retransmission of the digital information which would otherwise result in unacceptable further delays to the signal. An advanced algorithm is used for the correction mechanism. An example of such an algorithm is a Hamming or a Reed±

Solomon code.

To illustrate the principle of forward error correction we consider the form of FEC which is standardised as part of the Digital Video Broadcasting (DVB) standard.

A transmission frame is built around each block of so many consecutive bytes (say 187) of user information. To each block is added as a header a synchronisation pattern (typically of 1 byte in length). The synchronisation pattern identi®es clearly the beginning of each frame. Following the synchronisation pattern and user information block,a check sequence or Frame Check Sequence (FCS) is added (e.g. of 16 bytes in length). The overall frame length in our example is thus 204 bytes,subdivided as we illustrate in Figure 6.13.

The frame check sequence may take any of a number of different forms,whereby the most commonly used codes in radio systems are Hamming codes, Viterbi codes, BCH-codes and Reed±Solomon BCH-codes. We detail in Appendix 6 the various BCH-codes,but to get at

108 Basic Radio System Design and Functionality

Figure 6.13 Typical framing structure for forward error correction

least some appreciation of how we can detect and correct bit errors,consider the simple Hamming code error detection and correction methodology illustrated in Figure 6.14.

Figure 6.14 illustrates the addition of a three bit code integrated into the 4-bit user data pattern,which occupies bit positions 3,5,6 and 7. The three check bits are being used to make up even parity,respectively for the bit combinations 5±6±7; 3±6-7 and 3±5±7 (Figure 6.14a). Thus,because the values of bits 5,6 and 7 contain an odd number of values equal to

`1',the ®rst check bit is set to value `1'. This gives an even number of values `1' over the four bits 5-6±7 plus the ®rst check bit or even parity bit. The second check bit provides for similar even parity over the four bits 3±6±7 plus the second check bit. The third check bit provides for even parity over bits 3±5±7 plus the third check bit.

The code has been designed to detect and correct a single bit error during transmission, without the need to retransmit the sequence. We consider the case of a single bit error affecting the user data during transmission (i.e. corrupting one of the bits 3,5,6 or 7).

Figures 6.14b and 6.14c illustrate the received code in the case of bit errors either at position 6 (Figure 6.14b) or position 7 (Figure 6.14c).

During the detection process,the check bits are used to check the even parity again,and values are given to each of the check bits to indicate whether there is a parity error or not. In the case of incorrect parity,value `1' is assigned. Meanwhile `0' represents correct parity.

The checks of parity on receipt (in the case of Figure 6.14b) yield the following results:

. Check bit 1 of the received pattern plus bits 5±6±7: received:1 calculated:0 incorrect,set value 1 . Check bit 2 of the received pattern plus bits 3±6±7: received:0 calculated:1

incorrect,set value 1 . Check bit 3 of the received pattern plus bits 3±5±7: received:0 calculated:0

correct,set value 0

The resulting bits signal the binary value `110' --- in decimal value `6'. In other words,we have detected an error in the bit at position 6,so we are also able to correct the error.

Indoor Unit (IDU) or Radio Modem 109

Figure 6.14 Simple Hamming code for error detection and correction

In Figure 6.14c,the values of the received parity checks are `100'. This compares with the calculated values `011'. So that the check bits 1,2 and 3 are respectively 1,1 and 1 or

`111' (decimal value `7'). (As an aside,you might like to note that the comparison of the two values `100' and `011' is achieved easily by modulo 2 addition. This is an important assistance for electronic circuit designers.)

In the case of no errors,the receive end parity check will deliver the value `000'.

So,you might ask,what about an error occuring in the string of parity check bits? The answer is simple,we add another parity bit to check whether an error affected the other parity bits. If this parity bit is incorrect on receipt,then we ignore the other parity check bits and make no corrections to the user data bits. On the assumption that there will be a maximum of 1 bit in the sequence which is corrupted,we are able to detect and correct the error!

Since radio transmission is prone to burst errors (strings of consecutive bit errors) rather than single bit errors,it is normal to randomise the order of the user data bits and check bits within the user data block of Figure 6.14. This has the effect of distributing the errors across the different individual connections carried within the block. This has two advantages. First,it is easier to design correction codes to cope with individual error.

Secondly,it is less likely that a single end user will have to shoulder the burden of any unrecoverable errors. We illustrate this technique in Figures 13.3 and 13.4 of Chapter 13.

More advanced forward error correction codes work in a similar manner to our example.

The various codes have different relative strengths and are designed for different length user data blocks and different error rates.

Before we leave the subject,you might also have observed that we have `used' 4 of the 8 transported bits for the error correction and detection,and thus reduced the maximum user bitrate we can carry. This is a correct observation. Typically,we have to use more powerful FEC to improve the reliability of transmission across radio links using the higher modulation schemes which we discussed in Chapter 4. It is thus true to some extent that what we can in higher bitrate with higher modulation schemes we must give up (at least in part) to carry the extra bits needed for the FEC. It is up to the radio system designer to ensure the optimum mix of modulation,and FEC to achieve the required user data bitrate and received signal quality.

Modulation and Demodulation (the Radio Modem)

The radio modem unit of Figure 6.12 performs the modulation of the data signal to be transmitter,converting it from digital format into intermediate frequency using the techniques we discussed in Chapter 4. It also performs demodulation of the incoming intermediate frequency signal,for presentation to the input/output ports.

Terminal Control Unit

The terminal control unit of Figure 6.12 comprises the electronics which monitor the radio terminal as a whole,and respond to software commands received from a network management system or other equipment control or con®guration terminal. Modern equipment is typically designed to be largely software-based,and to conform with standard software and network management interfaces (including SNMP and CMIP) and is capable of being monitored and con®gured from a remote location.

110 Basic Radio System Design and Functionality

The control terminal or network management terminal may take a number of different forms. An external control terminal (e.g. a personal computer) is designed to be attached to the indoor unit of Figure 6.12 for purposes of monitoring and con®guring the indoor unit and the outdoor unit of the same terminal. In most systems,a communication channel via a network management wayside channel over the link also allows the same (local) control terminal to be used to monitor also the remote terminal (both indoor and outdoor units).

Telemetry Communication between Indoor and Outdoor Units

The telemetry modem is used to create a communications channel for network management,monitoring and con®guration purposes,allowing the outdoor unit to be controlled from a control terminal connected to the indoor unit. This allows even mast-top equipment to be diagnosed and recon®gured without having to climb the mast. The telemetry modem need only use a relatively simple modulation technique. The signal is transmitted only as far as the outdoor unit. Network management communications with the remote terminal,by contrast,need to be incorporated in wayside channels,as we discussed earlier in the chapter,in conjunction with the user port multiplexor.

Power Supply Unit

Most radio equipment is designed to work from a direct current power source,typically 24 volt DC (in North America) or 48 volt (in Europe). Typically,the equipment will be rated at for the given voltage (e.g. 48 volt),but also permit the use of an `unsteady' supply provided the supply remains within a given tolerance band (e.g. tolerance band typically  20%,say 39 volts to 62 volts). The power supply should conform with the nominal required voltage. A normal supply of 60 volts (given the above tolerance range) will not do!

Typical consumption of a radio terminal is about 50 Watt per modem. Much of this power is `wasted' in the electronics rather than transmitted as radio signal energy. The unit cooling or airconditioning therefore needs to compensate this heat dissipation. Particularly, the indoor unit may be equipped with fans for forced cooling or have a perforated casing to allow convective cooling. The heat dissipation can be a problem in restricted spaces packed with other equipment or modems. The ventilation holes in the casing can also be a problem if there is any possibility of water intrusion --- for example,due to the collection of condensation.

Sometimes indoor units are placed in `outdoor cabinets' (e.g. small weathertight air conditioned cabinets). Such installations need to be planned carefully in order to keep the indoor unit maintained within its speci®ed operating temperature and humidity range (typically 08C to ‡ 408C and humidity 0±95%,but non-condensing). The elimination of condensation can be a challenging problem!

Cable Multiplexor

The cable multiplexor (Figure 6.12) serves only to share the use of the indoor-to-outdoor connection cable between the modulated user data signal,the control telemetry communication and for the conveyance of power to the outdoor unit.

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