ALE Signal Structure

An ALE transmission is a contiguous sequence of ALE words called an ALE frame. The structure of the ALE frame will be described in Section 4.5. Here, we present the structure of the ALE word itself, the FEC applied to that word, and the modem used to carry ALE words over the HF channel.

4.2.1 ALE Modem

In Chapter 3, we discussed modem waveforms designed for carrying streams of data over the HF channels.

A modem for ALE must cope with the same challenging channel characteristics, but must also efficiently convey short bursts of information, the 24-bit ALE words. Consequently, long interleavers and convolutional coding were ruled out for the ALE modem. Furthermore, the limited DSP technology available in the early 1980s prompted selection of a simple modulation that could be implemented using analog filters: 8-ary frequency shift-keying (FSK). One of eight orthogonal tones (Table 4.1) is sent in each symbol time, at a rate of 125 symbols per second. Each tone (symbol) represents three bits of data, so the raw data rate of this waveform is 375 bps.

The relatively long symbol period of 8 ms per tone keeps the data rate low, but permits this modem to cope with delay spreading without a need for the advanced signal processing found in adaptive equalizers (Chapter 3). Instead, we can simply discard the edges of the received symbols (where the intersymbol interference is found) and use only the middle of each symbol for demodulation. (The long symbol period also gives the ALE modem a characteristic warbling sound, which has become familiar to shortwave listeners who tune to ALE channels.)

4.2.2 ALE Word

The 24-bit ALE word is divided into two parts: a 3-bit preamble and a 21-bit data field (seen in Figure 4.3).

In many cases, the data field carries three 7-bit ASCII characters representing all or a portion of a station address (call sign). The ALE word can also be used to convey orderwire commands over the HF link. In this case, the data section is partitioned in various ways, often using the first seven bits to specify a command and the remaining bits to carry parameters specific to that command.

Each ALE word is of one of eight types, as specified by the 3-bit preamble in that word. The word types and corresponding encodings of the preamble field are listed in Table 4.2.

The uses of the various types of ALE words will be presented in the discussion of the ALE protocols in Section 4.5.

4.2.3 Forward Error Correction

The goal of the FEC sublayer in Figure 4.2 is the error-free delivery of ALE words through the HF channel despite the noise, fading, and multipath spreading common in skywave propagation. Four steps of FEC and related processing are applied to the ALE word to improve its chances of error-free reception. This ALE FEC processing expands the ALE word from 24 bits to a 147-bit redundant word. When sent using the ALE modem, the resulting 49 symbols occupy 392 ms on the air. This fundamental time quantum in the ALE system is usually abbreviated T_rw (duration of a redundant word).

Table 4.1 8-ary FSK Modem for ALE

Figure 4.3 ALE word.

Table 4.2 ALE Word Types

Word Type Preamble Use

TO 010 Direct destination address

THRU 001 Group calls

TIS 101 Immediate source

TWAS 011 Immediate source; terminates link

FROM 100 Quick ID

CMD 110 Orderwire functions

DATA 000 Extension of preceding word(s)

REP 111 Duplicates preceding preamble

4.2.3.1 Golay Coding

At the sending station, the first step in encoding the ALE word is to split it into two 12-bit halves. Each half is then encoded using the extended Golay (24, 12) code, a perfect block code that is capable of correcting up to 6 errors per 24–bit ALE word (3 bits in each Golay word). The FEC code generator polynomial is:

g(x) = x¹¹ + x⁹ + x⁷ + x⁶ + x⁵ + x + 1

The generator matrix G derived from g(x) contains an identity matrix I₁₂ and a parity matrix P. That is, the Golay encoding is systematic, with the 12 bits from the ALE word sent unmodified, followed by 12 parity check bits. A simple approach for decoding this code, while adjusting its balance of error detection and correction, may be found in [6]. To assist the receiver in identifying word boundaries, the parity bits of the second Golay word are inverted before transmission.

4.2.3.2 Interleaving

As noted above, the span of error-correction efforts in the ALE waveform is limited to each individual ALE word. Thus, interleaving, which is used to spread channel errors for more effective processing by the Golay decoder, is applied only within the word (a 392-ms interleaver depth). In the original design, bits from the two Golay words were interleaved in a pseudo-random manner; however, this was found to offer little extra performance over a simpler perfect shuffle interleaving, so the latter was chosen for the standard.

4.2.3.3 Triple Redundancy

After interleaving the two Golay words, we have 48 bits of coded data. For added robustness, this coded/interleaved ALE word is sent three times in succession. At the receiver, majority voting is employed, both to correct some errors and to estimate the channel error rate. (Any nonunanimous vote indicates that at least one error has occurred.)

4.2.3.4 Stuff Bit

The original MITRE design for the FEC sublayer employed the three steps described so far. A fourth step was added to improve robustness for military uses. If a heterodyne tone (or a tone jammer) interfered with one of the ALE modem tones, this interference could be excised in the frequency domain, but we would still lose any bits sent using the affected tone. Triple redundancy wouldn’t help because the 48-bit word divides evenly over the 3 bits per symbol, so every repetition of the word would produce the same 16-tone sequence. Any tone lost in the first instance would also be lost in the second and third instances.

A simple solution was to add a 49th stuff bit (always set to 0) at the end of the 48-bit coded/interleaved word. This causes the coded ALE bits to rotate through symbol boundaries, resulting in some time- and in-band frequency-diversity [7]. In practice, adding the 49th bit was found to improve SNR robustness in fading channels by roughly 1 dB.

4.2.3.5 Receive Processing

At the receiver, the first step in synchronizing with a new incoming signal is for the modem to identify 8-ms symbol boundaries. Once the modem declares the presence of ALE tones and timing, the next step is to achieve word synchronization. Identifying the boundaries between the redundant words is a cooperative process between the FEC sublayer and the ALE protocol sublayer. As each symbol (tribit) is delivered from the modem to the FEC sublayer, error correction and word sync processing proceed together as follows (see Figure 4.4):

Figure 4.4 Word sync processing.

• Majority voting among tribits received at times T (the current tribit), T – 49, and T – 98 yields a majority tribit and a count of unanimous votes (0, 1, 2, or 3).

• This majority tribit is concatenated with the previous 15 majority tribits to form a 48-bit majority word (the 49th bit is discarded here). The total count of unanimous votes over the 16 majority tribits is compared to a threshold. If the unanimous vote total falls short of the threshold, it is unlikely that the 48-bit majority word is correctly framed, and processing halts until the next incoming symbol (tribit).

• If the unanimous vote threshold is met, the 48-bit majority word is de-interleaved into two 24-bit Golay words.

• The Golay words are decoded individually. If both words are correctable, the 12-bit results are concatenated to form a candidate 24-bit ALE word, and delivered to the ALE protocol for final determination of word sync. However, if either Golay word is uncorrectable, word sync will not be achieved at this symbol, and processing halts.

Once word sync is achieved, the FEC sublayer tasks of majority voting, deinterleaving, and Golay

decoding are executed only after 49 new symbols have been received (not after every symbol).