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CROSS-REFERENCES TO RELATED APPLICATIONS
Reference is hereby made to copending U.S. application Ser. No. 07/220,211
to Reed et al. and copending U.S. application Ser. No. 07/220,213 to
Goodwin, both assigned to the assignee of the present application.
FIELD OF THE INVENTION
The present invention relates to data transmission in an HF transmission
system and in particular to a data transmission protocol.
BACKGROUND ART
HF is generally recognized as the frequency range 2-30 MHZ. For the
purposes of this specification the term is intended to cover any frequency
up to 30 MHZ and may include frequencies above 30 MHZ. The term is
intended to designate a type of radio transmission usually involving
ionospheric reflection, from the F-layer, the maximum usable frequency
usually being somewhat less than 30 MHZ.
Commercial and military systems are known employing HF transmission between
a large number of ground stations. For example, an army may possess
portable transceivers carried on vehicles or as "man-packs" which rely on
HF data transmission. Because of the nature of HF transmissions, usually
involving ionospheric reflection, there is a high probability of errors in
data transmission caused by bursts, rapid fading and gaussian noise.
Further since a military system will be required to operate in all
conditions and any geographic location, e.g. the Arctic circle, where
disturbed ionospheric conditions of propagation may exist, these causes of
transmission error may be compounded.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a data transmission
protocol for data transmission in HF telecommunications systems which is
reliable and can adapt to changing transmission conditions.
The invention provides in one aspect a data transmission system comprising
a plurality of radio stations, each station including a transceiver with
associated encoding/decoding means for transmitting/receiving data,
wherein data signals are transmitted as packets including one or more
blocks of data encoded with an error correcting code, wherein each station
is arranged automatically to issue a request for repeat of a packet of
data should it receive a packet containing errors which cannot be
corrected, and wherein should the error rate in received packets be more
than a predetermined amount over a predetermined interval of data
transmission, the system is arranged automatically to reduce the rate of
data transmission within each packet and/or change the channel frequency.
The invention provides in a further aspect a method of data transmission
between a plurality of radio stations, each station including a
transceiver with associated encoding/decoding means for
transmitting/receiving data, the method comprising:
a first station transmitting data signals as a data packet to one or more
receiving stations, the packet including one or more blocks of data each
encoded with an error correcting code,
a receiving station receiving and decoding the transmitted data packet and
transmitting an acknowledgement signal with a request for retransmittal of
data should the packet contain more errors than can be corrected,
and the receiving station transmitting to the first station a request for
change of data transmission rate and/or channel frequency should the error
rate in the received packets be more than a predetermined amount over a
predetermined interval of data transmission.
The invention provides in a further aspect apparatus for data transmission
comprising a plurality of radio stations, each station including a
transceiver with associated encoding/decoding means for
transmitting/receiving data, wherein each station includes:
means for forming data packets for transmissions, each packet including one
or more blocks of data each encoded with an error correcting code,
means for decoding transmitted data packets and for correcting detected
errors, and means for transmitting an acknowledgement signal with a
request for a repeat of a data packet should errors not be correctable,
and means for assessing the error rate in received data packets and for
issuing a request in the acknowledgement signal for a change in data
transmission rate and/or channel frequency should the error rate be more
than a predetermined amount for a predetermined interval.
Thus in accordance with the invention measures are provided for corrections
of errors with received data packets, but if these errors are above a
predetermined level leading to conditions in which transmitted data may
not be recovered, then the system is arranged automatically to choose a
data rate which is lower thus enabling a clearer reception, or
alternatively to switch to another channel frequency which is determind to
be more free from interference and/or noise. Thus in practice when the
system initialises data transmission, a radio station will choose a data
rate which is the highest compatible with the noise and interference
levels which have previously been determined as present on the
communication channel frequency selected by the station. If during
transmission the error rate increases because of increased noise levels
more than can be dealt with by error correcting codes and repetition of
data packets, then the receiving station may signal to the transmitting
station to change data rate to enable clearer determination of received
data. If however the error rate is still unacceptably high (or if it was
clear that the error rate could not be contained by lowering the data
transmission rate) then the channel frequency may be changed to a value
which has previously been determined as acceptable in terms of noise
levels.
It will be understood that the system of this invention may be employed in
the context of a network control system described in copending U.S.
application Ser. No. 07/220,213 and a system for automatic selection of
acceptable channel frequencies disclosed in copending U.S. application
Ser. No. 07/220,211.
In addition to he measures forth above, further protection against burst
errors may be provided by interleaving two or more blocks of data within
each packet of data so that should burst errors occur these will be spread
equally over the interleaved blocks so that the blocks may nevertheless be
recoverable.
As preferred, the blocks are encoded with a Golay perfect code. This is a
known technique and has been found particularly suitable with the present
invention as offering a compromise between power at correction of error
whilst not being an unreasonably long code. However other block encoding
techniques may be employed capable of error correction.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention will now be described with
reference to the accompanying drawings wherein:
FIG. 1 is a schematic view of an HF data network with a plurality of radio
stations;
FIG. 2 is a diagram of FSK modulated signals formed in the present
invention;
FIG. 3 is a diagram of Data R-block which forms the basic unit of a packet
of data to be transmitted in a data protocol according to the invention;
FIG. 4 is a diagram of a packet of data comprised of a plurality of
concatenated S-blocks, each S-block being formed of a plurality of
interleaved R-blocks;
FIG. 5 is a diagram showing the issuance of automatic repeat requests (ARQ)
in a fixed cycle of operation;
FIG. 6 is a diagram of the components of an automatic repeat request block;
FIG. 7 is a diagram of the components of packets for different baud rates;
and,
FIG. 8 is a view of the component parts of a data packet for multiple
address messages wherein the packet is broadcast to a number of radio
stations.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The HF data transmissions system of the present invention employs a
technique for network control which is described in copending U.S.
application Ser. No. 07/220,213. Briefly this involves each radio station
contending over predetermined control channel frequencies for network
control, and control being assigned to the station with a highest
preallocated priority. During the initialisation control procedure, a
`pool` of frequency channels are automatically determined which have
relatively low levels of noise and interference, and from this pool a
radio station wishing to transmit selects a channel frequency. For each
channel frequency selected, one or more reserve channels are nominated
should the channel frequency become unusable. The method of automatic
frequency determination is described in copending U.S. application Ser.
No. 07/220,211.
A network of radio stations for HF data communication is shown in FIG. 1,
each station being schematically indicated as at 10. Each station is of
identical construction and comprises an aerial 12 coupled to a receiver 14
and transmitter 16. The receiver and transmitter are coupled to an
oscillator 18 which may synthesize the frequencies allocated for
transmission.
A data buffer/modulator 20 provides frequency shift keyed (FSK) modulated
signals to transmitter 16 and a data buffer/demodulator 22 demodulates
received FSK signals from receiver 14. The form of FSK modulation is shown
in FIG. 2. At the highest transmission rate used in the system 300
bauds.sup.-1, each data bit is represented within each modulation interval
by one pair of two pairs of tones spanning a 3 KHZ bandwidth. Thus it is
possible to transmit two bits of data within each time interval. For a
slower data rate of 150 bauds.sup.-1 each data bit is duplicated in the
two separate pairs of tones within each time interval, thus increasing the
inband diversity and lowering the probability of error. For the slowest
data rate used in the system, 75 bands.sup.-1, the two separate pairs of
tones representing a single data bit are lengthened to occupy two
modulation times intervals.
Data provided by data processor 30 is subject to an encoding technique in
encoder 32 prior to modulation, which is described in more detail below.
Similarly data receivd in receiver 14 and demodulated as at 22 is decoded
36 prior to processing in processor 30.
The encoding technique will now be described in more detail with reference
to FIG. 3 which shows data encoded into an R-block (repetition block) for
transmission to one other station (Single Address Message-SAM). the error
correcting code chosen is the Golay (23, 12) perfect code which is
described in W. W. Peterson "Error Correcting Codes", John Wiley, and
Clarke and Bibbcain "Error Correction Coding for Digital Communications",
Plenum.
For SAM transmission, the message data is encoded at the source for error
protection using the Golay (23, 13) perfect block code. A Golay codeword
is an error correcting code well known in this art, the codeword
comprising a 23 bit code of 12 data bits and 11 parity check bits. Using
this code to correct 2 error bits and detects 3 or more error bits per
codeword, it is possible to obtain a character (7 bits) error rate of less
than 1 in 10.sup.4 at a bit error rate of 3%.
The codewords are divided into numbered R-blocks (Repetition blocks) which
are the smallest units repeated in ARQ. An R-block consists of 14
codewords, FIG. 3. Two R-blocks are concatenated with two further
codewords to create an S-block (send block). The first extra codeword
contains the source address (8 bits) plus 4 flag bits. The second contains
two 6 bit labels which refer to the two R-blocks. During transmission a
data bit interleaving scheme is employed over each S-block (690 bits),
FIG. 4.
To minimise the overhead associated with the ARQ response, the forward
transmission (Packet) consists of several S-blocks, the exact number
depends on the baud rate in use. In addition the forward packet begins
with a 31 bit synchronisation sequence.
Each packet requires individual acknowledgement by the receiving station.
In the recipient's reply (ARQ block), the individual R-blocks are
acknowledged (effectively by number), and only those in error are
retransmitted in the next, constant length packet; the remainder of the
packet contains new blocks. If the ARQ packet is corrupted or lost, all
blocks are repeated. If the forward packet is not received by the
destination, an ARQ packet requesting all repeats is transmitted. To
prevent a possible collision between forward packets and ARQ packets,
there is a fixed cycle time between each transmission at both the source
and destination stations, FIG. 5.
The use of error correction and detection enables a quantitative assessment
of the channel to be made. This information is used by the recipient to
request data rate changes and, possible, a channel change. Changes in baud
rate are initiated only by the destination station, and signalled in an
ARQ packet, FIG. 6. Although the baud rate request is shown in this
example in the first codeword following the synchronisation block, the
request may be contained in any one of the codewords depicted. Each
forward packet is transmitted at the last requested baud rate in a a
received ARQ packet. Message transmission commences at a predetermined
baud rate, which is known to both stations. To maximise throughput, the
baud rate is increased on a good (low error) channel and decreased on a
poor (high error) channel. The recipient will request a lowering of the
data rate automatically if the quality factor of received blocks is
consistently less than a predetermined threshold. If with the lowest data
transmission rate, the quality factor is consistently less than the
threshold value, then a change to another frequency channel will be
requested. The change of frequency channel will be requested in the first
codeword in a similar manner to baud rate request, and a change will be
made to a nominated reserve channel.
For example, traffic transmission may commence at 150 baud but, depending
upon prevailing conditions, this can then be changed to 75 baud or 300
baud at the recipient's request. ARQ transmissions always use 75 baud data
rate. To maintain forward packet lengths of 20 s--which can be shown
theoretically to be optimum--the number of S-blocks per packet changes
with baud rate. There are two S-blocks per packet at 75 baud, four at 150
baud and eight at 300 baud, FIG. 7.
The numbering of each R-block enables the recipient to interpret the
received data correctly and so reconstruct the message exactly, even if
the channel has been disturbed or faded for a long period. A quality
factor is assigned to each received S-block based on a function of the
number of codewords corrected and the number of bits in error in that
S-block. If the quality factor is worse than a predefined threshold then
both R-blocks contained in that S-block are rejected, and a repeat
requested. The R-blocks are otherwise stored, but an R-block repeat is
still requested if it contains a codeword with uncorrectable errors.
An overlay technique is employed to improve the probability of forming an
error free block should an R-block be repeated, whether requested or not.
If the repeat R-block has a better quality factor than the equivalent
stored R-block, any of its codewords containing uncorrectable errors are
replaced with the corresponding codewords from the stored R-block.
Otherwise, the uncorrectable codewords of the stored R-block are replaced
with those from the repeat R-block. If the resultant R-block is error free
it is accepted. Otherwise, it replaces the stored R-block in the buffer
and a further repeat is requested. This process is continued until the
message is received in its entirety.
MAM (Multiple Address Message) transmissions use a similar modular
framework to the SAM transmissions (described above) with some tailoring
to maintain efficient throughput for multiple addresses.
The message to be sent is encoded and divided up in to a number of
sections, each of which consists of four concatenated SAM packets,
including the synchronisation sequences. For instance, at 150 baud, a MAM
section consists of 16 S-blocks with four synchronisation sequences, FIG.
8.
After each section is transmitted, ARQ packets are collected from each
recipient in a predetermined order. Due to the large forward packet size,
MAM ARQ packets acknowledge S-blocks and not individual R-blocks. As a
result, the S-block header format differs from that for SAM packets in
that the two 6 bit R-block labels are replaced by an 8 bit S-block
together with 4 check bits.
Having received the ARQ packets from the destination stations, the source
is able to compile a list of S-blocks to be repeated. If an ARQ packet is
not detected (e.g. due to interference) all S-blocks in the previous
section are added to the repeat list. The repeat lists are held by the
source until all sections of the message have been transmitted. The
requested repeats are then transmitted in the same format as the message
transmissions. Further repeat requests are collected after each section.
After the first repeat transmissions have been completed, the source
continues to service repeat requests from destinations requesting not more
than 50% of the message. Up to 3 repeats of each S-block are permitted,
except that if the number of requested blocks is less than the number of
blocks available in the section to be transmitted the spare blocks are
filled with additional repeats of the requested 8-blocks. If the source
location fails to detect an expected ARQ during the repeat phase, S-blocks
transmitted in the previous section shall be automatically retained in the
list for a further repeat only if they were previously requested by the
destination concerned.
Transmission is continued in this manner until no further repeat requests
are made (within the criterial outlined above).
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