U.S. patent application number 10/156195 was filed with the patent office on 2003-01-16 for carrier activation for data communications.
This patent application is currently assigned to International Mobile Satellite Organization. Invention is credited to Feldman, Howard Ray, Wong, Siu Wah.
Application Number | 20030012152 10/156195 |
Document ID | / |
Family ID | 26313222 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030012152 |
Kind Code |
A1 |
Feldman, Howard Ray ; et
al. |
January 16, 2003 |
Carrier activation for data communications
Abstract
In a radio frequency communications system, a data carrier
activation method is implemented such that the carrier is switched
off when no data is available for transmission. If repeated
signalling information is required to be transmitted, only a
predetermined number of repeats is transmitted before the carrier
is switched off. The data input for transmission are compared with
an idle sequence with different bit alignments to detect the
presence of idle signalling, and the carrier is switched off if a
match is found. When more user data is received, the carrier is
switched on and frames (SM) are transmitted in synchronization with
the timing of frames (SM) transmitted before the carrier
deactivation. After carrier reactivation, a constant power preamble
(P) may be transmitted to assist in level control in the
transmitter (27; 40).
Inventors: |
Feldman, Howard Ray;
(Middlesex, GB) ; Wong, Siu Wah; (West Wimbledon,
GB) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
International Mobile Satellite
Organization
London
GB
|
Family ID: |
26313222 |
Appl. No.: |
10/156195 |
Filed: |
May 29, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10156195 |
May 29, 2002 |
|
|
|
09262084 |
Mar 4, 1999 |
|
|
|
Current U.S.
Class: |
370/316 |
Current CPC
Class: |
H04L 1/0066 20130101;
H04L 1/0083 20130101; H04L 69/324 20130101; H04L 1/0078 20130101;
H04L 1/0042 20130101; H04W 48/08 20130101; H04W 84/06 20130101;
H04L 27/3466 20130101; H04B 7/18532 20130101; H04L 9/40 20220501;
H04W 72/00 20130101; H04W 4/18 20130101; H04L 1/0072 20130101; H04W
28/22 20130101; H04B 7/18513 20130101 |
Class at
Publication: |
370/316 |
International
Class: |
H04B 007/185 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 1998 |
GB |
9804640.2 |
Mar 4, 1998 |
GB |
9804639.4 |
Claims
1. Communications interface apparatus for connection between a
source of data, including both user data and signalling
information, and a transmitter, such that said user data is
transmitted by said transmitter on a modulated radio frequency
carrier, the apparatus being arranged to receive said data, to
detect the presence of repeated signalling information and the
absence of user data in said data, and to deactivate said carrier
if the number of repetitions of said signalling information is
equal to or exceeds a predetermined value, such that excess
repetitions are not transmitted.
2. Apparatus as claimed in claim 1, wherein said signalling
information is an HDLC line control message.
3. Apparatus as claimed in claim 1, wherein said signalling
information is a flow control message.
4. A method of carrier deactivation, comprising: receiving data,
including both user data and signalling information, and
transmitting said user data on a modulated radio frequency carrier,
the method including: detecting the presence of repeated signalling
information and the absence of user data in said data, and
deactivating said carrier if the number of repetitions of said
signalling information is equal to or exceeds a predetermined
value, such that said excess repetitions are not transmitted.
5. A method as claimed in claim 4, wherein said signalling
information is an HDLC line control message.
6. A method as claimed in claim 4, wherein said signalling
information is a flow control message.
7. Satellite communications interface apparatus for connection
between a source of data and an earth station transmitter, the
apparatus being arranged to format said data as a series of
constant length frames and to selectively output said frames to
said transmitter such that said output frames are transmitted on a
modulated radio frequency carrier in an SCPC format, the apparatus
being further arranged to detect whether at least an initial
portion of each of said frames contains no information or only
redundant information, to control the transmitter to deactivate the
carrier in response to a positive said detection, to reactivate the
carrier in response to a subsequent negative said detection, and to
transmit frames subsequent to said reactivation with a timing
synchronised with that of frames prior to said deactivation.
8. Apparatus as claimed in claim 7, wherein the apparatus is
arranged to detect whether the whole of each of said frames
contains no information or only redundant information.
9. A method of satellite carrier activation, comprising: receiving
data, formatting said data as a series of constant length frames,
and selectively transmitting said frames on a modulated radio
frequency carrier in an SCPC format, the step of selective
transmission comprising detecting whether at least an initial
portion of each frame contains no information or only redundant
information, and deactivating the carrier such that said portion of
the frame is not transmitted, wherein, after the carrier is
deactivated, subsequent frames are transmitted with a timing
synchronised with that of frames transmitted prior to said
deactivation.
10. A method as claimed in claim 9, wherein the detecting step
comprises detecting whether the whole of each of said frames
contains no information or only redundant information.
11. A method of transmitting a data burst via satellite to a
receiving terminal, comprising: transmitting the data burst in a
format comprising one or more frames having a variable power level
modulation, preceded by a preamble having a constant power
level.
12. A method as claimed in claim 11, wherein the power level of
said preamble is approximately equal to the average power level of
said one or more frames.
13. A data burst signal comprising a frequency carrier modulated by
a preamble having a constant power level, followed by one or more
data frames having a variable power level.
14. Radio frequency communications apparatus for connection between
a source of data and a radio frequency transmitter, the apparatus
being arranged to divide said data in sequence into blocks and to
compare a series of bits of a predetermined length at the end of a
first block with multiple sequential series of bits of said
predetermined length comprising a second block, and, if all of said
series are equal, inhibiting transmission of said second block.
15. Apparatus as claimed in claim 14, further comprising carrier
control means for deactivating or reducing the power level of a
carrier transmitted by the radio frequency transmitter for at least
approximately a transmission time corresponding to the length of
said second block.
16. Apparatus as claimed in claim 14, further arranged to format
the blocks into frames for transmission, each of said blocks being
of the same length and each of said frames comprising the same,
integral number of said blocks.
17. Apparatus as claimed in claim 15, further arranged to format
the blocks into frames for transmission, each of said blocks being
of the same length and each of said frames comprising the same,
integral number of said blocks.
18. A method of radio frequency communication, comprising: dividing
data for transmission into blocks in sequence; comparing a series
of bits of a predetermined length at the end of a first block with
multiple sequential series of bits of said predetermined length
comprising a second block, and if all of said series are equal,
inhibiting transmission of said second block.
19. A method as claimed in claim 17, further comprising
deactivating or reducing the power level of a carrier transmitted
by the radio frequency transmitter for at least approximately a
transmission time corresponding to the length of said second
block.
20. A method as claimed in claim 17, further comprising formatting
the blocks into frames prior to transmission, each of the blocks
being of the same length and each of said frames comprising the
same, integral number of said blocks.
21. A method as claimed in claim 18, further comprising formatting
the blocks into frames prior to transmission, each of the blocks
being of the same length and each of said frames comprising the
same, integral number of said blocks.
22. A satellite earth station including apparatus as claimed in
claim 1.
23. A satellite earth station including apparatus as claimed in
claim 7.
24. A satellite earth station including apparatus as claimed in
claim 14.
Description
[0001] This application is a divisional application of U.S.
application Ser. No. 09/262,084 filed Mar. 4, 1999, which in turn
claims the benefit of priority from foreign applications United
Kingdom 9804640.2 filed Mar. 4, 1998 and United Kingdom 9804639.4
filed Mar. 4, 1998.
[0002] The present invention relates to a data communication method
and apparatus, and in particular such an apparatus for carrier
activation in a satellite communication system.
[0003] In satellite voice communication systems, it is known to
switch the carrier off in one direction over the satellite link
when the party transmitting in that direction is not talking. This
technique is known as `voice activation` or more generally `carrier
activation` and is described for example on page 55, section 3.2 of
`Satellite Communications--Principles and Applications` by Calcutt
and Tetley, First Edition 1994, published by Edward Arnold. The
average English speaker only talks during about 40% of the time
during a telephone conversation, and therefore a satellite power
saving of up to 4 dB can be achieved by this technique.
[0004] The document U.S. Pat. No. 5,481,561 mentions that carrier
activation could be applied to voice, facsimile and data
communications, but recognizes that this is difficult to realize in
practice.
[0005] Carrier activation in fax calls has been implemented in the
Inmarsat-M.TM., Inmarsat-B.TM. and Inmarsat-mM.TM. satellite
services. The deterministic nature of the ITU T.30 protocols, to
which Group 3 fax terminals conform, is used to detect when one
terminal is about to receive page data and will therefore not be
transmitting; the carrier for transmission by that terminal is then
switched off.
[0006] However, duplex data calls are generally not considered
suitable for carrier activation, because data may be sent
continuously in both directions.
[0007] According to one aspect of the present invention, there is
provided a transmitter in a satellite communications system, which
receives input data in a format which may include an idle signal
indicating that there is no user data present, compares the input
data with a bit pattern corresponding to said idle signal in more
than one relative bit alignment, and ceases transmission if a match
is found.
[0008] An advantage of this aspect is that carrier activation may
be implemented even when byte alignment is not preserved between
transmitting and receiving applications.
[0009] According to another aspect of the present invention, there
is provided a transmitter in a satellite communications system
which assembles data and signalling information for transmission
over the satellite, determines which of said signalling information
need be transmitted in order to maintain the communications link
over the satellite and ceases transmission if there is no data and
only unnecessary signalling information to be transmitted.
[0010] According to another aspect of the present invention, there
is provided a single channel per carrier satellite communications
system in which signals are transmitted in a constant length frame
structure and carrier activation is implemented such that frames
transmitted after reactivation of the carrier are synchronised with
the frame timing of frames transmitted before the deactivation of
the carrier. The interval between transmission of frames may be an
integral number of frame periods, or an integral number of
fractions of a frame period, such as quarters of a frame
period.
[0011] An advantage of this aspect of the invention is that a
receiver may receive and decode the frames transmitted after the
reactivation of the carrier without having to reacquire the frame
timing. Furthermore, carrier activation may be implemented in this
way as an additional feature to an existing satellite SCPC system
without modification of frame formatting protocols.
[0012] According to another aspect of the present invention, there
is provided a method and apparatus of inhibiting transmission of a
block of repeated data by detecting whether the last byte of a
previous block is the same as each byte of the current block and
inhibiting transmission of the current block if this is the case.
Preferably, the carrier on which the blocks are transmitted is
deactivated or reduced in power during the period in which the
current block would otherwise be transmitted.
[0013] According to another aspect of the present invention, there
is provided a method of transmitting a burst of information after a
period of carrier deactivation, in which a constant power level
preamble is transmitted before the information. Advantageously,
this assists in automatic level control of the transmitter.
[0014] Specific embodiments of the present invention will now be
described with reference to the accompanying drawings, in
which:
[0015] FIG. 1 is a diagram of a communications link between data
terminals through a PSTN and a satellite network;
[0016] FIG. 2 is a functional block diagram of a mobile earth
station and its associated interface to a data terminal;
[0017] FIG. 3 is a functional block diagram of a fixed earth
station and its associated interface to a PSTN;
[0018] FIG. 4 shows the channel format used over the satellite link
in a first embodiment of the present invention;
[0019] FIG. 5 is a flowchart of a carrier activation algorithm in
the first embodiment;
[0020] FIG. 6 shows the timing of SCPC frames in the first
embodiment;
[0021] FIG. 7 is a diagram of the frame format used over the
satellite link in a second embodiment;
[0022] FIG. 8 shows an HDLC transmission and reception process
including zero insertion and removal;
[0023] FIG. 9 is a flowchart of a carrier activation algorithm in
the second embodiment;
[0024] FIG. 10 shows the timing of SCPC frames in the second
embodiment;
[0025] FIGS. 11a to 11c shows the timing of SCPC frames and the
contents of encoded blocks transmitted in those frames, in a third
embodiment;
[0026] FIG. 12 is a flowchart of an algorithm performed by the
transmitting MIU on each data block in the third embodiment;
and
[0027] FIG. 13 is a flowchart of an algorithm performed by the
receiving MIU in the third embodiment.
[0028] The overall layout of a satellite communications system,
when used for data communications, is shown in FIG. 1. One example
of such a system is the INMARSAT-B.TM. or INMARSAT-M.TM. satellite
communications system, as described for example in Chapters 12 and
14 of "Satellite Communications: Principles and Applications" by
Calcutt and Tetley, 1st edition, published by Edward Arnold. The
following system is also described in W096/31040, the contents of
which are incorporated herein by reference.
[0029] A mobile DTE 2 is connected via an RS232C interface to a
modem interface unit (MIU) 4. The MIU 4 simulates a
Hayes--compatible modem and is able to decode Hayes-type commands
from the mobile DTE 2, so that off-the-shelf communications
software may be used in the mobile DTE 2. The MIU 4 does not
perform modulation or demodulation in this case, since it is not
connected to an analog line. Instead, the MIU 4 provides an
interface to a mobile earth station (MES) 6 which allows
communication via a satellite 8 to a fixed or land earth station
(LES) 10. The LES 10 is connected to an LES MIU 12 which interfaces
the satellite link to a network 14, in this case a public switched
telephone network (PSTN), and functions as a modem to convert
analog signals on the PSTN 14 to digital signals on the satellite
link, and vice versa. A fixed DTE 18 is connected to the PSTN 14
through a modem 16 of standard type.
[0030] FIG. 2 shows the MES MIU 4 and the MES 6 in greater detail.
The MES MIU 4 comprises a DTE interface 20, which provides an RS232
physical interface and emulates an AT.PCCA type modem, i.e. it
complies with the minimum functional specification for data
transmission systems published by the Portable Computer and
Communications Association (PCCA), including the use of the AT
command set and responses.
[0031] Data received by the DTE interface 20 is sent to a buffer
22, which is in turn connected to an MES interface 24. The MES
interface 24 implements, in ARQ (automatic repeat request) mode, a
variant of the HDLC (High Level Data Link Control) protocol, as
defined in ISO recommendations ISO/IEC 3309, ISO/IEC 4335: 1993 and
ISO/IEC 7809: 1993. The particular version employed is ISO HDLC BAC
3.2, 4, 8, 10, 12 as defined in ISO 7809: 1993 (synchronous,
two-way simultaneous, duplex, non-switched). A controller 26
controls the operation of the interfaces 20 and 24 and the flow of
data through the buffer 22.
[0032] The MES includes an RF modulator/demodulator 27, connected
to an antenna 28, for RF modulating the output of the MES interface
24 and transmitting the output through the antenna 28 to the
satellite 8, and for RF demodulating RF signals received from the
satellite 8 through the antenna 28 and sending the demodulated
signals to the MES interface 24. The MES 6 also includes access
control and signalling equipment (ACSE) 30, for setting up and
clearing the satellite link, which exchanges data with the
controller 26 of the mobile MIU 4.
[0033] The MES ACSE 30 communicates with a network control station
(NCS) which allocates communications channels, supervises
communications traffic through the satellite 8 and communicates
with further ACSE at the LES.
[0034] The mobile MIU 4, MES 6 and ACSE 30 may be integrated in a
mobile unit and the antenna 28 may be integrated or connected
externally with the mobile unit.
[0035] FIG. 3 shows the LES 10 and the LES MIU 12 in greater
detail. The LES MIU 12 includes a modem 31 for demodulating analog
signals from the PSTN 14 and modulating digital signals for the
PSTN 14, and a modem interface 32 which supports modem protocols
such as V.42 error correction, for communication with the modem
16.
[0036] The modem interface 32 is connected through a buffer 34 to
an LES interface 36, which implements protocols compatible with the
MES interface 24, so that data can be exchanged between the LES MIU
12 and the MES MIU 4. A controller 38 supervises the operation of
the modem interface 32, buffer 34 and LES interface 36. The LES
interface 36 is connected to an RF modulator/demodulator 40 which
modulates signals for transmission to the satellite 8 through an
antenna 42, and demodulates signals received from the satellite 8
though the antenna 42. Call set-up and clearing are controlled by
an LES ACSE 44 within the LES 10 which exchanges signals with the
LES MIU 12, the MES ACSE 30, and the network control station
(NCS).
[0037] Although the system described above allows full duplex data
communications, many user applications such as file transfer,
database and email protocols communicate in half-duplex mode for
reasons of design simplicity, even if files are to be sent in both
directions. However, switching off the carrier during a call may
cause the receiver to lose synchronisation with the
transmitter.
[0038] Moreover, in existing satellite communications protocols,
some redundant signalling takes place when there is no user data to
be sent. The carrier could be switched off during this signalling,
but it must be determined which signalling is redundant and which
is necessary.
[0039] In the first embodiment, the MIU connected to both the LES
10 and MES 6 detects whether there is no information or only
redundant information to be transmitted, and if so, sends a signal
to the LES 10 or MES 6, which disables the transmitter thereof
until the MIU indicates that information is ready for transmission.
In the case where the LES 10 is receiving the carrier which is
deactivated, the LES 10 signals this to the LES MIU 12, which
maintains the connection with the PSTN modem 16. For example, if
the V.42 protocol is being used, the LES MIU 12 transmits
flags.
[0040] As described above, the MIU formats the data to be
transmitted into HDLG frames.
[0041] Multiple HDLC frames are formatted into one single channel
per carrier (SCPC) frame, as shown in FIG. 4. The transmission
begins with a header portion P, followed by a sequence of
fixed-length SCPC frames SM.sub.1, SM.sub.2, . . . SM.sub.n. The
end of the transmission is indicated by an end signal E.
[0042] Each SCPC frame SM is subdivided into four sections, each
containing a header H.sub.1, H.sub.2, H.sub.3, H.sub.4, a data
field D.sub.1, D.sub.2, D.sub.3, D.sub.4, and dummy bits (shaded).
The data fields D.sub.1 and D.sub.2 together form one or more HDLC
frame, which is repeated in the data fields D.sub.3 and D.sub.4, to
increase the energy per bit. The contents of each HDLC frame depend
on whether data or control information is being sent.
[0043] If data is being sent, the HDLC frame has an information (I)
format formed from the concatenated data fields D.sub.1 and
D.sub.2. The HDLC frame includes control bytes C containing
acknowledgement and frame number information indicating the
sequence number of the transmitted frame and the sequence number of
the last frame received correctly.
[0044] Line control messages are sent as unnumbered information
(UI) HDLC frames, more than one of which may be contained within
the data fields D.sub.1 and D.sub.2. Flow control messages are sent
in a supervisory (S) HDLC frame format.
[0045] The LES MIU 12 and the MES MIU 4 are programmed to generate
either RR (Receive Ready) or RNR (Receive Not Ready) HDLC flow
control frames when no user data is received and no other HDLC
signalling is required. The flow control frames indicate whether
the MIU is ready to receive more data over the satellite link. In
order to maintain this function, while implementing carrier
activation, the MIU follows the algorithm shown in FIG. 5. The
algorithm is intended as a modification of an existing MIU
functionality and is therefore applied after the framing of data
into HDLC and SCPC frames, including the generation of RR and RNR
frames. The algorithm determines the carrier state which is
signalled to the earth station to which the MIU is connected, in
order to switch off the carrier.
[0046] At the first iteration of the algorithm, at the beginning of
a call, initial values of variables are set as follows:
[0047] Flow control flag, FC=cleared
[0048] Number of redundant flow control frames to be sent, X=1 (or
a higher integer)
[0049] Number of `Establish LCM` to be sent, N.sub.E=3 (or another
positive integer)
[0050] Variable for detecting change in N(R), N.sub.rp=0.
[0051] At step S10, it is detected whether a new SCPC frame has
been composed. At step S20, it is detected whether the SCPC frame
is empty. If so, the carrier state is set as `OFF` (S30) and the
algorithm restarts.
[0052] If the SCPC frame is not empty, the MIU detects (S40)
whether the new SCPC frame is an `Establish LCM` (line control
message) which is transmitted during call set-up to establish the
parameters of the call. If so (S50), the MIU sets the carrier state
as `OFF` (S55) if the counter N.sub.E (number of Establish LCM) is
zero; if N.sub.E is not zero, it is decremented (S60) and the
carrier state is set `ON` (S65). In either case, the algorithm
restarts. As a result, sufficient `Establish LCM` frames are
transmitted to ensure that one is received, before the carrier is
deactivated.
[0053] If the SCPC frame is not an `Establish LCM`, the MIU next
detects (S70) whether N.sub.rp=N(R), where N(R) is a variable
defined in the HDLC protocol and represents the serial number of
the next expected I (information) frame. If the current SCPC frames
contains more than one HDLC frame each having an N(R) value, the
most advanced N(R) value is taken. If N.sub.rp#N(R), Nr, is set to
N(R) (S80), the carrier state is set as `ON` (S95) and the
algorithm proceeds to step SI00.
[0054] If N.sub.rp=N(R), the MIU detects (S90) whether the SCPC
frame contains only RNR or RR HDLC frames. If not, the carrier
state is set as `ON` (S95) and the algorithm proceeds to step S100.
At step S100, the MIU detects whether the SCPC frame includes an RR
frame. If so, the flow control flag FC is cleared (step S110) and
the algorithm restarts. If not, the MIU detects (S120) whether the
SCPC frame includes an RNR frame and sets the FC flag (S130) if it
does. In either case, the algorithm then restarts.
[0055] If the MIU detects at step 90 that the SCPC frame does
contain only RR or RNR frames, this means that no user data is
present, but the MIU must still determine whether the RR or RNR
frames need to be sent to ensure flow control. At step 140, the MIU
determines whether the last frame inside the HDLC frame is an RR or
an RNR frame. If the frame is RNR, the MIU detects (S150) whether
FC is set and if not, sets it (S160) and proceeds to step 190. If
the frame is RR, the MIU detects whether FC is set, and if so,
clears it (S180) and proceeds to step 190.
[0056] At step 190, the carrier state is set as `ON`. The variable
N.sub.FC, which is used as a counter of the number of redundant
flow control indications remaining to be sent, is set (S200) to
X-1, and the algorithm restarts.
[0057] If FC is detected as set at step S150 or as clear at step
170, the MIU then detects (S210) whether N.sub.FC is zero, i.e.
whether no more flow control indications need to be sent. If so,
the carrier state is set to `OFF` (S220) and the algorithm
restarts. If not, the carrier state is set to `ON` (S230), N.sub.FC
is decremented (S240) and the algorithm restarts.
[0058] The state of the carrier is redetermined for each SCPC frame
and a decision is made as to whether to switch the carrier off for
that SCPC frame. The SCPC frame length is constant. Thus, when the
carrier is switched off and then on, the next SCPC frame timing is
aligned with that of the previous transmitted frame, as shown in
FIG. 6. In other words, the period for which the carrier is
switched off is an integral number of SCPC frames.
[0059] A second embodiment of the present invention will now be
described, in which a 64 kbit/s channel is provided by the
satellite link and is used by an ISDN application. In this
embodiment, the network 14 is an ISDN and the satellite 8 has a
multibeam user antenna for communication with the MES 6, in order
to increase the gain of the user link and support a higher data
rate. In this embodiment the LES MIU 12 provides an ISDN interface
to the network 14, while the MES MIU 4 simulates an ISDN terminal
adapter for the mobile DTE 2. Since the MES MIU 4 does not simulate
a modem in this embodiment, it does not decode the Hayes.TM. AT
command set and is preferably integrated with the MES 6. In the
second embodiment, a 16 QAM modulation scheme is used for
transmission, such that transmitted data has a variable power
envelope. Further details of the modulation and coding schemes are
described in co-pending application GB 9804639.4, the contents of
which are incorporated by reference in so far as they relate to a
64 kbit/s satellite channel.
[0060] As shown in FIG. 7, the format used for data transmission in
this embodiment comprises SCPC frames SM.sub.1, SM.sub.2 . . .
SM.sub.n each having as a header a unique word UW to assist
synchronisation in the receiver. The end of a sequence of SCPC
frames is indicated by an end of data signal E. Each SCPC frame
contains two subframes SF1 and SF2. Each subframe SF is encoded
from an input frame IF1, IF2 which contains a data field D of fixed
length. (in this case 2560 bits) and a signalling field S. Each
data field D contains HDLC frames transmitted by an ISDN
application on the mobile DTE 2 or the fixed DTE 18.
[0061] In ISDN applications, an idle state is indicated by
transmitting a continuous sequence of HDLC flags (binary 01111110
or hex 7E). However, user data may coincidentally contain this bit
sequence. Therefore, the applications follow a procedure as shown
in FIG. 8. At P10, the user data is assembled for transmission. At
P20, any sequence of 5 set bits together (11111) is detected and a
zero (0) is inserted after them. The following bits are all shifted
one bit position to allow the zero to be inserted. This technique
is known as `zero insertion`. As a result, the user data cannot
replicate the flag sequence. At P30, HDLC flags are generated if
there is no user data to send and the HDLC frames are
transmitted.
[0062] At P40, the HDLC frames are received by the receiving
application, flags are detected and the user data is separated from
them. At P50, a zero is removed after every set of 5 sequential set
bits, in a reverse operation to that of P20, to restore the user
data to its original form for input to the application at P60.
[0063] The user data is formatted 8-bit bytes and the data field D
comprises an integral number of bytes (320 in this case). However,
zero insertion destroys the original byte alignment of the user
data, so that HDLC flags may no longer appear as binary 01111110.
Instead, the HDLG flags may appear as any of the following bytes
shown in Table 1:
1TABLE 1 HDLC Flag Representation with Bit Shift Number of Bits
Shifted Binary Hex 0 01111110 7E 1 00111111 3F 2 10011111 9F 3
11001111 CF 4 11100111 E7 5 11110011 F3 6 11111001 F9 7 11111100
FC
[0064] In this embodiment, the MIU performs the algorithm shown in
FIG. 9 in order to detect an SCPC frame consisting entirely of
flags, which therefore need not be transmitted. At step T10, the
MIU assembles the data content of the input frames IF1 and IF2 of
the current SCPC frame. At step T20, the MIU checks whether the
value of the last data byte of the preceding SCPC frame had any of
the hex values shown in Table 1 above. If so, the MIU then detects
(T30) whether all of the data bytes in the current SCPC frame are
equal to the last data byte of the preceding SCPC frame. If so,
this indicates that the entire current SCPC frame consists of HDLC
flags and an `idle` state is set (T40). If either of the tests of
T30 and T40 are not satisfied, the `idle` state is not set
(T50).
[0065] If the `idle` state is set, the MIU controls the MES 6 or
LES 10 to which it is connected to switch off the carrier for the
duration of the current SCPC frame. When a transition occurs to the
`idle` state, the MIU appends an end signal E to the end of the
last transmitted SCPC frame, as shown in FIG. 10. Subsequently,
when a transition out of the `idle` state occurs, the new SCPC
frames are transmitted with the same frame timing as the previously
transmitted SCPC frames, so that the start of the new SCPC frame
occurs an integral number of frame periods after the start of the
previously transmitted SCPC frame.
[0066] The receiving MIU, on detecting the end signal E without an
indication from the ACSE that the call has been cleared, determines
that the transmitting MIU has detected an idle state. Since ISDN is
a synchronous protocol, the receiving MIU must continue to transmit
signals to its associated DTE. The receiving MIU repeats the last
byte of the SCPC frame received before the end signal. Since this
has previously been detected by the transmitting MIU to be an HDLC
flag or a bit-shifted version thereof, the repeated bytes will be
detected as HDLC flags by the receiving user application.
[0067] In an alternative to the second embodiment, the MIU
continuously checks the input user data without waiting for
sufficient user data to be received to form a complete SCPC frame,
and an idle state is detected as soon as any 8 consecutive bits
have the binary value `0111110`, for example by reading the input
bits into an 8-bit shift register and continuously comparing the
contents with hex 7E. However, the transmission of the current SCPC
frame cannot be interrupted immediately when a flag is detected
without violating the frame format, so this option does not confer
any advantage in implementing carrier activation and requires a
greater processing overhead than the second embodiment.
[0068] An optional feature of the frame format of FIG. 10 is shown
in dotted outline. In this arrangement, a short preamble P is
transmitted at the beginning of a burst of frames SM, as soon as
the carrier has been reactivated.
[0069] The preamble P comprises a repeated sequence of the same 16
QAM symbol, having a power level equal to the average power level
of the 16 QAM constellation. The sequence comprises 16 symbols
transmitted at a rate of 33.6 kSymbol/s, having a total duration of
476 .mu.s.
[0070] The transmission of the preamble assists in automatic level
control using a feedback loop in a high-power amplifier (HPA) in
the MES RF modulator 27 and the LES RF modulator 40, so that the
transmit power can be ramped up to the required level in 500 .mu.s
or less.
[0071] If the preamble P were not transmitted at the beginning of
each burst, the transmission would begin with a unique word which
does not have a constant power level, and would then not allow the
HPA level to stabilise in the required time.
[0072] In another alternative to the second embodiment, when the
carrier is switched off and new user data is input to the MIU, the
next SCPC frame is transmitted as soon as sufficient data has been
received for one subframe SF and that subframe has been encoded.
Thus, the previous frame timing is lost and the receiver must
acquire the new timing by detecting the unique word UW.
[0073] In a third embodiment illustrated with reference to FIGS. I1
to 13, the MIU divides the baseband data for transmission into
blocks d1 to dn each equivalent to 20 ms duration, shown in FIG.
11a. The blocks containing no user data are shaded. Each frame SM
is of duration 80 ms and so contains four blocks. The MIU performs
a carrier activation algorithm as shown in FIG. 12 on each block,
prior to scrambling and encoding the data for transmission. As
described in GB9804639.4, the coding is performed by a Turbo
encoder including an interleaver into which one 20 ms block is
loaded at a time. The Turbo encoder is reset every 40 ms so that
the Turbo encoding algorithm is performed on 40 ms blocks
corresponding to two 20 ms blocks or one subframe SF. Because the
interleaver has a constraint length of half the total interleaver
size, the Turbo encoder incurs only a 20 ms delay as shown in FIG.
11a. This technique is described in more detail in PCT/GB97/03551.
Hence, the 20 ms blocks are convenient subdivisions of a whole
frame on which to perform carrier activation detection.
[0074] At step U10, the MIU starts processing the next 20 ms block
d. At step U20 the MIU detects whether the block is the first block
in a frame SM. A position pointer X counts the position of the
current block within the frame, so that at step U20, the MIU
detects whether X=0. If X is not zero, this indicates that a
previous block in the current frame has already been sent for
transmission. Because the MIU cannot interrupt a frame SM once
transmission has begun, the current block is then output for
scrambling and encoding at step U30 and the counter X is
incremented modulo 4 at step U40, to indicate the frame position of
the next block to be checked.
[0075] If X is zero, indicating that the block, if transmitted,
will be first block of a frame, then the MIU detects at step U50
whether the last byte of the previous block was equal to hex 7E,
3F, 9F, CF, E7, F3, F9, or FC. If not, this indicates that idle
flags may not be present in the current block and the current block
is output for transmission, at step U60. At step U70, X is set to
1, indicating that the next block will be the second block in the
frame.
[0076] If, on the other hand, the result of the test at step U50 is
positive, the MIU detects at step U80 whether each byte of the
current block is identical to the last byte of the previous block,
as detected at step U50. If not, this indicates that the current
block probably contains at least some data other than flags, so the
data is output for transmission at step U90, and X is set to 1 at
step U100. Otherwise, if the result of the test at step U80 is
positive, the current block is not output for transmission at step
U110, the carrier is turned off, and X is set to zero at step U120.
As shown in FIG. 11b, the 20 ms slot d5 which would have been
output at the beginning of a new frame is not transmitted, and
instead an end signal E is transmitted and the carrier is turned
off for the rest of the 20 ms period. In this case, the block d6
contains user data so that the carrier is turned on and a new frame
Sm.sub.n+1 is transmitted, beginning with block d6. In this way,
although frame synchronisation is not maintained on carrier
reactivation, synchronisation is maintained with blocks which
represent a fraction of the total frame length, so that the
receiver does not need to resychronise to any great extent.
[0077] FIG. 13 shows an algorithm used by an MIU receiving the
transmissions represented by FIG. 11, every time a new frame SM is
received. At step V10, a new frame is demodulated and decoded. At
step V20, the MIU detects whether the frame is followed immediately
by an EOD signal. If not, at step V30 the contents of the received
frame are output to the DTE 2 or 18, but otherwise the MIU detects
at step V40 whether the last byte of the current frame is equal to
hex 7E or its bit-shifted versions. If it is equal to one of these,
at step V50 this last byte is repeatedly output to the DTE 2, 18
until the next frame is received or the call is cleared; this has
the effect of transmitting a continuous series of flags to the DTE.
If the result of step V40 is negative, the MIU outputs hex 7E flags
continuously to the DTE at step V60 until the next frame is
received or the call is cleared.
[0078] The algorithms of FIGS. 9, 12 and 13 are designed
specifically to look for an HDLC hex 7E flag, but may be modified
to look for any repeating byte entirely filling a frame or block,
and to turn the carrier off if the repeating byte is also the last
byte in the previous transmitted frame or block. The receiving MIU
would then output the repeated byte a number of times corresponding
to the period for which the carrier is switched off. Thus, power
can be saved by not transmitting repeated user data, as well as
repeated flags. The receiving MIU infers that the last byte of the
previous frame should be repeated if the carrier is switched off,
but must maintain timing synchronisation to calculate the correct
number of repetitions. However, since the carrier is switched off
for an integral number of blocks or frames, the receiving MIU need
only be able to detect the carrier deactivation interval with a
resolution of one block or frame, so that the local clock reference
of the receiving MIU would be sufficient.
[0079] The above embodiments have been described with reference to
an 8bit HDLC protocol, but are applicable to other communications
protocols with different idle sequences. For example, in a 16-bit
variant of HDLC, the idle flag is hex 7FFE, so the carrier
activation algorithm would look for bit-shifted versions of that
flag instead. Alternatively, some protocols may use an all-zero or
all-one byte (e.g. hex 00 or FF) as an idle flag. In that case,
there would be no need to look for bit-shifted versions of the idle
flag, but the carrier would be deactivated if a block or frame
contained all zeros or all ones. Other protocols use a repeating
sequence of different bytes to indicate an idle state; for example
MPEG-4 uses a repeating sequence of a pseudo-random synchronisation
sequence and a header. If transmitting data under those protocols,
the MIU stores at least the quantity of data from a previous block
or frame corresponding to one repeat period of an idle sequence and
compares this to the contents of the current block or frame to see
if the sequence is repeated throughout the block or frame.
Optionally, the MIU's may be operable with more than one protocol,
each having a different byte length or flag sequence, and the
protocol type is then signalled from the transmitting FIU to the
receiving FIU during call set-up so that the parameters of the
carrier deactivation algorithms can be set appropriately at the the
receiving MIU.
[0080] In the embodiments described above, the carrier transmitted
by either the LES 10 or the MES 6 can be deactivated; in the former
case, satellite power efficiency is improved, while in the latter
case, MES battery power is saved. However, it is not essential that
carrier activation should be implemented in both directions. For
example, carrier activation may be an optional feature of the MES,
so long as the LES 10 is able to perform the necessary reception
protocols if carrier activation is implemented at the MES. The
present invention is not limited to present or proposed
Inmarsat.TM. satellite services, but may be applied to other
satellite data services employing HDLC or other protocols.
[0081] In the above embodiments, a carrier is deactivated
completely if there is only redundant data to be sent.
Alternatively, however, the power level of the carrier could be
reduced and optionally a synchronising sequence such as a unique
word transmitted at reduced power during the deactivation period;
this reduces the power requirements of an MES if implemented on an
MES MIU and of a satellite if implemented on an LES MIU. Hence,
references herein to `deactivating` a carrier encompass the
continued transmission on a carrier at reduced power while not
transmitting any user data or level signalling information.
[0082] In the specific description above, the apparatus is
illustrated in terms of functional blocks, for ease of explanation.
However, these blocks do not necessarily correspond to discrete
physical units.
* * * * *