Note: Descriptions are shown in the official language in which they were submitted.
CA 02050437 1999-04-30
A METHOD AND SYSTEM FOR UNIQUELY IDENTIFYING
CONTROL CHANNEL TIME SLOTS
FIELD OF THE INVENTION
The present invention is directed to a mobile
cellular radio telephone system having digital
voice/traffic capacity wherein digital control channels
may occupy the same radio channel time slots as
voice/traffic channels. More specifically, the present
invention relates to a method and system in which radio
channel time slots are identified as control channel time
slots.
BACKGROUND OF THE INVENTION
The first cellular mobile radio systems in
public use were generally analog systems for the
transmission of speech or other analog information. The
systems comprised a plurality of radio channels for
transmitting analog information between base and mobile
stations by transmitting analog modulated radio signals.
In general, the first cellular mobile radio systems had
comparablx large coverage cells. More recently, digital
cellular mobile radio systems for public use have been
designed.
Digital cellular mobile radio systems coatprise
digital channels for transmitting digital or digitized
analog information between base and mobile stations, by
transmitting digitally modulated radio signals. Digital
cellular mobile radio systems offer substantial advantages
over analog cellular mobile radio systems.
One digital mobile radio system intended to be
used as a common system for many European countries is the
Global System for Mobile Communication (GSM) system. In
European countries already having an analog cellular mobile
system, the new digital GSM system is intended to be
introduced as a new system which is
~~5~~ra~~~
-2-
independent of any existing analog system. The GSM system
base and mobile stations have not been designed to be
compatible with existing systems, however, they are
designed to give optimum performances in various aspects
in and of the system itself. Aecnrdingly, there has been
a comparatively great freedom of choice in technical
matters when designing the GSM system.
Rather than introduce a new independent digital
cellular mobile radio system, like the GSM system, in an
to area with an existing analog cellular system, it has been
proposed to introduce a digital cellular mobile radio
system which is designed for cooperation with the existing
analog cellular mobile radio system. Tn order to obtain
digital channels within the frequency band allotted to
cellular mobile radio systems, there have been proposals
to withdraw a number of radio channels allotted to the
present analog mobile radio systems and use them in the
digital cellular mobile radio system. Due to the proposed
design of the digital mobile radio system, three or
possibly six digital channels may occupy the same
frequency band of one previous analog radio channel by
using time division multiplexing. Accordingly, replacing
some analog channels by digital channels in time division
multiplex may increase the total number of channels.
The intended result is to gradually introduce
the digital system and to increase the number of digital
traffic channels while decreasing the number of analog
traffic channels in the coexisting cellular systems.
Analog mobile stations already in use will then be able to
cantinue to use the remaining analog traffic channels.
Meanwhile, new digital mobile stations will be able to use
the new digital traffic channels. Dual-mode mobile
stations will be able to use both the remaining analog and
the new digital traffic channels.
With the addition of the new digital traffic
~~~f~4~'~
-3-
channels, a corresponding need for new digital control
channels arises. The conventional dual-mode systems for
the most part utilize existing analog chaxrnels, such as
dedicated frequencies, as the control channel.
~~Y S~ TNT INVI~N
The present invention has an objective to allow
for a more flexible use of the existing traffic channels
and the introduction of new control channels for control
signalling. The present invention achieves this objective
by providing a method and system wherein the control
channels of an associated base station directly correspond
to certain time slots of a message frame, which also
comprises time slots being utilized for traffic channels.
The time slots being utilized as control channels are
~.5 readily detected because these time slots are marked with
uniquely defined control channel indicators. In one
embodiment, the control channel indicators may be uniquely
defined synchronization words. In another embodiment,
they maybe uniquely defined control channel
identification words. The mobile station recognizes the
uniquely defined control channel indicators and identifies
those particular time slots as control channels, thus
distinguishing those time slots not being used as control
channel. This operation allows for the channels which
arm riot control channels to be skipped more rapidly, and
therefore the process c~f going through a ranking list of
the strongest signals to be used as control channels will
be executed at a faster rate. Each of the unique control
channel indicators associated with a specific tide slot is
distinsJuishable from corresponding bits in corresponding
fields associated with adjacent time slots.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1. illustrates a part of a cellular mobile
radio system with cells, a mobile switching center, base
stations and mobile stations;
Fig. 2 illustrates a block diagram of a mobile
station utilized in accordance with the present invention;
Fig. 3 illustrates a block diagram of a base
station utilized in accordance with the present invention;
Fig. ~A illustrates the frame structure of a
r0 radio channel as utilized in accordance with the present
invention;
Fig. 4B illustrates a digital control channel
time slot format for transmissions from a mobile station
to a base or land station in ane embodiment of the present
invention;
Fig. 4C illustrates a digital control channel
time slot format for transmissions from a base or land
station to a mobile station in one embodiment of the
present invention;
Fig. ~D illustrates a digital control channel
time slot format for transmissions from a mobile station
to a base or land station in another embodiment of the
present invention;
Fig. ~E illustrates a digital control channel
time slot format for transmissions from a base or land
station to a mobile station according to another
embodiment of the present invention;
Fig. 5 illustrates an example of a radio channel
frame having six time slots in which control channels and
traffic channels are mixed;
Figs. 6A and 6B illustrate examples of a message
frame which comprises control channels and traffic
channels utilizing the unique Sync Words in accordance
with one embodiment of the present invention; and
Figs. 6C and fD illustrate examples of a message
_5_
frame which comprises control channels and traffic
channels utilizing unique control channel identification
words in conjunction with the time slot sync words to form
unique extended Sync Words in accordance with another
embodiment of the present invention.
DETAILED DESCgtIpTIOId OF THE ILLUSTRATED EMBO_DIMEPdTS
Fig. 1 illustrates ten cells C1 to C10 in a
cellular mobile radio system. In actual practice, the
method and means according to the present invention are
implemented in a cellular mobile radio system comprising
many more cells than ten. However, for the purpose of
explaining the present invention, ten cells is deemed to
be sufficient.
For each of the cells C1 through C10 there is a
base station B1 through B10, respectively, with the same
number as the cell. Fig. 1 illustrates base stations
situated in the vicinity of the center of the cell and
having omni-directional antennas. The base stations of
adjacent cells may, however, be allocated in the vicinity
of cell borders and have directional antennas as is well
known to those of ordinary skill in the art.
Fig. 1 also illustrates ten mobile stations M1
through M10 which are movable within a cell and from one
yell to another cell. In actual practice, the method and
means according to the present invention are implemented
in a cellular mobile radio system comprising many more
mobile stations than ten. In particular, there are
usually many more mobile stations than there are base
stations. However, for the purpose of explaining the
present invention, the use of ten mobile stations is
deemed to be sufficient. The system of Fig. 1 also
includes a mobile switching center MSC. The mobile
switching center is connected to all ten illustrated base
stations by cables. The mobile switching center is also
CA 02050437 1999-04-30
-6-
connected, by cables, to a fixed public switching
telephone network or similar fixed network with ISDN
facilities. All cables from the mobile switching center
to the base stations and cables to the fixed network are
not illustrated.
In addition to the mobile switching center
illustrated, there may also be another mobile switching
center connected by cables to other base stations than
those illustrated in Fig. 1. Instead of cables, other
means may be utilized for base to mobile station switching
center communication, e.g., fixed radio links.
The cellular mobile radio system illustrated in
Fig. 1 comprises a plurality of radio channels for
communication. The system is designed both for analog
information, e.g., speech, digitized analog information,
digitized speech, and pure digital information. According
to the system, the term connection is used for a
communication channel established between a mobile station
and another mobile station in the same system or another
system, or a fixed telephone or terminal in a fixed
network connected to the cellular mobile radio system.
Thus, a connection may be defined as a call where two
persons are able to talk to each other, but also may refer
to a data communication channel where computers are
exchanging data.
Referring now to Fig. 2, an embodiment of a
mobile station that can be utilized in a cellular
telephone system that operates in accordance with the
present invention is illustrated. A speech coder 101
converts the analog signal generated by a microphone into
a bit data stream. The bit data stream is then divided
into data packages, according to the Time Division Multiple
Access (TMDA) principle. A fast associated control channel
(FACCH) generator 102 generates control and supervision
signaling messages between the system and the mobile
station and messages
-7-
between the mobile station and the system. The FACCH
message replaces a user frame (speech/data) whenever it is
to be transmitted. A slow associated control channel
(SACCH) generator 103 provides a continuous channel for
the exchange of signalling messages between the base
station and the mobile station and vice-versa. A fixed
number of bits, e.g., twelve, is allocated to the ~ACCH
for each time slot of the message train. Channel collars
104 are respectively connected to the speech collar 101,
FACCH generator 102, and SACCH generator 103 for
manipulating the incoming data in order to carry out error
detection and correction. The techniques used by the
channel collars 104 are convolutional encoding, which
protects important data bits in the speech code, and
cyclic redundancy check (CRC), wherein the perceptually
significant bits in the speech collar frame, e.g., twelve
bits, are used for computing a seven bit check.
A selector 105 is connected to the channel
collars 104 associated with the speech collar 101 and the
FACCH generator 102, respectively. The selector 105 is
controlled by the microprocessor controller 130 so that at
appropriate times user information over a particular
speech channel is replaced with system supervision
messages aver FACCH. A two-burst interleaver 106 is
coupled to the output of the selector 105. Data to be
transmitted by the mobile station is interleaved over two
distinct time slots. The 260 data bits, which constitute
one transmitting word, are divided into two equal parts
and are allotted two consecutive time slots. The effects
of RAYLEIGH fading will be reduced in this manner. The
output of the two-burst interleaver 106 is provided to the
input of a modulo-two-adder 107 so that the transmitted
data is ciphered bit by bit by logical modulo-two-addition
of a pseudo-random bit stream.
The output of the channel collar 104 associated
CA 02050437 1999-04-30
-g-
with the SACCH generator 103 is connected to a 22-burst
interleaver 108. The 22-burst interleaver 108 interleaves
data transmitted over SACCH over 22 time slots each
consisting of 12 bits of information. The 22-burst
interleaver 108 utilizes the diagonal principle so that as
two SACCH messages are transmitted in parallel, the second
message is displaced eleven bursts from the other message.
The mobile station further includes a Sync Word
- DVCC generator 109 for providing the appropriate
synchronization word (Sync Word) and DVCC which are to be
associated with a particular connection. The Sync Word is
a 28 bit word used for time slot synchronization and
identification. The DVCC (digital verification color
code) is an 8-bit code which is sent by the base station
to the mobile station and vice-versa, for assuring that
the proper channel is being decoded.
A burst generator 110 generates message bursts
for transmission by the mobile station. The burst
generator 110 is connected to the outputs of the
modulo-two-adder 107, the 22-burst interleaver 108, the
Sync Word/DVCC generator 109, an equalizer 114, and a
control channel message generator 132 generating channel
coded control messages. A message burst comprising data
(260 bits), SACCH (12 bits), Sync Word (28 bits), coded
DVCC (12 bits), and 12 delimiter bits, combined for a
total of 324 bits are integrated according to the time
slot format specified by the standard Electronic Industries
Alliance/Telecommunications Industry Association (EIA/TIA)
IS-54. Under the control of the microprocessor 130, two
different types of message bursts are generated by the
burst generator 110: control channel message bursts from
the control channel message generator 132 and voice/traffic
message bursts. The control channel message is generated
in accordance with commands from the microprocessor 130 and
is sent on a digital control channel having the same burst
formats as traffic channels but where the SACCH as
CA 02050437 1999-04-30
_g_
well as the speech data normally generated in a
voice/traffic burst are replaced by control information.
The transmitting of a burst, which is equivalent
to one time slot, is synchronized with the transmitting of
the other two time slots, and is adjusted according to the
timing provided by the equalizer 114. Due to time
dispersion, an adaptive equalization method is provided in
order to improve signal quality. For further information
regarding adaptive equalization techniques, reference
should be made to U.S. Patent 5,088,108, issued February
11, 1992, and assigned to the same
assignee. A correlator adjusts to the timing of the
received bit stream. The base station is the master and
the mobile station is the slave with respect to frame
timing. The equalizer 114 detects the incoming timing and
synchronizes the burst generator 110. The equalizer 114
also checks either the Sync Word and DVCC or a control
channel identification word for identification purposes as
will be explained in greater detail below.
A 20ms frame counter ill is coupled to the burst
generator 110 and so is the equalizer 114. The frame
counter 111 updates a ciphering code utilized by the
mobile station every 20ms, once for every transmitted full
rate frame. It will be appreciated that according to this
particular example, three time slots make up one frame. A
ciphering unit 112 is provided for generating the
ciphering code utilized by the mobile station. A pseudo
random algorithm is preferably utilized. The ciphering
unit 112 is controlled by a key 113 which is unique for
each subscriber. The ciphering unit 112 consists of a
sequencer which updates the ciphering code.
The burst produced by burst generator 110, which
is to be transmitted, is forwarded to an RF modulator 122.
The RF modulator 122 is operable for modulating a carrier
frequency according to the ~r/4DQPSK method (~/4 shifted,
~~~~~t~~ ~ . . .
_vo_
Differentially encoded Quadrature Phase Shift Iteying).
The use of this technique implies that the information is
differentially encoded, i.e., 2 bit symbols are
transmitted as four possible changes in phase; ~ ~r/4 and ~
3~rJ4. The transmitter carrier frequency supplied to the
RF modulator 122 is generated by a transmitting frequency
synthesizer 124 in accordance with the selected
transmitting channel. Before the modulated carrier is
transmitted by an antenna, the carrier is amplified by a
power amplifier 123. The RF power emission level of the
carrier frequency is selected on command by a
microprocessor controller 130. The amplified signal is
passed through a time switch 134 before it reaches the
antenna. The time switch 134 is synchronized to the
transmitting sequence by the microprocessor controller
130.
A receiver carrier frequency is generated in
accordance with the selected receiving channel by
receiving frequency synthesizer 125. Incoming radio
frequency signals are received by receiver 126, the
strength of which are measured by signal level meter 129.
The received signal strength value is then sent to the
microprocessor controller 130. An RF demodulator 127
which receives the receiver carrier frequency from the
receiving frequency synthesizer 125 and the radio
frequency signal from receiver 126, demodulates the radio
frequency carrier signal, thus generating an intermediate
frequency. The intermediate frequency signal is then
demodulated by an IF demodulator 128, which restores the
original n/4-DQPSK - modulated digital information.
The restored information provided by IF
demodulator 128 is supplied to the equalizer 114. A
symbol detector 115 converts the received two bit symbol
format of the digital data from the equalizer 114 to a
single bit data stream. The symbol detector 115 in turn
CA 02050437 1999-04-30
-11-
produces three distinct outputs. Control channel messages
are sent to a control message detector 133 which supplies
channel decoded and detected control channel information
to the microprocessor controller 130. Any speech
data/FACCH data is supplied to a modulo-two-adder 107 and
a two-burst deinterleaver 116. The speech data/FACCH data
is reconstructed by these components by assembling and
rearranging information from two consecutive frames of the
received data. The symbol detector 115 supplies SACCH
data to a 22-burst deinterleaver 117. The 22-burst
deinterleaver 117 reassembles and rearranges the SACCH
data, which is spread over 22 consecutive frames.
The two-burst deinterleaver 116 provides the
speech data/FACCH data to two channel decoders 118. The
convolutionally encoded data is decoded using the reverse
of the above-mentioned coding principle. The received
cyclic redundancy check (CRC) bits are checked to
determine if any error has occurred. The FACCH channel
decoder furthermore detects the distinction between the
speech channel and any FACCH information, and directs the
decoders accordingly. A speech decoder 119 processes the
received speech data from the channel decoder 118 in
accordance with a speech decoder algorithm (Vector Sum
Excited Speech Coding, or VSELP), and generates the
received speech signal. The analog signal is finally
enhanced by a filtering technique. Messages on
the fast associated control channel are detected by FACCH
detector 120, and the information is transferred to the
microprocessor controller 130.
The output of the 22-burst deinterleaver 117 is
provided to a separate channel decoder 118. Messages on the
slow associated control channel are detected by SACCH
detector 121, and that information is transferred to the
microprocessor controller 130.
The microprocessor controller 130 controls the
mobile station activity and the base station
communication, and also handles the terminal keyboard
input and display output 131. Decisions by the
microprocessor controller 130 are made in accordance with
received messages and measurements made. The keyboard and
display unit 131 enable an information exchange between
the user and the base station.
Fig. 3 illustrates an embodiment of a base
station that can be utilized in a cellular telephone
system that operates in accordance with the present
invention. The base station incorporates numerous
component parts which are substantially identical in
construction and function to component parts of the mobile
station illustrated in Fig. 2 and described in conjunction
therewith. Such identical component parts are designated
in Fig. 3 with the same reference numerals utilized
hereinabove in the description of the mobile station, but
axe differentiated therefrom by means of a prime
designation.
There are, however, minor distinctions between
the mobile and base stations. For instance, the base
station has two receiving antennas. Associated with each
of these receiving antennas are a receiver 1261, an RF
demodulator 1271, and an IF demodulator 1281.
Furthermore, the base station does not include a user
keyboard and display unit 131 as utilized in the mobile
station. Another difference is that a base station
handles the communication of many mobiles as is
represented in Fig. 3 as three channel controllers 1, 2,
and 3, each of which handles one of three time slots of
one frequency.
6~hen power is applied to the mobile station, the
microprocessor controller 130 executes an initialization
procedure. Initially, the serving system parameters are
retrieved, meaning that the preferred system, e.g.,
wire-line (B) or nonwireline (A), is selected. Depending
:~~~~~~ R
~13°
on the choice made, the scanning of the dedicated control
channels belonging to the preferred system starts.
The receiving frequency synthesizer 125 is
ordered by the microprocessor controller 130 to generate
the frequency which corresponds to the first dedicated
control channel. When the frequency is stable, the signal
level meter 129 measures the signal strength, and
thereafter the microprocessor controller 130 stores the
signal strength value. The same procedure is performed
for the frequencies corresponding to the remaining
dedicated control channels, and a ranking based on the
signal strength of each is made by the microprocessor
contraller 130. The receiving frequency synthesizer 125
is then ordered to tune to the frequency with the highest
signal strength level so that the mobile station will be
able to make attempts to synchronize to that channel.
The radio signal is captured by the receiver J126
and is demodulated according to the selected carrier
frequency by Rf modulator 127, and then demodulated by Tf
demodulator 128. Synchronization and primary analysis of
the digital information in the radio signal is made in the
equalizer 114. If the equalizer 114 manages to detect a
Sync Word or control channel identification word identical
to the Sync Word or control channel identification word
stored in Sync Word generator 109, respectively, the
equalizer 114 will lock to the time slot associated with
that Sync Word or control channel identification word.
The mobile station waits for the system parameter overhead
message decoded by the control channel message detector
1.33 and transferred to the microprocessor controller 130.
This message contains information about the identification
of the system, the protocol capability, the number of
available paging channels (PC), and their specific
frequency allocation.
In the situation where the equalizer 114 is not
_1~_
able to recognize the Sync Word or control channel
identification word within a specified period of time, the
receiving frequency synthesizer 125 is ordered by the
microprocessor controller 130 to tune to the channel with
the next strongest signal. If the mobile station is
unable to synchronize at this second choice, the
microprocessor controller 130 orders a change of the
preferred systems, e.g., from A to B or vice versa.
Thereafter, the scanning of the dedicated control channels
of the new preferred system will begin.
When the mobile station has received the system
parameter overhead message, the paging channels are
scanned in the same manner as the dedicated control
channels, i.e., by measuring the signal strength and
selecting the frequency with the strongest signal.
Synchronization to the paging channels is then performed
accordingly.
Upon successful synchronization on a paging
channel, the mobile station will leave the initialization
procedure and start an idle mode. The idle mode is
characterized by four states, which are controlled by the
microprocessor controller 130, and which are sequentially
looped through as long as no access to the system is
initiated. Tt should be noted that the scanning of the
paging channels is performed whenever the bit error rate
on the current paging channel increases above a certain
level in order assure that the mobile station is listening
to the paging channel with the strongest signal strength.
The first state associated with the idle mode is
a continuous updating of the mobile station status, e.g.,
the number of and the location of existing access channels
(AC). This information is carried to the mobile station
in the system parameter overhead message on the paging
channel, often referred tc as a digital forward control
channel (DFOCC). This message is decoded in the control
channel message detector 133 and sent to the
microprocessor controller 130. Certain messages
transmitted from the base station in the system parameter
overhead message demand responding actions from the mobile
station, e.g., a rescan message will order the
microprocessor controller 130 to restart the
initialization procedure. As another example, a
registration identity message from the base station will
force the mobile station to make a system access in order
to register in accordance with the system access mode
described hereinbelow.
The second state associated with the idle mode
relates to the situation where the mobile station attempts
to match page messages transmitted by the base station.
These mobile station control messages, which are sent over
the DFOCC, are decoded in the control channel message
detector 133 and analyzed by the microprocessor controller
7.30. if the decoded number matches the identification
number of the mobile station, a connection to the base
station will be prepared in the system access mode.
The third state of the idle mode involves
listening to orders sent by the base station over the
DFQCC. Decoded orders, such as an abbreviated alert, will
be processed by the mobile station accordingly.
The fourth state in the idle mode involves the
microprocessor controller 130 supervising the input from
keyboard 131 for user activity, e.g., call initiating. A
call origination results in the mobile station leaving the
idle mode and starting the system access mode.
one of the primary tasks in the system access
mode of the mobile station is the mobile station
generating an access message. The access channels (P.C)
available to the mobile, which were updated during the
idle mode, are now examined in a manner similar to the
measuring of the dedicated control channels as previously
described. A ranking of the signal strength of each is
made, and the channel associated with the strongest signal
is chosen. The transmitting frequency synthesizer 124 and
the receiving frequency synthesizer 125 are tuned
accordingly, and a service request message is sent over
the selected channel in order to inform the base station
about the type of access wanted, e.g., call origination,
page response, registration request or order confirmation.
After completion of this message, the amplifier 123 of the
mobile station is turned off and the mobile station waits
for further control messages on the DFOCC. Depending on
the access type, the mobile station will then receive an
adequate message from the base station.
If the access type is origination or paging, the
mobile station is assigned a free traffic channel by the
base station, and the mobile station leaves the system
access mode to occupy that traffic channel. The mobile
station will then tune the transmitting frequency
synthesizer 124 and the receiving frequency synthesizer
125 to the frequencies associated with the chosen traffic
channel. Thereafter, the equalizer 114 starts
synchronizing. A time slot alignment procedure is
controlled by the base station and is based on time delay
measurements which are performed at the base station on
the received signal. From this moment on, control
messages exchanged between the base station and the mobile
station are transferred over the fast associated control
channel (FACCH) and the slow associated control (SACCH).
Piessages from the microprocessor controller 130
are generated by the FACCH generator 102 or the SACCH
generator 103, and data is error protection coded in the
channel coder 104. The FACCH data is time multiplexed
with speech data in the multiplexer 105, and interleaved
over two bursts by the two burst interleaver 106. The
data is then encrypted in the madulo-2 adder 107, which is
~~~~4~~
-17- . .
controlled by the ciphering algorithm generated by
ciphering unit 112. The SACCH data is interleaved over 22
bursts by 22 burst interleaver 108, and is then supplied
to the burst generator 110 where the SACCH data is mixed
with speech data, FACCH data, the Sync Word, and the Dt7CC
from DACG generator 109. The RF modulator 122 modulates
the bit pattern according to the n/4--DQPSK principle. The
power amplifier 123 is activated, and the power level is
controlled by the microprocessor controller 130 wring the
time of the transmitted time slot.
Control messages from the base station to the
microprocessor controller 130 of the mobile station axe
also transferred via the FACCH and SACCH. The symbol
detector 115 converts the received four symbol pattern to
a bit data stream which is directed to the speech decoder
119, FACCH detector 120, or SACCH detector 121, depending
on the type of data used. Speech data and FACCH data are
decrypted by the modulo-2 adder 107, and the two burst
deinterleaver 116. The channel decoders 118 detect bit
errors and inform the microprocessor controller 130
accordingly. The SACCH is deinterleaved over 22 bursts by
22-burst deinterleaver 117 before error detection is
carried out in channel decoder 118.
Messages transmitted from the base station to
the mobile station typically include alerting orders,
requests to perform channel quality measurements, release
call, and hand-off orders. Messages transmitted in the
opposite direction are those initiated by the mobile
station user, e.g., the release order. The last order
implies that the user is finished with the call, and the
mobile station will leave the control of the traffic
channel and return to the initialization mode of
operation.
Figs. 4A - ~E illustrate the structure of the
digital channels according to the present invention and
~d~~~~-~ i'
_ig_
compatible with the digital traffic channels according to
the HIA/TIA standard IS-54. Fig. 4A depicts the frame
structure of a radio channel. According to this example,
one radio channel frame consists of typically six time
slots which include a total of 1,944 bits or 972 symbols.
The frame is 40 ms in length with a data transmission rate
of 25 frames per second. each of the time slots are
typically numbered from 1-6, each respectively including
Sync Words of 28 bits as defined above.
Figs. 418 and 4C illustrate a time slot format
far transmissions from the mobile station to the land
station and from the land station to the mobile station,
respectively, for one embodiment of the present invention.
The time slot formats commonly include 260 bits reserved
fox data transmission, 12 bits for a digital verification
color code (DVCCj, 12 bits for a slow associated control
channel (SACCHj, and 28 bits for synchronization and
training data (SYNCj. The slot format from the mobile
station to the land station includes two 6 bit blocks for
guard time (Gj and ramp time (Rj information. The slot
format from the land station to the mobile station
includes a 12 bit block which is reserved for future uses.
In a half rate alternative, each half rate
voice/traffic channel utilizes one time slot of each
frame. This implies that one frame comprises six half
rate traffic channels with the slots being numbered
sequentially 1, 2, 3, 4, 5, 6. According to the full rate
alternative, each full rate traffic channel utilizes two
equally spaced time slots of the frame, e.g., 1 and 4, 2
and 5, or 3 and 6. In this alternative, the time slots
are numbered 1, 2, 3, and the configuration of one frame
will therefore be 1, 2, 3, 1, 2, 3. It will be
appreciated that for purposes of explanation it will be
assumed that the present invention utilizes the full rate
alternative for the following examples.
CA 02050437 1999-04-30
-19-
Figs. 4D and 4E illustrate a time slot format
structure of the digital control channels according to
another embodiment of the present invention. Fig. 4E
depicts the frame structure of the Digital Forward Control
Channel (DFOCC) downlink from base to mobile and Fig. 4D
depicts the structure of the Digital Reverse Control
Channel (DRECC) uplink from mobile to base. In comparing
the uplink and downlink control channel formats in Figs.
4D and 4E with the corresponding traffic channel formats
in Figs. 4B and 4C, it can be seen that the formats are
the same except for the SACCH and Coded Digital
Verification Color Code (CDVCC) fields.
The present invention addresses the problems
which occur when the control channels of an associated
base station are utilizing time slots in radio channel
frames which also comprise time slots being utilized for
traffic channels. According to the operation of the
mobile station of the present invention, there are four
distinct occasions when the mobile station is required to
scan the control channels. The first occasion is during
the initialization procedure when the dedicated control
channels are scanned. The second occasion is during the
initialization procedure and in the idle mode when the
paging channels are being scanned. The third occasion is
during the system access mode when the access channels are
scanned. Finally, the paging channels are also scanned
after a call termination.
It is now assumed that the mobile station during
these paging occurrences is aware of which radio channel
frequencies should be scanned and that the scanning
procedure as described above is performed. A ranking of
the signal strength of each radio channel frequency is
made as previously described, and preferably more than two
frequencies should be stored in the ranking list. The
next processing step will be to synchronize the mobile
station to one of the stored frequencies. However, in
:~U:~~~-~ a°
-20-
this example, a problem arises in that the control
channels searched for are mixed with the traffic channels
as illustrated in Fig. 5. Fig. 5 illustrates a
standardized frame which comprises both control channels
and traffic channels. In this example, time slots I and 4
are utilized as control channels and time slots 2, 3, 5
and S are utilized as traffic channels.
A conventional approach to this problem of
distinguishing which slot is a control channel and which
slot is a traffic channel, is programming the
microprocessor controller x.30 to detect this distinction.
Unfortunately, this approach requires significant signal
processing of the received signal including channel
decoding and ARC calculations before a control channel may
be distinguished from a traffic channel. In addition, if
the data transmitted is ciphered, channel distinction
would further be delayed by the deciphering required of
the received signal.
The present invention offers alternative
solutions to this problem. In a first preferred
embodiment, the time slot being used as a control channel
may be detected faster if these time slots are marked with
uniquely defined Sync Words generated by the Sync Word
generator 1091 of the base station. In this situation,
the equalizer 3.14 of the mobile station recognizes the
uniquely defined Sync Wards and identifies those
particular time slats as control channels, thus
distinguishing the time slots which are not being used as
traffic channels. This operation allaws for those
channels which are not control channels to be eliminated
rapidly so that the process of ranking the strongest
control channel signals may be executed at a much faster
rate.
Figs. 6A and 6~ illustrate examples of the
implementation of the unique Sync Words (ESW) which define
~C~,~U~~'~
-~ 1- . . .
the time slot as a control channel. As the Sync Words
associated with the six time slots in the frame according
to the standard IS-54 are referred to as Sync Words 1, 2,
4, 5, 6, it is proposed that the unique control channel
Sync Words be referred to as Sync Words 7, 8, and 9 for
purposes of explanation. Generally, unique Sync Word 7
will relate solely to time slots 1 and 4, Sync Word 8
relates solely to time slots 2 and 5, and Sync Word 9
relates solely to time slots 3 and 6.
1~ Fig. 6A illustrates one frame which comprises
one control channel and two traffic channels. Time slots
1 and 4 are utilized as control channels, and therefore
have associated therewith the unique Sync Word 7. Time
slots 2, 3, 5 and f are being used as traffic channels,
and therefore have associated therewith the conventional
Sync Words 2 and 3. Fig. 5B illustrates one frame which
comprises two control channels and one traffic channel.
Time slots 1, 2, 4 and 5 are used as two separate control
channels and have associated therewith unique Sync Words 7
and 8, respectively. Time slots 3 and 6 are being
utilized as a traffic channel, and therefore have
associated therewith the conventional Sync Word 3.
The number of unique Sync Words required to
implement the present invention varies depending on the
number of time slots of the radio channel frame and the
maximum number of time slots available for use as separate
traffic channels or as separate control channels. The
following examples demonstrate this point.
In a preferred embodiment described having a
six--slot, radio channel frame using full- or half-rate
traffic channels and full-rate control channels, nine
unique Sync Words are required if every time slot can be
used either as a traffic channel or a control channel.
however, if only four out of six time slats can be used as
~~5~~~';v' ..
--22-
two full rate control channels and all six time slots can
be used either as full--rate or half-rate traffic channels,
only eight unique Sync Words are required. Tn a half-rate
traffic channel, including two radio channel frames, each
having six time slots, twelve unique Sync Words are
required if every time slot is to be used either as a
full-rate or half-rate traffic ar control channel.
When the radio channel frame includes only four
time slots and every time slot can be used as either a
full-rate or a half-rate traffic ar control channel, then
eight unique Sync Wards are required. Tn a situation
where the radio channel frame comprises only four time
slots and only two out of four time slots can be used
either as full-rate or half-rate traffic or control
channels, then only six unique Sync Words are required.
In a similar fashion, when the radio channel frame
includes only three time slots and every time slot can be
used either far a full-rate, separate traffic channel or a
full-rate, separate control channel, then six unique Sync
words axe required. When the radio channel frame includes
more than six time slats, e.g., eight slots, more than
twelve unique Sync Words may be required depending on the
number of slots per channel and the number of time slots
that may be used as either traffic or control channels.
Thus, for example, sixteen unique Sync Words may be
required.
The set of unique Sync Wards performs dual
functions of synchronization and channel identification.
7Ln general, every Sync Word should have gaod correlatian
SO properties, meaning that each Sync Ward should have a low
correlation with other Sync Wards regardless of whether
they are in-phase or out-of-phase with each other.
Moreover, each Sync Word should also exhibit low
correlation with itself when cut of phase. Thus, Sync
Words need to be determined within the context of a set of
-23-
Sync Words. Eecause longer Sync Words provide better
correlation properties, the larger the number of unique
Sync Words required, the longer the Sync Words must be to
meet the necessary correlation requirements.
Unfortunately, longer Sync Words require more space in a
burst and reduce space for traffic information, like
speech or data. Thus, the determination of a set of Sync
Words is often a compromise between correlation properties
and length.
While there are various methods for correlating
Sync Words, one method for correlating binary Sync Words
is as follows. If each binary Sync Word includes n
digits, there are 2° possible Sync Word combinations. A
total of m Sync Words having good correlation properties
may then be chosen from the 2° possible Sync Words. A
computer may be programmed to test all possible Sync Word
combinations provided that n is relatively small when
compared to the number of arithmetic operations the
computer can perform per unit time. Such testing is
accomplished efficiently by first excluding Sync Words
that have a high correlation with themselves when out of
phase before correlations of different words are tested.
Of course, another method for determining a set
of Sync Words is to use Sync Words already known and
disclosed in the prior art literature. For example, if
six or less 28-bit unique Sync Words are required, the
Sync Words disclosed in the TIA IS-54 standard on U.S.
cellular systems may be used. If eight or less 26-bit
Sync Words are required, the Sync Words of the
pan-European digital cellular system, abbreviated GSM, may
be used.
For a given set of known Sync Words, various
adaptations can be made to that set. For example, when a
large number of Sync words are required and long Sync
Words are acceptable, but only a small group of shorter
~Y~~4~ a
-24- . . .
Sync Words are available, the large number of Sync Words
may be obtained by uniquely combining the shorter Sync
Words. From P shorter unique Sync Words, a maximum number
of P (P+1)/2 unique Sync Words may be obtained which are
twice as long as the short Sync Words. Thus, from the
short Sync Words A, B, C and D, long Sync Words AA, AB,
AC, AD, BB, BC, BD, CC, CD, and DD may be obtained.
It may be desirable or necessary in some
situations not to include additional Sync Words to
designate those time slots which are control channels.
Another preferred embodiment of the present invention
offers an effective approach to resolve such a necessity
as well as the problems with the conventional approach
described above. A time slot being used as a control
channel may be detected simply and quickly if those
contral channel time slots are identified with an
unencrypted uniquely defined control channel
identification word or words (CCIW), as illustrated in
Figs. 4D and 4E. The SACCH and CDVCC fields of the
control channels include a control channel identification
word (CCIW)~. The SACCH and the GDVCC are not required on
the control channel and their associated fields are used
in this embodiment of the present invention to identify a
time slot as a control channel. Although not illustrated,
any number of the control related bits, including the
SACCH and/or CDVCC bits, may be used for the purpose of
identifying time slots being used as control channels as
opposed to a traffic channel. One advantage of this
embodiment is that control fields are never encrypted so
that any de-encryption is unnecessary. Another advantage
is that no additional Sync Words need be defined.
As an example, a control channel identification
ward may be generated using simply the 12 bits of the
CDVCC field. Out of the total naunber of possible CDVCC
codes which may be generated in the CDVCC field, one or
"~~~U~~'a'
-25° . . .
more of those codes is reserved for use as control channel
identification word(s). Of course, for purposes of
implementing the present invention, only one CCIW is
necessary to designate a time slot as a control channel.
However, if more than one CCIW is used, the number of
different control channel identification words should not
be too great, otherwise the number of CDVCC codes will be
too restricted. To avoid bit patterns that are typically
generated by the SACCH and/or CDVCC codes and to achieve
low correlation between the CCIW's and the SACCH's and/or
CDVCCIS, only two different CCIW~s are generated from the
24 bits of the SACCH and the CDVCC. Accordingly, the 24
bit control channel identification word (CCIW) is used to
indicate that a time slot is a control channel. The 24
bit Sync Word is used only as a synchronization pattern to
indicate the specific time slot number (time slot 1 to
time slot 6).
If two or more different 24 bit control channel
identification words are designated, each CCIW not only
indicates that a channel is a control channel but also
transmits information from the mabile to the base station
or vice versa. In other words, for example, a first and
second CCIW may differ. This difference is equivalent to
one bit of information which may be used to communicate
any number of useful messages between the mobile and base,
e.g., when using several control channels including
access, authentication, and ciphering functions requiring
more than one time slot. The two patterns of 24 bits each
must be chosen. to have good correlation properties,
meaning that each CCIW should have a low correlation with
the other CCIW regardless of whether they are in-phase or
out-of-phase with each other. Moreover, each CCIW should
also exhibit low correlation with itself when out of
phase.
The control channel identification word or words
~s~~C~~a~ a~ .. _
are generated by the Sync Word generator 1091 of the base
station in the downlink direction from base to mobile
station in order to allow the mobile station to quickly
scan the control channels. As described earlier, the
CCIW's allow for thane channels which are not control
channels to be eliminated rapidly to increase the speed of
selecting the strongest control channel signals.
Moreover, no additional Sync Words are required to
identify which time slots axe control channels.
Figs. 6C and 6D illustrate examples of the
implementation of the control channel identification words
which define a time slot as a control channel. As the
Sync Words associated with the six time slots in the frame
according to the standard IS°54 are generally referred to
as Sync Words 1, 2, 3, ~, 5, 6, for purposes of describing
Figs. 6C and 6D in a manner similar to the description of
Figs. 6A and 5E, the unique control channel indicators on
the control channels are referred to as extended Sync
Words (ESW) X1, X2, and X3. Specifically, the extended
Sync Words (ESW) include not only the standard Sync Words
1°6 but also the control channel identification words in
the SACCH and CDVCC fields.
Fig. 6C illustrates one frame which includes one
control channel and two traffic channels. Time slots 1
and 4 are utilized as control channels, and therefore have
associated therewith the unique extended Sync Word (ESW)
X1. Time slots 2, 3, 5 and 6 are used as traffic channels
and therefore have associated therewith Sync Words 2 and
3. Fig. 5D illustrates one frame which comprises two
control channels and one traffic channel. Time slots 1,
~ and 5 are used as two separate control channels and
have associated therewith unique extended Sync Words (ESW)
X1 and X2, respectively. Time slots 3 and 6 are utilized
as a traffic channel, and therefore have associated
therewith the conventional Sync Word 3.
,~e~~~~a~ ~i . .
Accordingly, the present invention presents a
method and system which allows for a more flexible use of
the existing radio channels and the introduction of new
control channels for control signalling. The method and
system involve the use of control channels of an
associated base station which directly correspond to
certain time slots of a radio channel frame, which also
comprises time slots being utilized for traffic channels.
The time slots to be used as control channels are marked
with a uniquely defined control channel indicator either
in the form of a unique sync Word or a unique control
channel identification word. A mobile station is able to
recognize the uniquely defined control channel indicator
and identify the particular time slots as control
channels, thus distinguishing the time slots which are not
being used as control channels.
While a particular embodiment of the present
invention has been described and illustrated, it should be
understood that the invention is not limited thereto since
modifications may be made by persons skilled in the art.
The present application contemplates any and all
modifications that fall within the spirit and scope of the
underlying invention disclosed and claimed herein.
