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BACKGROUND OF THE INVENTION
The present invention relates to wireless digital telephone systems wherein
a base station is in communication with a plurality of subscriber stations
connected thereto by RF channels in a manner to permit simultaneous
communications with the subscriber stations over a given channel having
multiple sequentially repetitive time slots, with particular time slots
being assigned to particular subscriber stations.
SUMMARY OF THE INVENTION
The present invention relates to a wireless digital telephone system that
includes a base station in communication with a plurality of subscriber
stations connected thereto by RF channels in a manner to permit
simultaneous communication with the subscriber stations over a given
channel having multiple sequentially repetitive time slots, with
particular time slots being assigned to particular subscriber stations.
The present invention provides means to automatically change either the
frequency or the time slot configurations, or both, in the event of the
entrance of additional subscriber units which have gone off-hook or in the
event of deterioration of transmission quality due to modulation change,
frequency channel interference, equipment failure, or the like. The change
means includes a remote-connection processing unit and an exchange unit,
preferably in the form of a concentrator, which includes a switch that
responds to a control signal from the remote-connection processing unit by
physically connecting a selected port in a communication circuit to a
selected slot. The control signals are supplied in response to status
messages received by the remote-connection processing unit.
This invention relates to communications systems of the wireless digital
telephone type, and it more particularly relates to means in such a system
for automatically adjusting the system to compensate for undesirable
effects during operation such as frequency interferences, fading, link
quality deterioration and equipment failure.
The present invention is utilizable in a system such as disclosed in U. S.
patent application Ser. No. 713,925, filed Mar. 20, 1985 now issued as
U.S. Pat. No. 4,675,863, dated June 23, 1987. Such a system comprises a
base station and a plurality of subscriber stations connected thereto by
RF channels. It provides for communication between the subscriber stations
and an external communication network having a plurality of ports. The
base station includes a communication circuit for enabling simultaneous
communications between a plurality of the ports and the subscriber
stations over a given communication channel having multiple sequentially
repetitive time slots, with predetermined time slots being assigned
respectively to predetermined subscriber stations. The system includes a
central office terminal (COT) in which is provided a remote-connection
radio processing unit (RPU) for directing communications between the time
slot assigned to a given subscriber station and a given port and an
exchange for connecting the communication circuit to the ports. The
exchange, which is preferably a concentrator of a type hereinafter more
fully described, includes a switch which responds to a control signal from
the RPU by physically connecting a selected port to a selected
communication channel time slot assigned to a given subscriber station.
The communication circuit includes a plurality of channel control units
(CCUs) for coupling the assigned communication channel time slot to the
corresponding subscriber station in response to a command signal
communicated by the RPU to the CCU's over a baseband control channel
(BCC). These command signals are in response to status messages provided
over the BCC to the RPU to indicate the usage status of the communication
channel time slots and the subscriber stations. The assigned communication
channel time slot is coupled to the corresponding subscriber station by an
assigned time slot in an assigned radio frequency (RF) channel. The BCC is
provided over lines separately connected from the RPU to each of the
CCU's. Control commands and status messages are communicated between the
CCUs and the subscriber stations over a radio control channel (RCC)
assigned to a predetermined time slot of a predetermined RF channel.
The primary object of the present invention is to provide frequency and
slot agility to overcome any difficulties in transmissions between the
base station and its associated subscriber units. Another object of the
present invention is to provide the aforesaid agility in a relativley
simple and inexpensive manner.
The various other objects and attendant advantages of the present invention
will be readily apparent from the following detailed description when read
in conjunction with the following drawings wherein:
FIG. 1 is a block diagram of a base station utilized in the present
invention, the connected subscriber stations being generally indicated.
FIG. 2 is a block diagram of a single subscriber unit embodying the present
invention.
FIG. 3 is a block diagram of a dual subscriber unit embodying the present
invention.
FIG. 4 is a block diagram of a multiple single-subscriber unit embodying
the present invention.
FIG. 5 is a block diagram of a multiple dual-scriber unit embodying the
present invention.
FIG. 6 is illustrative of a radio processing unit matrix embodying the
present invention.
FIG. 7 is a block diagram showing the details of a channel module utilized
in the present invention.
FIG. 8 is a block diagram of a radio processing unit and its connecting
elements embodying the present invention.
FIG. 9 is a functional illustration showing the layers of communication
between the base station and a subscriber station.
GLOSSARY OF ACRONYMS
BCC: baseband control channel
BER: bit error rate
CCT: channel control task
CCU: channel control unit
CO: central office
COT: central office terminal
CRC: cyclic redundancy check
DBM: data base module
LQ: link quality
MF: malfunction
MPM: message processing module
MTU: master timing unit
MUX: multiplexer
PCM: pulse code modulation
RCC: radio control channel
RF: radio frequency
RPU: radio processing unit
RRT: remote radio terminal
SCT: subscriber control task
SDLC: synchronous data link control
SM: scheduler module
VCU: voice codec unit
Referring to the drawings wherein similar reference characters refer to
similar parts, there is shown in FIG. 1 a base station, generally
designated 10 which includes a central office terminal (COT), generally
designated 12 and a remote radio terminal (RRT), generally designated 14.
The COT 12 includes a remote connection processing unit (RPU) 16 coupled
through an interface 18 to an exchange unit 20. The exchange is preferably
in the form of a concentrator such as "Model 1218C" digital image
concentrator provided with a switch, which is presently available from ITT
corporation, New York, N.Y., U.S.A.
The exchange 20 is in circuit with the central office (CO) 22 over a
plurality of two-wire appearance lines 24 and is also in circuit, via echo
cancellers 26 and a plurality of span trunks 28, with a plurality of
multiplexers (MUX's) 30 coupled to a master timing unit (MTU) 32 in the
remote radio terminal 14. Each of the span trunks 28 carries a plurality
of multiplexed time slots provided by its respective MUX 30. A baseband
control channel (BCC) 34, which occupies one of the time slots on one of
the span trunks 28, provides communication between the MUX's 30 and the
channel modules 36.
Each MUX 30 is embodied in a modular card that is capable of handling up to
24 pulse code modulation (PCM) simultaneous circuits or 23 PCM
simultaneous circuits plus the BCC. The MUX's act to extract the data from
the trunks 28 and to distribute it to the channel modules 36 that are in
circuit with the transmitter receiver network 38 via power amplifier 40.
The RPU 16 has ultimate control of both the exchange (concentrator) 20 and
the RRT 14 and acts to process subscribers' requests to set up the
required transmission path between the subscriber stations, indicated
generally at 42, and the central office (CO) 22. The subscriber stations
42 may each comprise either a single subscriber unit, a dual subscriber
unit or a multiple subscriber unit. FIG. 2 shows a single subscriber unit
that comprises a transceiver 50 coupled between an antenna 52 and modem 54
that is in circuit with a codec 56 for encoding or decoding signals to and
from a subscriber set 58. In FIG. 3, the same general type of unit is
shown except that it comprises a dual subscriber unit in which the antenna
60, transceiver 62, modem 64 and codec 66 are in circuit through buffers
68 and 70 with separate subscriber sets 72 and 74.
In the dual subscriber unit of FIG. 3, if two conversations are transmitted
and received in one slot, the transceiver, modem and codec may be single
reversible elements that only have to perform one set of operations at a
time, e.g. transmit or receive over each line. Furthermore, since the unit
is never receiving and transmitting at the same time, no duplexer is
required since the only duplication that takes place is through the
buffers 68 and 70 which are connected to interface circuitry (not shown)
for the subscriber sets.
In FIG. 4 there is shown a multiple system of single subscribers that
comprises a combining, deploying and duplexing network, generally
indicated at 80, coupled to an antenna 82 and to a multiplicity of single
subscriber units 84a, 84b, 84n. Each subscriber unit is coupled to a
respective subscriber set indicated at 86a, 86b and 86n.
FIG. 5 shows the same type of multiple system as in FIG. 4 except that the
network 88 is coupled to dual subscriber units 90, 92, 94, etc., each of
the type shown in FIG. 3 and each being coupled to respective dual
subscriber sets, as at 96 and 98, for unit 90, at 100 and 102, for unit 92
and at 104 and 106 for unit 94.
During a conversation, some problems may arise due to various factors which
may generally be classified as follows: (a) multisubscriber configuration,
(b) modulation change (c) interference avoidance and (d) equipment failure
of an individual channel.
FIG. 6 illustrates an example of an RPU matrix where the various
subscribers are designated by the letters A through G, the horizontal rows
representing frequencies and the vertical columns representing slots. The
number "1" represents "in use" and "0" represents "not in use". In this
matrix only the transmit slots need to be stored (as shown) since receive
slots are always offset by 2 slots.
During operation, it frequently occurs that all four slots of a particular
frequency are occupied by various subcribers. Assume, for example, that
there is one dual subscriber unit while the others are single subscriber
units. In the case of the dual subscriber unit it is necessary that the
two dual subcribers thereof be in adjacent slots. If one of the two dual
subscribers is occupying a particular frequency and slot while the other
slots in that frequency are occupied by other single subscribers, and if
the other of the two dual subscribers requires a connection because he has
either gone off-hook to make a call or an incoming call is to be routed to
him from the central office, either the single subscriber in the adjacent
slot of that frequency must be moved to another frequency and/or slot to
make room for the second of the two dual subscribers or the first dual
subscriber occupying the particular frequency and slot must be moved to
another frequency where an adjacent slot is available. The RPU, in this
situation, acts to set up the current connections and store them in its
memory and it can, therefore, determine where within the matrix a transfer
can be effected.
The above type of transfer is accomplished in the following manner: Assume,
for example, that the first of the dual subscribers is currently engaged
in a conversation on frequency D in transmit slot 1 (receive slot 3). When
the second of the dual subscribers goes off-hook, an alert signal (one
bit) is sent back to the base station as part of the control bits
transmitted in addition to the conversation. The alert bit is detected in
the channel module 36 which then sends a message back to the RPU 16 via
the MUX 30, span trunk 28, exchange 20 and interface 18, either along with
the reverse conversation path or along a separate path. Conversely, an
incoming signal from the control office (CO) 22 is routed through the
exchange 20 and interface 18 to the RPU 16.
In the event of the occurence of either signal, e.g. from the off-hook set
or from the central office, a slot must be made available for the second
of the dual subscribers to connect to the base station in a slot adjacent
to the slot already in use by the first of the dual subscribers. While it
would be possible to move the first dual subscriber from frequency channel
D to G, that would be the less preferable move since two empty adjacent
slots on the same frequency are not often available. It is, in any event,
at least as easy to move the adjacent single subscriber unit from
frequency D, slot 2 to a different frequency. In the example shown, the
adjacent subscriber could be moved to any one of the eight open slots.
Since the single subscriber being moved does not require adjacent slots
for transmission and reception, if the move is made in frequency only
(e.g. from channel D to B or G) it can be accomplished while the slot is
unused and is, therefore, glitchless. If a change is made in both slot and
frequency, then one slot's worth of transmission data is either duplicated
or lost.
The analagous situation exists with multiple single unit subscribers, such
as in FIG. 4. Since each unit has a complete set of elements, more than
one unit is capable of transmitting at one time. It is, in fact,
preferable to have all transmit simultaneously and receive simultaneously
in order to prevent receiver front-end overload. Therefore, in the matrix
of FIG. 6, if two subscribers are occupying slot 1 on frequencies B and D
and a call set-up is required for a third subscriber, then the RPU 16 must
assign the third subscriber to slot 1 on either frequency F or G.
Where the system is that of FIG. 5, where there are multiple dual
subscriber units, the first subscribers of each pair of dual subscribers
must be in the same slot but on different frequencies while the second of
each pair must be in adjacent slots on the same frequencies as their
respective first subscribers.
The RPU, in the above situations, acts to set up the current connections at
each interval of time and stores them in its memory. It can, therefore
determine, at any time, where within the matrix a transfer can be
effected.
In all of the above situations where the concern is with the assignment of
slots and frequencies in multi-subscriber situations, the stimulus for
change originates outside the base station (either from the central office
or the subscriber set). In the following situations which involve
modulation change due to fading, or interference avoidance or change due
to equipment failure, the stimulus originates or is detected in the base
station itself.
In the base staton, each channel module 36, as shown in FIG. 7, includes a
channel control unit (CCU) 110 and a modem 112, each being coupled to a
voice codec unit (VCU) 114 by respective compressed data link 116 and
control status link 118. The VCU is interfaced to the MUX 30 by pulse code
modulation (PCM) data channels 120, while the CCU is interfaced with to
the MUX 30 by synchronous data link control (SDLC) 122.
A number of buffers (not shown) are associated with the various elements.
In this respect, each channel module contains an AGC buffer for each slot
as well as a link quality (LQ) buffer that measures the differential of
received phase error. In addition, the VCU is provided with a bit error
rate (BER) buffer. A cyclic redundancy check (CRC) is transmitted as part
of each conversation encoded burst which provides a BER count to the BER
buffer.
For the present purposes, a high AGC level means that a high signal level
is being received and the receiver gain is small. Conversely, a low AGC
level indicates a low received signal level and a large receiver gain.
This AGC level is used to indicate whether a deterioration of signals is
due to fading of the signal or whether there is interference. Interference
is indicated by a higher AGC level because the AGC level is based on the
sum of the transmitted signals plus any interfering signals. In either
instance correction can be made by modulation change. In this respect, as
more fully described hereinafter, each channel module 36 contains a modem
that is capable of transmitting and receiving on voice slots with either
16 or 4 phase modulation. There are only half as many bits passed per
symbol in 4 phase modulation as there are in 16 phase modulation, but the
bit rate is the same. Therefore, when 4 phase modulation is used, two
adjacent slots, as shown in the matrix of FIG. 6, are combined into one
elongated slot containing the same number of bits per elongated slot as
would occur in a slot during 16 phase operation.
When, for example, a dual subscriber system is in operation, if either the
BER increases beyond a certain threshold (e.g. 0.1%) or the LQ decreases
below a certain threshold (e.g. 6 degrees) the contents of the
corresponding buffers are reported to the RPU via the
MUX-spanexchange(concentrator)-interface link. The RPU thereupon sends a
control signal back via the same linkage to the control module to change
from 16 to 4 phase operation. At the same time, if two subscribers are
currently in operation in adjacent slots on the same frequency, one of the
subscribers is changed to another frequency in the manner described above.
If after the modulation level has been changed in the above manner, the AGC
level is still high for that particular subscriber, it is assumed that the
reason for the poor BER and LQ is interference rather than transmission
fade. The outputs of all the AGC buffers are reported to the RPU which
determines which frequencies (if any) have a low AGC level and the RPU
then initiates a change in frequency for that subscriber from the
frequency with a high AGC level to one having a low AGC level. This acts
to eliminate the interference if it is present.
The base station is also provided with malfunction (MF) buffers which
provide the operational status of each link in the base station chain
including the exchange (concentrator), span trunk, MUX, channel module,
power amplifier, transmitter and receiver. If the MF buffer senses a
malfunction or equipment failure, a signal is sent to the RPU indicating
which frequency is involved. On receipt of this MF signal, the RPU
initiates a frequency (and, if necessary, a slot) change for each
conversation (if any) currently taking place on the affected channel.
The RPU 18 is a primary element in controlling the assignment of frequency
channels and slots and its general structure is shown in FIG. 8. It
comprises a message processing module (MPM) 130 coupled to a data base
module (DBM) 132 and a scheduler module (SM) 134. The MPM manages all
interface between the SM and DBM within the RPU as well as with the
channel control unit (CCU) 110 and a subscriber control task (SCT) 136
situated outside the RPU. The CCU is located within the channel module 36,
as shown in FIG. 7, while the SCT is in circuit with the subscriber
stations.
In operation, if, for example, during a call, the link quality (LQ) goes
down below a predetermined level, the RPU is actuated to send a message to
the SCT 136, which, thereupon, communicates with the MFM 130. The MFM then
(a) sends a signal to the DBM 132 to identify an available channel, and
(b) checks with the scheduler module 134 for schedule control and timing
(the scheduler module serving to arrange the messages received by it in
the order that they are received). If there is an available channel, the
MPM sends a message back to the SCT to reconfigure the matrix to move the
call to the available channel. If there is no available channel, the call
is retained in the same channel.
If during a call, all four slots are occupied but, because of an
interference on a particular frequency, it is necessary to move all the
subscribers to another frequency with the same respective slots, the
following sequence occurs: The RPU, upon being actuated by the lowering of
the link quality, sends a message to the SCT requesting a frequency
change. The SCT forwards that message to the channel control task (CCT),
located at the subscriber station, for frequency change processing, and
also communicates with the MPM at the base station for allocation of an
available CCU channel. The MPM sends a signal to the DBM to identify an
available channel and also checks with the scheduler module to obtain a
time position in the schedule. If there is an available channel, the MPM
sends a message back to the SCT to reconfigure the channel position. If
there is no available channel, the SCT sends a busy siqnal to the RPU and
no change is made.
FIG. 9 illustrates the RCC protocol. The RPU 16 is in communication with
the CCU 110 and modem 112. At the subscriber station the SCT 136 is in
communication with the CCT 138 which is provided with modem 140. The RCC
protocol consists of two layers, a data link layer 142 and a packet layer
144. The data link layer is the physical link between the parts while the
packet layer comprises the messages being passed over this physical link.
The data link layer is responsible for error detection, word
synchronization and framing, detection and resolution of collisions
(conflicting contention for the same time slot on the same RF channel).
Referring to FIG. 9, an an example, the RPU processes two paths of
protocol: In the first protocol, the RPU sends a message to the CCU at the
base station to request a modulation change. In the second protocol, the
RPU sends a message to the SCT to request a frequency change. In the third
protocol, the SCT sends a message back to the RPU to indicate that all
channel frequencies are occupied.
In another example, the RPU sends a message protocol to the SCT to request
a channel frequency change. This message initiates two functions: First,
the message requests that the SCT at the subscriber station obtain a route
to another frequency from the RCC. The SCT thereupon communicates with the
CCT. The CCT then searches the channels and, when it finds an available
slot and frequency, it switches thereto. In the second function, the
message protocol causes the SCT to call the centraL office to stand by for
a connection.
If, during a conversation, the link quality falls below a predetermined
level, the RPU forwards this information to the CCU and requests a
modulation level change. The CCU acknowledges this message through the
SCLC link via the MUX to the RPU.
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