Note: Descriptions are shown in the official language in which they were submitted.
2153327
W O 95/15665 ^ PCTrUS94112419
COMMUNICATION ACROSS REGIONAL ENTITIES
Related Inventions
The present invention is related to the following invention which
is assigned to the assignee of the present invention. Method and
10 Apparatus for Passing Network Device Operations Between Network
Devices by Bonta having U.S. Serial No. 08/123,61~, and filed on
September 17, 1993.
Field of the Invention
The present invention relates to communication systems having a
plurality of transcoders and, more particularly, to a method and
apparatus for communication across regional entities in a
communication system.
Background of the Invention
The following description is directed for use in a direct sequence
code division multiple access (DS-CDMA) communication system. One
25 such DS-CDMA system is described in the communication standard
WO 95/15665 PCT/llS94/12419
~153327
-2 -
known as IS-95 or"Mobile Station-Base Station Compatibility Standard
for Dual-Mode Wideband Spread Spectrum Cellular System" as well as
IS-96 or "Speech Service Option Standard For Wideband Spread
Spectrum Digital Cellular System" and published by the
Telecommunications Industries Association (TIA), 2001 Pennsylvania
Ave, N.W., Washington, D.C. 20006. However, it will be appreciated by
those skilled in the art that the principles taught herein can readily be
extended to other types of communication systems including but not
limited to frequency division multiple access (FDMA) and time division
multiple access (TDMA) communication systems.
Referring now to FIG. 2, because DS-CDMA cellular
communication system equipment typically has an inherent limit on the
number of communication channels that can be supported by each
transcoder/selection system entity (XC), seams 200 form between the
cells (e.g., cell 1 and cell 4) supported by different regional
transcoder/selection entities (e.g., XC system A 106 and B 108,
respectively). Each transcoder/selection entity exchanges speech
information with voice coding devices (i.e., vocoders) and other
communication network devices higher up in the system hierarchy (e.g.,
a mobile switching centers (MSC) or the public switched telephone
network (PSTN)). In addition, each transcoder/selection entity
eliminates copies (by discarding or combining) of frames of voice traffic
information. Copies may exisf at the transcoder/selection entity,
because a frame may have traveled along two or more signaling paths
through a cellular infrastructure before reaching the transcoder. Each
transcoder/selection entity also notifies vocoders of bad or missing
frames (e.g., due to a signaling blank and burst operation). Finally, each
transcoder may also duplicate frames for simultaneous transmission to
multiple base transceiver stations (BTS).
One method to handoff across the seam 200 utilizes a hard
handoff instead of soft handoff (i.e., the soft handoff technique described
in IS-95). In a hard handoff, a mobile communication unit is instructed to
change to a completely new set of pilot channels which implies that no
diversity selection function can be accomplished during the transition
from the old transcoder/selection entity to the new one.
However, it is desirable to perform a soft handoff across the
seam. Soft handoff (SHO) maintains the benefits of a smooth transition
WO 95/15665 ~1 ~ 3 327 PCT/US94/12419
even while passing operations across the seam 200 between a source
to target transcoder/selection regional entity. These benefits include
providing potential signal receiver gains by adding diversity paths. It will
be appreciated by those skilled in the art that the source and target
5 transcoder may be located proximate one another in a central site
location scheme or remotely located from one another in a distributed
site location scheme. Whenever a transition is made (i.e., the source
cell drops out of a soft handoff connection), "ownership" of the call is
passed on to one of the target cells' controller (i.e., a link and call
10 managementdevice) and associated transcoding/selection entity. The
target cell's controller receiving the ownership then becomes the source
cell controller, and makes all subsequent decisions regarding soft
handoff until it drops out. Therefore, a need exists for a method
implementing such a desirable soft handoff across a transcoder seam
1 5 200.
Summary of the Invention
These needs and others are substantially met through provision
20 of a method and apparatus for use in a radio communication system
having a first and second base site communication unit operatively
coupled to a first and second transcoder, respectively. This system also
includes a mobile communication unit which is requesting to enter a
linked-communication mode with the first and second base site units. In
25 order to perform a linked-communication, a transcoder-base site
interface link is established between the second base site and the
second transcoder. In addition, the second transcoder is configured to
operate in a bypass mode such that it relays information within the
transcoder-base site interface link through a Communication Across
30 Regional Entity Link (CARE-Link) in conjunction with a CARE-Control-
Link between the first and second transcoder. Finally, the first
transcoder is configured to operate in linked-communication mode by
relaying information w.thin the CARE-Link which is controlled by the
CARE-Control-Link.
W 0 95/15665 `~ ~ 1 5 ~ 3 2 ~ PCT~US94/12419
Brief Description of the Drawings
FIG. 1 is a block diagram showing a preferred embodiment
communication system having several different transcoder device
configurations in accordance with the present invention.
FIG. 2 is a cellular coverage diagram for a communication system
operating in accordance with any of the preferred embodiment shown in
FIG. 1.
FIG. 3 is a communication flow diagram for adding a handoff
setup in accordance with one preferred embodiment transcoder device
configuration shown in FIG. 1.
FIG. 4 is a communication flow diagram for a dropping a handoff
setup in accordance with one preferred embodiment transcoder device
configuration shown in FIG. 1.
Detailed Description
Shown in FIG. 1 is a preferred embodiment communication
system having several different transcoder device configurations in
accordance with the present invention. The transcoderlselection entity
100 (i.e., a network device) is a key component to a DS-CDMA system,
and as such, infrastructure vendors are all interested in supplying this
equipment imbedded in their own system architecture. The architecture
consists of having this equipment located between a BTS 110 and an
MSC 112 (e.g., transcoder rack 100, 102, or 104). One configuration
consists of having the transcoder rack physically located between the
BTS 110 and the MSC 112. Another configuration consists of having
the transcoder rack physically adjunct to the MSC and providing
additional communication links between the transcoder rack and the
BTS through the MSC.
Each transcoder (XC) 106 along with a mobility manager (MM)
114 (collectively referred to as the transcoder rack 100) provides an
interconnection between other components iFl the communication
system. For example, an MSC 112 is located on one side and the
BTS's 110 and 116 on the other side of the XC 106. The XC 106
includes Highway Span interfaces, switching functions, shelf controllers,
and CDMA unique functions. The interface/controller cards in the XC
WO 95/15665 ~ ~ 5 3 3 2 7 PCT/US94/12419
frame 106 are configured around a time division multiplexed (TDM) bus
which carries voice and system message traffic. The heart of the switch
fabric is a 4092 port kiloport switch (KSW). Four KSWs can be space
switched with kiloport switch extenders (KSWXs). The KSWX supports
5 subrate switching which allows the transcoder/selector 106 to place
four sixteen kilo bits per second (16 kbps) encoded channels on one 64
kbps digital span (DS). Fault management information and intra-frame
control messages travel on a separate communication application
protocol (CAP) bus.
The XC tO6 includes one or more transcoder cards (XCDR card).
Each XCDR card converts 64 kbps ~l-law pulse code modulated (PCM )
signals into encoded voice and vice versa (e.g., this encoded voice may
preferably be coded according to IS-96). The XCDR card exchanges
encoded voice and PCM with other cards in the transcoder rack 100,
i.e., encoded voice to the KSW for routing to the proper BTS and PCM
with an interface card, via the TDM bus (i.e., the backplane). Its primary
functions encompass soft handoff (SHO) support (i.e., transcoder
selection), voice coding, and message processing to sub-multiplex
control message information to a physical layer stream under the MM
direction via a CAP interface.
The Supercell Transcoder/Rate Adaptor Unit (STRAU) produces
20 millisecond (ms) frames which are transferred at a 16 kilobits per
second (kbps) rate using a modified RA1 rate adaptation format
specified in CCITT V.110 as well as the RA2 rate adaptation format
specified in CCITT 1.460 (which is available from Comite Consultatif
International Telegraphique Et Telephonique (CCITT) now known as
International Telecommunication Union - Telematic Services (ITU-TS),
Place des Nations, CH 1211 Geneve 20, Switzerland). The modified
RA1 rate adaptation format consists of 320 bits wherein 260 are used for
information traffic (13 kbps), 21 (1.05 kbps) for control, 35 (1.75 kbps) for
frame synchronization, and four for time alignment.
In the DS-CDMA implementation, a XCDR outputs 160 bits and
appends 11 parity check bits. This leaves an excess of 7.45 kbps of the
13 kbps total at these times, while at the low end the excess rises to 12.2
kbps (16 bits out of the XCDR every 20 ms - no parity).
The XCs send these STRAU frames to each BTS through the
KSW switch and TDM Bus which then connects them to appropriate
WO 95/15665 2 1 ~ 3 3 2 7 PCTtUS94/12419
MSis (multiple Serial Interface Boards) which provide the T1 link(s) to a
BTS. For communications originating at the subscriber unit and being
received by the BTS (also known as the reverse link), the DS-CDMA
demodulator at the BTS determines the vocoding rate and sends only
the number of speech bits corresponding to the determined vocoding
rate within the STRAU format. A T1 link carries 24 DSOs, which
currently allows for up to ninety-six 16 kbps (compressed speech) links
or traffic channels using RA2 multiplexing. In a DS-CDMA system, the
frames present on the T1 traveling to the same cell are generally
synchronized in time to each other (except during soft handoff), because
the cell air interface timing is the same for all the channels.
However, a need exists for a way to coordinate information flow
through the XCs 106 and 108 during soft handoff (SHO) in the 16 kbps
STRAU links at the boundary locations or seams 200 (shown in FIG. 2).
16 This need is fulfilled by Communication Across Regional Entity (CARE)
- links as described below. These CARE links, as shown in phantom line
101, couple two XCs together so that information can readily be
transferred between them. It will appreciated by those skilled in the art
that these CARE links may be provided through links between the XCs
as routed through one or more MSCs 112 and 122, an MM 120, and/or
the PSTN 124. Whenever a XCDR is connected to a cell (e.g., cell 1
associated with BTS 116) located near a seam 200, the potential
situation exists for a link being required to another XC 108 connected to
a neighbor cell (cell 4 associated with BTS 118). If a subscriber unit
2~ reports a strong pilot in a cell (e.g., cell 4) across the seam 200, then the
XCs 106 and 108 coordinate and arbitrate the soft handoff across their
CARE link boundary using a segmented XCDR configuration. The
XCDR may eventually be handed over to another resource more local to
the subscriber current location. CARE links use STRAU to frame the
data for communication between XC's and a CAP interface for
communication to the higher (MM) entity.
The source XCDR (SXCDR) associated with BTS 116 in XC
system 106 terminates the traffic channel while in SHO. The
destination XCDR (DXCDR) associated with BTS 118 in XC 108 routes
3~ the physical link that it receives from its BTS through a CARE link to the
SXCDR in a bypass mode. In parallel, the DXCDR determines proper
time adjustment in preparation for assuming responsibility if the
WO 95/15665 215 3 3 2 7 PCT/US94tl2419
subscriber unit moves entirely into its cell. The SXCDR transmits the
voice and adjusts its transcoding window for proper delay minimization
using feedback through the STRAU from the BTS 118. The selector to
the DXCDR informs the destination transcoder to time adjust its window
5 by monitoring the time alignment information in the STRAU from the
BTS 118. In CARE links, added delay occurs for the other, passive
XCDR, path which routes STRAU in a bypass mode. If the MSC 112 or
122 is employed to route STRAU between the XC's 106 and 108, then
additional delay may be added to the CARE link. Therefore, any slight
10 discrepancy in the second BTS Span time must be accommodated.
This is preferably done at the SXCDR and is automatically controlled by
a feedback time alignment protocol. The added delay results in the
SXCDR advancing its transmission to its BTS's.
Since the MSC may be set up in a three party conference circuit
15 configuration, the vocoder chip must be programmed to send muted
PCM speech frames. The selecting function on the two links during
SHO is performed at the SXCDR. Physically the sub-systems may be
connected with a 1.544 kbps link between the two XC's 106 and 108.
Alternatively, the connection may be built into an open interface
20 standard such as the Motorola proposed "A+ interface" or other
communication protocol standards such as IS-41 (published by the
Telecommunications Industries Association (TIA), 2001 Pennsylvania
Ave., N.W., Washington, D.C.~20006), by using the MSC 112 or 122 to
route to the other XC 108. These connections at the MSC 112 or 122
25 could be nailed or a packet scheme may be used.
The DXCDR during the CARE link process monitors the STRAU
information at its selector, while also monitoring STRAU framing. It
reads the data from the BTS and immediately writes the data towards
the SXCDR as long as the in-band control bit within the STRAU framing
30 is set active for bypass mode. The SXCDR maintains active control of
the traffic channel until the MM 114 orders it to pass control to the
DXCDR, which occurs when the subscriber unit moves singularly into
the new XC 108 region coverage area. Active control means that a
XCDR performs selecting, transcoding, and subscriber unit call
3~ processing. The SXCDR always passes active control of the traffic
channel over to the DXCDR in the same known state. The SXCDR
continues the voice decoding while the DXCDR starts for
W O 9S/15665 2 ~ 5 ~ 3 ~ 7 PCTrUS94/12419
synchronization purposes. The MSC 112 or 122, which is functioning
in three party conference mode, must also know if its resources are in
SHO in order to determine blocking.
A typical call flow diagram is shown in FlGs. 3 and 4 depicting the
5 messaging necessary for adding and dropping BTS's across regional
entities. It will be appreciated by those skilled in the art that these
procedures also may be extended to scenarios involving handoffs to
more separated equipment, i.e., a different MM as well as other XCs.
Referring now more particularly to FIG. 3, a communication flow
10 diagram for adding a handoff setup is shown. A subscriber unit or
mobile station (MS) sends a Pilot Strength Measurement Message
(PSMM) to the BTS which passed the PSMM to the SXCDR. If the
PSMM indicates that another BTS pilot signal is greater than a
predetermined threshold, then a HO is recognized and indicated to the
15 MM. The MM sets up a CARE link between the SXCDR and the
DXCDR. This CARE link is confirmed by the generic processor
(GPROC) at each transcoder rack. In addition, the MM requests a three
party conference from the MSC or switch (SW) entity to smooth out the
audio quality of the call being handed off. The SW acknowledges this
20 request. Subsequently, the MM sends a SHO bypass request to the
DXCDR on the CARE link. This SHO bypass request is acknowledged
by the DXCDR on the CARE link . At the same time or any time
thereafter, the MM also requests a HO channel (i.e., a radio channel) to
be assigned by the destination transmitter/receiver (DXCVR) apparatus.
25 The DXCVR acknowl~dges the assignment of a communication
resource (i.e., radio channel). Once all of this is completed, the MM
initiates handoff at the SXCDR. The SXCDR begins to transmit the
subscriber communication information on the CARE link to the DXCDR
as well as monitor the subscriber communication information itself. The
30 SXCDR then sends a handoff direction message to the MS. When the
MS is acquired by the DXCVR, the MS sends a handoff completion
message to the SXCDR. Once the SXCDR receives the handoff
complete message, the SXCDR notifies the MM that the HO was
successful. At this point, the MS is in a SHO state and is communicating
35 with both the SXCDR and the DXCDR as well as the SXCVR and the
DXCVR.
WO 95/15665 2 ¦ 5 3 3 2 ~ PCT/US94/12419
g
Referring now more pa~ticularly to FIG. 4, a communication flow
diagram for a dropping handoff setup is shown. The MS sends a PSMM
to the SXCDR which indicates that the source BTS has a pilot signal
strength below a predetermined threshold. If the pilot signal strength
5 remains below the predetermined threshold for a set time period (at the
expiration of a timer), then the SXCDR sends a drop SHO request to the
MM. The MM then sends a vocoder/control release message to the
SXCDR. Subsequently, the SXCDR sends in the CARE link a start
vocoding and communication controlling message to the DXCDR. This
10 can be accomplished by the SXCDR with in-band signaling in the
STRAU for a synchronized swap of XCDR control. The DXCDR
acknowledges swapping of the control to the SXCDR. As a result, the
SXCDR notifies the MM that SHO Control has begun by the DXCDR and
requests that SHO control be released at the MM from the SXCDR. In
15 addition, the SXCDR preferably will send a mute audio signal to the SW
to smooth any possible audio hole created by a transcoder handoff.
Subsequently, the MM sends a HO direction message to the DXCDR
which passes the message to the MS. The MS responds with a
confirmation of the HO to the DXCDR and the DXCDR passes the HO
20 successful message to the MM. At approximately the same time, the
SXCDR releases the radio channel at the SXCVR. In addition, the MM
releases the three party conference circuit at the SW. The SW
acknowledges the release of the three party conference. The MM then
places the SXCDR in standby mode which is confirmed by the SXCDR
25 which completes the dropping of the SXCDR from the SHO
communication state.
In an alternative preferred embodiment, SHO may be extended
by connecting BTS's which are near transcoder seams to two XC sub-
systems (e.g., XC 108 and 132). Soft handoff between BTS 126 and
30 BTS 134 can be accomplished through the use of CARE signaling
techniques. Normally, a handoff between BTS 126 and BTS 134
would be required to be a hard handoff (i.e., without communication
between transcoders). But, with this configuration, since BTS 126 and
BTS 134 are both connected to XC-C 132 by dedicated spans 128
35 and 136, respectively, a soft handoff can occur between these cells. It
can employ the same CARE signaling with the slightly different physical
linking scheme. This scheme minimizes the bypass mode bandwidth
WO 95/1~665 Z 1 ~ 3 ~ 2 PCT/US94/12419
-10-
requirement during CARE signaling at the expense of having redundant
dedicated spans 128 and 130 to multiple transcoders per BTS.
The principles described herein can be summarized as follows.
A radio communication system (shown in FIG. 1 preferably includes a
first 106 and a second 108 transcoder which are operatively coupled to
a first 116 and a second 118 base site communication unit,
respectively. In addition, a mobile communication unit (not shown)
which is operating in the system requests to enter soft handoff mode
with the first 116 and the second 118 base site communication units. A
method for performing the soft handoff is provided which includes
establishing a transcoder-base site interface link between the second
base site 118 and the second transcoder 108. In addition, the second
transcoder 108 is configured to operate in a bypass mode such that the
second transcoder 108 relays information on the transcoder-base site
interface link through a CARE-Link in conjunction with a CARE-Control-
Link between the first 106 and the second 108 transcoder. In the
preferred embodiment, the second transcoder 108 synchronizes the
CARE- Link to the transcoder-base site interface link such that a lower
delay occurs for communications relayed between the two links. Finally,
the first transcoder 106 is configured to operate in soft handoff mode by
relaying information on the CARE-Link which is controlled by the CARE-
Control-Link. In an alternative embodiment, the second transcoder 108
can only operate in the bypas's mode to relay information within the
transcoder-base site interface link through a CARE-Link and monitor
control messages on the CARE-Control-Link between the first
transcoder 106 and the second transcoder 108.
To achieve a transcoder operation handoff (i.e. a SHO), control
information is passed to the second transcoder 108 which indicates that
the second transcoder 108 should take over communications with the
mobile communication unit, in response to a determination that
communications with the first base site 116 should be eliminated. The
second transcoder 108 monitors a communication link for subsequent
control information. The particular communication link which is
monitored may be the CARE-Link, CARE-Control-Link, or the
transcoder-base site interface link. Finally, the second transcoder 108
takes control of the communications with the mobile communication unit,
in response to the second transcoder 108 receiving control information
W 0 95/15665 2 15 3 3 ~ ~ PCTrUS94/12419
in the monitored communication link. This step of taking control may
also include configuring the first transcoder 106 to operate in a bypass
mode such that the first transcoder 106 relays information on a
transcoder-base site interface link between the first base site 116 and
5 the first transcoder 106 through the CARE-Link as well as setting up the
first transcoder to monitor the CARE-Control-Link for subsequent control
signals. This monitoring may consist of either detecting a change in the
content of the control information or waiting for a time out event. Further
the step of taking control may include the second transcoder 108
10 acknowledging on the CARE-Control-Link it's having taken control of the
communications with the mobile communication unit. At the same time,
the first transcoder 106 may be released from communications with the
mobile communication unit.
In the alternative embodiment, the second transcoder 108
1~ switches transcoding operations to another transcoder capable of
encoding and decoding voice information.
Alternatively, the transcoder operation handoff may be aborted by
passing control information to the second transcoder 108 which
indicates that the second transcoder 108 should terminate
20 communications with the mobile communication unit, in response to a
determination that communications with the second base site 118
should be eliminated. In addition, the second transcoder 108 monitors
a communication link for contfol information. The particular
communication link which is monitored may be the CARE-Link, CARE-
25 Control-Link, or the transcoder-base site interface link. Finally, the
second transcoder 108 is released from communications with the
mobile communication unit, in response to the second transcoder 108
receiving control information in the monitored communication link.
It will be appreciated by those skilled in the art that these steps of
30 passing transcoder operations are accomplished through the CARE-
Link and the CARE-Control-Link which each comprise a logical
connection between the first 106 and the second 108 transcoder. The
logical connection may be formed by an operative coupling between at
least two of the following system entities including: the first transcoder
3~ 106, the second transcoder 108, a first mobility manager 114, a
second mobility manager 120, a first base site controller 116, a second
base site controller 118, a first communication network switch 112, a
WO 95/15665 ~ 7 PCT/US94/12419
-12-
second communication network switch 122, a location register, and a
public switched telephone network 124.
Also, it will be appreciated by those skilled in the art that the SHO
may include enabling a three party conference circuit operatively
5 coupled to pulse code modulated information signals associated with
the first 106 and the second 108 transcoder, respectively, as well as at
least one communication network switch 112. Along with this enabling
step, an audio mute should be provided to the three party conference
circuit during the call handoff to reduce spurious audio noises in the call.
Further, it will be appreciated by those skilled in the art that,
although a linked-communication mode of a handoff between regional
entities is specifically described, the principles contained herein may be
readily applied to other linked-communications including diversity
combining and communication link encryption without departing from
15 the scope and spirit of the present invention.
Although the invention has been described and illustrated with a
certain degree of particularity, it is understood that the present
disclosure of embodiments has been made by way of example only and
that numerous changes in the arrangement and combination of parts as
20 well as steps may be resorted to by those skilled in the art without
departing from the spirit and scope of the invention as claimed. For
example, the network device operations have been described in the
context of a transcoder hando~ff operation. However, it will be
appreciated by those skilled in the art that the present invention
25 teachings can be readily adapted for use with other types of network
device operations such as diversity combining and communication link
encryption processes. In addition, the source and target network
devices may be devices other than a transcoder such as a mobility
manager, a base site controller, a communication network switch, or a
30 location register. Finally, the radio communication channel could
alternatively be an electronic data bus, wireline, optical fiber link,
satellite link, or any other type of communication channel.
