Abstract:
There is disclosed a system and method for sending intermittent messages on the A3 interface to provide, for use in a base station, a message transfer system for transmitting and receiving intermittent messages. The transfer system comprises i) a controller for inserting an intermittent message into a data frame; ii) a selection distribution unit in the base station for receiving and processing the intermittent messages; and iii) a transmitter for sending data frames with the intermittent message inserted to other base stations via the A3 interface.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention is directed, in general, to cellular communications and, more specifically, to a system for transmitting messages within a broad-band cellular communication system. 
   BACKGROUND OF THE INVENTION 
   Communication in cellular communication systems is conducted between a transmitting device and a receiving device over a Communication resource, commonly referred to as a communication channel. Code division multiple access (CDMA) is a mode of cellular communications that allows communication (both voice and data) between multiple users, simultaneously, on a single frequency. The simultaneous operation is accomplished by utilizing different codes for each user and an individual signal is distinguishable only after decoding. A received signal at a base station or mobile unit comprises many frequency and time overlapping coded signals from individual mobile units or base stations. Additionally, in a CDMA wireless system the mobile unit is linked to more than one base station during communication. As the mobile unit travels through the cellular regions, the mobile unit is always seeking to link to the base station with the strongest signal. Switching between base stations may be accomplished via hard handoff or soft handoff. During soft handoff, a mobile may be sending and receiving to three different base stations at the same time. It still has a source base station, but there are other linked base stations that are sending and receiving the same signals. 
   Generally, in a CDMA system user messages are being sent back and forth between the mobile unit and multiple base stations. These user messages are used, typically, for bringing up a supplemental data channel or reporting power strength that the mobile unit is receiving from the base station in the area. The mobile unit sends pilot strength measurement messages (PSMM) to the source base station and other base stations that the mobile unit is in communication with. 
   As with all communication systems CDMA has evolved since its debut. The most recent adopted standard, CDMA2000 (IOS V4.0), provides for 5 millisecond (ms) messages to carry essential signaling information to allow for rapid interaction between the mobile and the base station. The 5 msec messages are used for resource requests (SCRM) and allocation messages (SCAM), as well as pilot measurement update. These messages are carried on the Fundamental or Dedicated Control Channel and are always sent at the same rate: 9.6 kpbs. As such, the messages are always 24 bits long (there is an additional 8 bit tail and 16 bit CRC added at the physical layer). 
   The A3 interface, as defined in the IOS V4.0, carries coded user information (voice/data) and signaling information between the selective distribution unit (SDU) function and the channel element component of the BS (BTS). The A3 interface is composed of two parts: signaling and user traffic. The signaling information is carried across a separate logical channel from the user traffic channel, and controls the allocation and use of channels for transporting user traffic. Currently there is no way for the 5 msec messages to be passed to all the base stations that are in contact with the mobile unit during soft handoff. The IOS V4.0 text does not include support for the 5 msec messages on the A3 interface between base station. The existing 20 millisecond message format cannot be modified to contain the 5 millisecond messages. A frame sequence number (FSN) is included when a 20 msec message is transmitted to a mobile unit because it must be sent from all the base stations involved with the signals synchronized so that they send the frame at the same time to the mobile unit. Not only does the base station have to specify a particular 20 msec slot, but the particular 5 msec slot within the 20 msec slot must be specified. Since there are no extra bits in the 20 msec message there is no way to change the existing messages or use the existing messages to send 5 msec messages during soft handoff. 
   There is therefore a need in the art for a system that will provide synchronized passage of 5 msec messages between base stations and between base stations and a mobile unit during soft handoff. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a base station, a message transfer system for transmitting and receiving intermittent messages. The transfer system comprises i) a controller for inserting an intermittent message into a data frame; ii) a selection distribution unit in the base station for receiving and processing the intermittent messages; and iii) a transmitter for sending data frames with the intermittent message inserted to other base stations via the A3 interface. 
   According to one embodiment of the present invention, there is provided, for use in a base station, program instructions for expanding frame sequence number information in the data frames. 
   According to another embodiment of the present invention, there is provided, for use in a base station, instructions for providing a position identifier for the intermittent message. 
   According to still another embodiment of the present invention there is provided, for use in a base station, instructions for making a plurality of copies of the modified data frame and transmitting all the copies to target base stations. 
   According to yet another embodiment of the present invention, there is provided, for use in a base station, a time hack for causing the target base stations to transmit the data frame with the intermittent message to a mobile unit. 
   According to a further embodiment of the present invention, there is provided, for use in a base station, utilizing the A3 interface between base stations to transfer an intermittent message to the selection distribution unit from a mobile unit. The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
   Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
       FIG. 1  illustrates a general overview of the exemplary wireless network according to one embodiment of the present invention; 
       FIG. 2  depicts in greater detail exemplary base station according to one embodiment of the present invention; 
       FIG. 3  illustrates a high-level block diagram of a mobile unit in communication with a source base station and a target base station according to an embodiment of the present invention; 
       FIG. 4A  depicts the layout of a five millisecond Forward message sent from a source BS to a target BS over the A3 interface in accordance with an embodiment of the present invention; 
       FIG. 4B  is a layout of a five millisecond Reverse message sent from a target BS to a source BS over the A3 interface in accordance with an embodiment of the present invention; 
       FIG. 5A  depicts the layout of a Forward Layer  3  five millisecond message in accordance with an embodiment of the present invention; 
       FIG. 5B  illustrates the layout of a Reverse Layer  3  five millisecond message in accordance with an embodiment of the present invention; and 
       FIG. 6  depicts a high-level block diagram of a process for transmitting intermittent messages via A3 interface in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 6 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless office network. 
     FIG. 1  illustrates a general overview of an exemplary wireless network according to an embodiment of the present invention. Wireless network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  are operable to communicate with a plurality of mobile units (MU)  111 - 114 . Mobile units  111 - 114  may be any suitable wireless communication devices, including conventional cellular telephones, PCS handset devices, portable computers, telemetry devices, and the like. 
   Dotted lines show the approximate boundaries of the cell sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites also may have irregular shapes, depending on the cell configuration selected and both natural and man-made obstructions. 
   In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  each may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver station, for specified cells within a wireless communications network. A base transceiver station comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers, as well as call processing circuitry. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells  121 ,  122 , and  123  and the base station controller associated with each base transceiver station are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
   BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131  and mobile switching center (MSC)  140 . Mobile switching center  140  is well known to those skilled in the art. Mobile switching center  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system and/or the Internet. Communications line  131  may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network backbone connection, and the like. In some embodiments of the present invention, communications line  131  may be several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
   In the exemplary wireless network  100 , MU  111  is located in cell site  121  and is in communication with BS  101 , MU  113  is located in cell site  122  and is in communication with BS  102 , and MU  114  is located in cell site  123  and is in communication with BS  103 . MU  112  is also located in cell site  121 , close to the edge of cell site  123 . The direction arrow proximate MU  112  indicates the movement of MU  112  towards cell site  123 . At some point, as MU  112  moves into cell site  123  and out of cell site  121 , a “handoff” will occur. 
   As is well known, the “handoff” procedure transfers control of a call from a first cell to a second cell. “Soft” handoff is a type of handoff that provides for a transfer between cells (base stations) without notice to the MU  112  user. Direct BS to BS signaling and traffic connections between bas stations is supported by the A3 interface. For example, if MU  112  is in communication with BS  101  and senses that the signal from BS  101  is becoming unacceptably weak, MU  112  may then switch to a BS that has a stronger signal, such as the signal transmitted by BS  103 . MU  112  and BS  103  establish a new communication link and a signal is sent to BS  101  and the public telephone network to transfer the on-going voice, data, or control signals through BS  103 . The call is thereby seamlessly transferred from BS  101  to BS  103 . An “idle” handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels. 
     FIG. 2  depicts in greater detail exemplary base station according to one embodiment of the present invention. Base station  101  comprises base station controller (BSC)  210  and base transceiver station (BTS)  220 . Base station controllers and base transceiver stations were described previously in connection with FIG.  1 . BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  220  comprises BTS controller  225 , channel controller  235 , which contains representative channel element  240 , transceiver interface (IF)  245 , RF transceiver unit  250  and antenna array  255 . 
   BTS controller  225  comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS  220  and communicates with BSC  210 . Under normal conditions, BTS controller  225  directs the operation of channel controller  235 , which contains a number of channel elements, including channel element  240 , that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile unit and a “reverse” channel refers to inbound signals from the mobile unit to the base station. In an advantageous embodiment of the present invention, the channel elements operate according to a code division multiple access (CDMA) protocol with the mobile units in cell  121 . Transceiver IF  245  transfers the bi-directional channel signals between channel controller  240  and RF transceiver unit  250 . 
   Antenna array  255  transmits forward channel signals from RF transceiver unit  250  to mobile units in the coverage area of BS  101 . Antenna array  255  also sends to transceiver  250  reverse channel signals received from mobile units in the coverage area of BS  101 . In a preferred embodiment of the present invention, antenna array  255  is multi-sector antenna, such as a three sector antenna in which each antenna sector is responsible for transmitting and receiving in a 120 degree arc of coverage area. Additionally, RF transceiver  250  may contain an antenna selection unit to select among different antennae in antenna array  255  during both transmit and receive operations. 
     FIG. 3A  illustrates a high-level block diagram of a mobile unit in communication with a source base station and a target base station according to an embodiment of the present invention. Mobile unit  302  is in communication with base station  304  via base transceiver  306 . Mobile unit  302  is also in communication with a source base station  310 . In  FIG. 1 , mobile unit  112  is preparing to enter a cell supported by BS  103 . BS  103  becomes the target base station and soft handoff may begin. 
   Efficient inter-BS soft handoff is supported via direct BS to BS signaling and traffic connections between base stations. The A3 and A7 interfaces are used to support this form of inter-BS soft handoff which is based on packet technologies. The A3 interface, composed of signaling and user traffic subchannels, provides the ability to establish and remove A3 traffic connections. The A3 interface also provides support for operational procedures, such as turning on/off voice privacy or changing the service configuration of a call. The A3-Connect message is sent from the target BS to the source BS to initiate or add cells to one or more A3 user traffic connections. An A3-Connect Acknowledge (Ack) message is expected in response. 
   Upon receiving the A3 Signaling Address information, the receiving BS begins the process of establishing the A3 signaling connection with the source BS. The A3 Signaling Address is used by the target BS to allocate a logical circuit to be used for A3 signaling. The SDU ID identifies the particular instance of the SDU function. Following the establishment of the A3 signaling link, the A3-Connect message is sent from the target BS to the entity at the other end. The target BS expects an A3-Connect Ack message indicating the result of processing the A3-Connect message. When the A3 user traffic connection(s) is established, traffic packets are exchanged to verify that the two entities can communicate via the A3. (Section 3.4.1.1.1, Inter-operability Specification (IOS V4.0) for CDMA 2000 Access Network Interfaces, June 2000). 
   The signal is passed through the A3 interface (interface between source and target base stations as defined by IOS V4.0) back to the source base station  310 . The signal from mobile unit  302  is passed to selection distribution unit (SDU)  312 . SDU  312  receives signals from all the base stations that receive the originating signal from mobile unit  302 . SDU  312  then chooses from among the received signals the signal that has the most likelihood of being correct and discards the other signals. In the opposite direction, when source base station  310  is sending packets to mobile unit  302 , SDU  312  makes copies of whatever frame of data is to be sent and sends the copies out to the other bases stations, such as BS  304 . Additionally, SDU  312  sends the time to transmit the frame. A more clear signal is likely to occur during communications with mobile unit  302  if mobile unit  302  is sending and receiving from multiple base stations at one time. All the information received by the linked base stations is passed to SDU  312  which processes all the messages before transmitting the strongest signal to mobile unit  302 . 
     FIG. 3B  is a diagram of a 20 msec frame indicating a 5 msec slot for receiving a 5 msec message in one embodiment of the present invention. Since 5 msec messages can be sent/received on any 5 msec sub-interval boundary during a 20 msec frame, the sequence number information in the A3 message must be expanded to include information that indicates which 5 msec sub-interval the message is to be sent (or was received). This is accomplished with a 2-bit field. 
     FIG. 4A  depicts the layout of a 5 msec Forward message sent from a source BS to a target BS over the A3 interface in accordance with an embodiment of the present invention. This A3 message is sent from the source BS to the target BS over A3 IS-2000 user traffic subchannel of type IS-2000 Fundamental Channel (FCH) or IS-2000 DCCH (Dedicated Control Channel) It is used to send a Forward Link ms message to the target BS for transmission to the mobile station. 
     FIG. 4B  is a layout of a 5 msec Reverse message sent from a target BS to a source BS over the A3 interface in accordance with 20 an embodiment of the present invention. This A3 message is sent from the target BS to the source BS over A3 IS2000 user traffic subchannel of type IS-2000 FCH or IS-2000 DCCH. It is used by the target BS to send a decoded Reverse Link 5 msec message and control information to the source BS. 
     FIG. 5A  depicts the layout of a Forward Layer  3  5 msec message in accordance with an embodiment of the present invention. Element  500  contains the CDMA Forward 5 msec Message Frame and control information for packets flowing in the selection/distribution unit to base transceiver station direction. The SDU sets Reserved slot  502  to ‘00’. The SDU sets 5 msec slot  504  to the number of 5 msec sub-intervals that the target BS is to offset the message. For example, the value ‘00’ indicates that the 5 msec message is to be sent at the start of the 20 msec interval, while the value ‘01’ indicates that the message is to be sent in the second 5 msec slot of the 20 msec frame interval. The SDU sets FSN field  506  to system time in frames, Modulo  16  corresponding to the transmission time of the frame over the air in the forward direction. Forward Link Information  508  is set by the SDU and sent to the MS. The SDU sets the information bits in forward link information  508  to the information bits supplied by the multiplex options sublayer and the bit order is as specified in ISO V4.0. 
     FIG. 5B  illustrates the fields layout of a Reverse Layer  3  5 msec message in accordance with an embodiment of the present invention. Element  510  contains the CDMA reverse 5 msec Message frame and control information for packets flowing in the BTS to SDU direction. Reserved slot  512  is set by the BTS to ‘0’. Frame quality indicator (FQI)  514  is set to ‘1’ if the Reverse Traffic Frame CRC (not shown) passes. Otherwise, the BTS sets this field to ‘0’. 
   5 msec slot  516  is set to the 5 msec sub-interval in which the message was received. For example, the value ‘00’ indicates that the 5 msec message was received at the start of the 20 msec interval, while the value ‘01’ indicates that the message was received in the second 5 msec slot of the 20 msec frame interval. FSN  518  is set by the BTS to CDMA system time in frames, modulo  16  (see IOS v4.0) corresponding to the receive time of the air interface frame in the reverse direction. 
   Reverse link information  520  is set by the BTS to the information that the BTS receives from the MS. Th BTS sets the Information Bits to the information bits received from the MS which correspond to the Multiplex Option in us (see IOS V4.0). The bit order used is the order specified in IOS V4.0. 
     FIG. 6  depicts a high-level block diagram of a process for transmitting intermittent messages via A3 interface in accordance with an embodiment of the present invention. The process begins when the SDU punctures 5 msec message into a 20 msec data frame. The frame sequence number in the data frame is expanded for the position identifier of the 5 msec message (process step  600 ). 
   The selection/distribution unit in the source base station copies the data frame and then transmits the 20 msec message to associated target base stations via the A3 interface. Included in the 20 msec message is a specific time that the target base stations transmit the data frame, with the 5 msec message, to a mobile unit. The target base stations then transmit the 20 msec message to the mobile unit at th same time (process step  602 ). 
   The source base station may receive and process 5 msec messages transmitted by a mobile unit to target base stations. In this instance, the process works in the reverse in that the target base stations each transmit a copy of 20 msec data frame from the mobile unit, each 20 msec message including a 5 msec message, to the source station SDU (process step  604 ). The SDU processes each of the 20 msec messages and determines the 20 msec message with the strongest signal. The remaining messages are then discarded (process step  606 ). 
   In the CDMA2000 standard, 5 msec messages that are received during the transmission of a 20 msec frame are punctured into the 20 msec frame on a 5 msec boundary. A 20 msec frame contains quite a bit of repetition because the air interface is so uncertain. The bits in the 20 msec message are interleaved or repeated very often within the 20 msec message. When the 5 msec message is punctured into the 20 msec message there is enough repetition in the rest of the message to assure an accurate frame. Further, it is possible to send a blank or non-data 20 msec message and puncture the 5 msec message into the frame. 
   It is still possible for the receiver to correctly receive and decode both the 5 msec message and the 20 msec message. As such, the source BS may send a 5 msec message with a sequence number that overlaps with the sequence number for a previously or yet to be sent, 20 msec frame. The 5 msec slot parameter identifies where, in the 20 ms frame, is to be punctured. Similarly, the target BTS may receive a 5 msec message within a 20 msec frame, in which case the target will send both the 5 msec message and 20 msec frame to the source with the same sequence number. Again, the 5 msec slot will identify where in the 20 msec frame the 5 msec message was received. 
   Because the 5 msec messages are sent/received within 20 msec frames, it is not necessary to include the typ of information in the A3 messages normally associated with 20 msec frames (e.g., power control information and reverse link quality). 
   Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.