CA2595365A1 - Base transceiver station (bts) synchronization - Google Patents

Abstract

In a network overlay wireless location solution for a GSM or UMTS
communications network, spectrum utilization can be made far more efficient by synchronizing the BTSs, which can require distributing a timing signal to all BTSs, or installing a satellite-based timing unit in each site. The present invention provides an architecture in which Location Measurement Units (LMUs) are installed at some or all of the BTS sites for the purpose of locating wireless devices. The LMUs are used to measure the timing of various uplink and/or downlink signals in the cellular network in support of various location techniques. These LMUs may include a GPS-based timing reference module, which may be used to synchronize the time bases of all LMUs. To reduce the overall cost of BTS synchronization, the LMU distributes timing signals, including a periodic electrical pulse as well as time description information, on a serial or other interface, which is available for other nodes to use for synchronization. The format of the electrical pulse and time description information is modified through hardware and software to adapt to the various formats required by various BTS types. For example, the BTSs with co-located LMUs can receive a synchronization signal with little or no hardware cost. The External Interface Unit (EIU) described herein may be used to adapt to various BTS hardware formats. For BTS sites not equipped with an LMU, a Timing Measurement Unit (TMU) can be used. The TMU has the single function of providing BTS time signals in the same formats as provided by the LMUs. The time signals provided by the TMUs are synchronous to the signals provided by the LMUs. This timing-only TMU has a lower cost than the LMU because it does not support the uplink or downlink signal measurement functions. This approach allows a cellular operator to synchronize BTSs at a relatively low cost.

Description

2 PCT/US2005/014188 BASE TRANSCEIVER STATION (BTS) SYNCHRONIZATION
CROSS REFERENCE
[0001] The present application claims priority to U.S. Provisional Application No. 60/652,265, filed on February 11, 2005, entitled "Base Transceiver Station (BTS) Synchronization," which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION
[0002] The present invention relates generally to the field of wireless location and associated wireless communications systems, and more particularly, but not exclusively, to a system for synchronizing Base Transceiver Stations (BTSs) of a GSM or UMTS network coupled with an overlay wireless location system (WLS).
BACKGROUND OF THE INVENTION

[0003] The present invention is especially suited, but not necessarily limited to, use with GSM and UMTS systems and the like. GSM stands for Global System for Mobile communication and is a digital mobile telephone system widely used in Europe and otlier parts of the world, whereas UMTS stands for Universal Mobile Telecommunications System and is a third-generation (3G) broadband system based on the GSM standard. This specification describes systems and methods to provide Global Positioning System (GPS)-derived timing information to base stations of a wireless communications system, for the purposes of network synchronization.
For exaniple, GSM network synchronization can benefit a wireless carrier in several ways. In unsynchronized GSM networks, the co-channel interference created by frequency reuse can be reduced by synchronization. A reduced noise/co-channel interference level allows for tighter frequency reuse patterns, thus allowing the carrier to increase system capacity (e.g., Erlang capacity) or improve voice/data quality.
SUMMARY OF THE INVENTION

[0004] The following statements summarize several important aspects of the present invention, which are described in greater detail herein:
1. In a network'overlay wireless location solution for a wireless communications system comprising a network of Base Transceiver Stations (BTSs), for example GSM or UMTS communications network, a method and system of improving spectrum by synchronizing the BTSs.
2. A method and system as recited above, wherein a timing signal is provided to each BTS by either a Location Measurement Unit (LMU) or a Timing Measurement Unit (TMU).
3. A method and system as recited above, wherein each LMU and TMU
comprises a GPS-based timing reference module and means for generating a periodic timing signal which is synchronized within a pre-specified degree of accuracy witll the timing signals generated by each other LMLI and TMU.
4. A method and system as recited above, wherein the LMUs are used to measure the timing of various uplink and/or downlink signals in the cellular network in support of various location techniques.

5. A method and system as recited above, wherein the LMUs and TMUs distribute timing signals, including a periodic electrical pulse as well as time description information.

6. A method and system as recited above, wherein the format of the electrical pulse and time description information are modified through hardware and software to adapt to the various formats required by various BTS types.

7. A method and system as recited above, wherein the BTSs with co-located LMUs receive a synchronization signal with little or no hardware cost, and wherein BTS sites not equipped with an LMU are equipped with TMU that has the single function of providing BTS time signals in the same formats as provided by the LMUs, wherein the time signals provided by the TIV1Us are synchronized to the signals provided by the LMUs and the timing-only TMU has a lower cost than the LMU
because it does not support uplink or downlink signal measurement fiuictions.
[0005] It should be noted that the concept of the time signals being "synchronized" is not limited to signals of substantially identical shape or occurring simultaneously. For example, two signals may be considered sufficiently synchronized, for the purposes of the present invention, where they are offset in time but have a known relationship.

BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Figure 1 schematically depicts an illustrative embodiment of an emergency-only overlay location solution.

[0007] Figure 2 depicts several ways of deploying base station synchronization products (LMUs and TMiJs) in accordance with the present invention.

[0008] Figure 3 depicts an illustrative embodiment of a TMU's internal architecture and external interface.

[0009] Figure 4 depicts an illustrative relationship between a 1 PPS timing signal and synchronization data.

[0010] Figure 5 depicts an exemplary GSM/UMTS network including a mixture of synchronized/location-enabled BTSs and synchronized/not location-enabled BTSs.

[0011] Figure 6 depicts an exemplary architecture of an External Interface Unit (EIU).

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
1. Overview

[0012] The present invention is particularly suited for use in connection with a network overlay solution for a GSM communications network. The GSM network is specified by the European Telecommunications Standards Institute (ETSI) and extended by the 3rd Generation Partnerslii.p Project (3GPP). In a fully integrated, GSM specification-compliant Location Services solution, the SMLC (Serving Mobile Location Center) depends on the existing BSC (Base Station Controller) or PCU
(Packet Control Unit) to provide RF assignment information for the MS (Mobile Station, i.e., the mobile unit to be located). By modifying the LMU to monitor the uplink and or downlink control channels, it is possible to implement an emergency-only overlay location solution that satisfies the FCC's E911 mandate and does not require any modifications to the existing GSM handsets or network. An exemplary architecture for such a solution is illustrated in Figure 1. (For further information about this architecture, see U.S. Patent Application No. 20040203429, filed on September 3, 2002 and published on October 14, 2004, "E911 Overlay Solution for GSM, for Use in a Wireless Location System.")

[0013] As shown in Figure 1, the E911 overlay solution comprises the following elements:
1. A GSM communications network 100, including receive/transmit antennae 102A coupled to a Base Transceiver Station (BTS) 104; a Base Station Controller (BSC) 106; a Mobile Switching Center (MSC) 108; and a Gateway Mobile Location Center (GMLC) 110. All of these components and subsystems are well known in the art. See, e.g., 3GPP TS 03.71 V8.6.0 (2002-06).
2. A Location Measuring Unit (LMU) 200A, which as indicated by the dashed line may be co-located with the BTS 104, so as to share antennae 102A
for receiving RF signals from the Mobile Stations. The LMU 202A may include an internal GPS receiver and so a GPS antenna 202A may also be provided. The LMU
may also provide the ability to monitor and demodulate the forward channel signals transmitted by the BTS to the MS. This forward link monitor port may be connected to a separate antenna, or directly to the BTS forward link path. In addition, the system may be configured such that, for a given call, there will be a Primary LMU, in this case LMU 200A, and one or more Cooperating LMUs, e.g., the LMU designated 200B. The Cooperating LMUs are generally configured the same as the Primary LMU, and so they are coupled to a GPS antenna 202B and are typically co-located with a BTS.
3. The LMUs are coupled to a Serving Mobile Location Center (SMLC) 300, which in turn is coupled to a Gateway Mobile Location Center (GMLC) or Mobile Positioning Center (MPC) 400. The concept of the LMU, SMLC, GMLC, and MPC are well known, as can be seen from the above-cited GSM specification documents.
4. Figure 1 also shows a Mobile Station 500. Of course, there will typically be many such units in operation within a geographic region, and more than one may be engaged in an emergency call at a given time.

[0014] In a cellular/wireless system, such as GSM or UMTS system, spectrum utilization can be made far more efficient by synchronizing the BTSs.
For example, 10-20% more voice calls per unit bandwidth can be achieved through BTS
synchronization. Synchronizing a large number of BTSs in a network to an adequate level of accuracy is difficult and requires distributing a timing signal to all BTSs, or installing a satellite-based timing unit in each site. Satellite-based timing units are expensive and take up precious power and space at the BTS sites.

[0015] The present invention provides an arcliitecture in which Location Measurement Units (LMUs) are installed at some or all of the BTS sites for the purpose of locating wireless devices. The LMUs are used to measure the timing of various uplink and/or downlink signals in the cellular network in support of various location techniques. These LMUs may include a GPS-based timing reference module, which is used to synchronize the time bases of all LMUs. This allows relative time difference measurements to be made in support of location.

[0016] To reduce the overall cost of BTS synchronization, the LMU
distributes timing signals, including a periodic electrical pulse as well as time description information, on a serial or other interface, which is available for other nodes to use for synchronization. The format of the electrical pulse and time description infonnation is modified through hardware and software to adapt to the various formats required by various BTS types. For example, the BTSs with co-located LMUs can receive a synchronization signal with little or no hardware cost.
The EI[J described later is used to adapt to various BTS hardware formats.

[0017] Not all BTS sites will be equipped with the LMUs. For those sites without an LMU, a Timing Measurement Unit (TMU) can be used. The TMU has the single function of providing BTS time signals in the same formats as provided by the LMUs. The time signals provided by the TMUs are synchronous to the signals provided by the LMUs. This timing-only TMU has a lower cost than the LMU
because it does not support the uplink or downlink signal measurement functions.
This set of products allows a cellular operator (wireless carrier) to synchronize the BTSs at a relatively low cost.

2. BTS Synchronization

[0018] In accordance with the present invention, the LMUs may contain a high performance GPS receiver to provide a cornmon high accuracy timing reference for all LMUs within the location system. The GPS receiver can provide a timing reference to a co-located base station for the purposes of synchronizing the base station network, i.e., to synchronize the BTSs to within a specified degree of precision. In one exemplary implementation of the invention, the LMU contains a network synchronization interface that may be adapted to be compatible with the corresponding interface on the associated BTS. Thus, through the addition of software modifications, the existing LMUs can be upgraded to a configuration compatible with the BTS interface. This software upgrade is termed the BSS Timing Option (BTO), and can be installed into existing LMU/ BTS installations and shipped with new LMUs.

[0019] For BTS sites without an installed LMU, a Timing Measurement Unit (TMU) may be employed. The TMU contains a GPS receiver and necessary software to conform to the BTS timing interface. A market can contain a mix of LMUs with BTO and TMU timing modules or the carrier may elect to use only the TMU to synchronize markets where LMUs are not yet installed.

[0020] The Timing Measuring Unit is a standalone product that can be deployed independent of the Wireless Location Systems. The TMU contains a built-in GPS receiver, including GPS antenna, for the purpose of establishing precise timestamps. The clocking output includes a 1 pulse per second (PPS) signal and timing information. The TMU provides data in a pre-specified ASCII format developed for use with the particular BTS equipment deployed.

[0021] TruePosition base station synchronization products can be employed in several ways, as recited below and depicted in Figure 2:
1. In green-field deployments which have neither location nor synchronization capability.
2. When upgrading an already synchronized BTS to include location capability.
3. When upgrading a location-enabled BTS to incorporate synchronization.

3. Timing Measurenaent Unit (illustrative esnbodiment)

[0022] To enable synchronized GSM operation by the wireless carrier, a TMU can be deployed to provide a periodic signal and related timing data information to the BTSs. The TMU preferably includes a GPS receiver designed to provide this periodic signal and related timing data information to the BTS over, e.g., an communications interface.

[0023] In one exemplary embodiment, the TMU is a standalone device containing a GPS receiver/engine (GPS), an 80C51 microcontroller (C51), a serial interface for supplying timing information to a BTS, and a console interface.
The purpose of the TMU is to obtain accurate time information from the GPS and supply it to the BTS. Timing is provided to the BTS in the form of a pulse per second (PPS) signal that is preceded by a serial message announcing the precise time at the rising edge of the pulse.

[0024] The TMU attempts to maximize the amount of time that it can supply accurate timing information to the BTS. To this end, the TMU takes measures to bring the GPS on-line as quickly as possible after a power outage and to keep it on-line whenever possible.

[0025] To support maintenance and test, the TMU has three modes of operation, boot mode, test mode and operational mode. Boot mode allows the TMU
firmware to be updated after production. Test mode supports testing and diagnostics of the TMU hardware platform. Operational mode provides the primary TMU
functionality of supplying timing to the BTS.
[00261 The TMU provides synchronization information as described above for two primary reasons:
1) When an LMU is not present in the BTS. When an LMU is present, synchronization information is provided by the LMU through an External Interface Unit (EIU). The External Interface Unit takes the 1 PPS signal and related timing information signal and converts both signals to an RS-422 communication format for interface to the BTS.
2) When an LMU is deployed with equipment already utilizing its signal output capability, such that is it unable to provide the timing signals.
[0027] Figure 3 shows an illustrative embodiment of the TMU internal architecture and external interfaces. The received GPS satellite signal is input to the TMU internal GPS receiver. An internal micro-controller provides the following capabilities:
1) Format GPS timing data in a serial format as may be required.
2) TMU firmware upgrade through the external RS-232 Console port.
3) Control the tri-colored LED that indicates TMU health, and synchronization status.
4) Reset capability through a front panel reset switch.

[0028] The 1 PPS signal output from the GPS receiver, and the formatted serial timing data signal output from the micro-controller are both converted to RS-422 signal levels, and output to the BTS. The 1 PPS and serial data signals are fanned out to 4 four ports that comprise a quad output connector. Each output port provides both a 1 PPS and a serial data output in RS-422 signal levels.

[0029] The TNW micro-controller firmware is capable of upgrade through the RS-232 console port.
[0030] The TNW will transmit the synchronization timing data messages, and the 1PPS signal to the BTS in RS-422 signal levels as shown in Figure 3.
The synchronization timing data interface to the BTS may be a serial communications link.
[0031] The 1 PPS signals distributed by the TNW at each of the 4 output ports may have a frequency of 1 Hz and an accuracy 100 ns RMS with respect to UTC time.
[0032] The serial communications link physical layer is based on a RS-422 UART. Specific characteristics are as follows:

= RS-422 interface with 100 ohm termination in the BTS
= 9600 bits/s = No parity = One start bit = 8 bits data length = One stop bit [0033] A RS-422 transmitter in the TMU drives the one PPS signal. The 10 - 90 % rise time may be less than 10 ns at each of the TNW output ports. The BTS
may include a built in 100 ohm termination.
[0034] The synchronization data is preceding the one PPS pulse. See Figure 4 for timing details. The arrows in Figure 4 show the rising edge of the PPS
pulse-pulse. The data signal containing the timing information is preceding the corresponding PPS-pulse.
[0035] Figure 5 is a schematic diagram showing a GSM or UMTS network in which the BTSs are synchronized using the timing information obtained from an LMU or a TMU. The LMUs may or may not require an ElU, depending on the BTS
interface requirements as discussed herein.

TMU Operational Description (illustrative enzbodinaeut) [0036] As discussed, the TMU provides timing for a BTS that will enable the BTS to synchronize its operation with other BTSs in its network. The TMU

derives timing information from its integral GPS receiver and provides the BTS
with a PPS signal and Periodic PPS Report and Position Data messages. The TMU is deployed at locations where there is no LMU present or where timing signals are unavailable from the deployed LMU. Where the L1VIU is deployed, the L1VIU can supply the same timing functionality as the TMU by employing an EIU.
Synchronized BTSs can increase network capacity through precise management of radio resources.
[0037] The TMU software, in a preferred implementation, supports three modes of operation: boot mode, test mode and opeYatioyaal mode. Although each mode provides a mechanism that allows switching to the others, each mode is independent and mutually exclusive. That is, boot mode does not support test mode functionality, test mode does not support boot mode functionality, neither boot nor test mode provide any operational functionality, and operational mode does not support any functionality of the other two modes.
[0038] To utilize the functionality of any mode, the TMU must first be switched to that mode by an appropriate mechanism (usually a console command).
Once switched into a particular mode, it is understood that the functionality of the other modes is unavailable. For example, when switched to test mode, the time synchronization to the BTS is disabled since this f-unctionality is only supported by operational mode. BTS timing synchronization cannot be resumed until the TMU
is returned to operational mode.
[0039] Certain conditions can prevent the switching from one mode to another. For example, it is not possible to switch out of boot mode if a valid program image is not present. In addition, certain conditions can cause an automatic switch to a mode. For instance, the T1VIU will automatically switch to boot mode on reset if a valid program image is not present.
[0040] The current mode of the TMU may be identified by the console prompt. The console prompt enumerates the current mode as follows.
= "T1VIU>" for operational mode = "Boot>" for boot mode = "Test>" for test mode Boot Mode [0041] Boot mode allows the TMU software to be update in the field. In boot mode, a software image can be downloaded through the console port. The downloaded image will replace the image stored in flash memory. Only the test mode and operational mode portions of the image can be replaced using this method.
The boot mode portion of the image can only be replaced during production or through a JTAG port.
[0042] Boot mode may be entered by console command or it may be automatically invoked following a reset if a valid program image is not found.
Certain failure conditions, such as a watchdog timeout, can produce a reset that may then result in the boot mode being invoked. Boot mode is exited by a reset when a valid program image is present. Reset can be implemented by pushing the reset button, cycling power or by console command. Boot mode cannot be exited if a valid program image is not present. Wllen boot mode is exited successfully, the TMU
returns to the operational mode.

Test Mode j0043] Test mode supports console commands that directly exercise the TMU hardware. Commands are generally either low-level commands or high-level commands. Low-level commands directly manipulate TMU hardware and provide little or no translation for the operator. Low-level commands are useful for board-level test and troubleshooting. High-level commands provide signal interpretation and manipulate combinations of signals to support interaction with the hardware by the operator. These commands are useful when diagnosing operational issues.
[0044] Test mode is intended for use during manufacturing testing, installation, diagnosing of field failures and repair. Test mode is intended for use by a trained technician. Test mode may be entered from operational mode by console command. Test mode is exited by any reset and the TMU returns to the operational mode (as long as a valid program image is present).

Operational Mode [0045] Operational mode is the primary mode for the TMU. When in operational mode, the TMU functions autonomously toward its primary goal, supplying precise time synchronization information to the BTS. While in operational mode, the TMU may send alarms and status information to the console port. In addition, operational mode supports console commands that allow query of operational conditions and manipulation of operational parameters.
[0046] Operational mode is entered automatically following any reset, if a valid program image is present. Operational mode may be exited by invoking test mode or boot mode via console coinmand. Operational mode may be exited automatically if certain failure conditions are detected.

Operational States [0047] The TMU's front panel status LED reflects the TMU's current state.
The state of the TN1U is determined by its mode of operation and the exiting conditions. Of the ten (10) possible LED states, only the following are defined as valid. LED states always indicate existing conditions.

= SOLID RED (failure) - This indicates a failure, such that the TMU is unable to function normally and must be replaced or repaired by a qualified technician.
= FLASHING GREEN (initializing) - This indicates that the TMU is operational and no unexpected conditions have been detected. This state may only exist immediately following reset and indicates that the conditions necessary to provide timing to the BTS have not yet been established. If the required conditions cannot be established within two minutes following reset, the state will proceed to FLASHING AMBER. Once this state has been exited, the TMU will not return to this state until it is again reset = SOLID GREEN (full functionality) - This indicates that the TMU is operating normally, there are no outstanding alarm conditions, and the BTS is being supplied with accurate timing.

= FLASHING AMBER (impaired) - This indicates the TMU is completely functional but that there exist conditions or alarms that prevent the TMU from supplying the BTS with timing. This state is always the result of external influences, such that replacing the TMU itself will not circumvent the issue.
When all outstanding conditions clear, the TMU will return to the SOLID
GREEN state.

Alarsns and Status Messages [0048] In operational mode, the TMU monitors conditions that may affect its ability to provide accurate timing information to the BTS. In addition, it also notes exceptions or conditions that it encounters in the execution of its programming.
Messages concerning these conditions will be sent to the console. These messages are either alarms or status. A status message is purely informative and can indicate anything of interest. The issuance of a status message has no effect on the TMU.
Alarms indicate conditions that may impact the performance of the TMU. The existence of alarms may result in a change of the TMU state. When multiple alarms are indicated, the most severe state is assumed.

Table 1 - TMU Alarms No. Alarm State De9c'ription 1 CPU clock failure Solid Red The CPU external oscillator is not functioning Raised: During software initialization Cleared: Only be reset Indication to BTS:
No messages 2 TMU Initialization Solid Red The TMU CPU has encountered an error during failure initialization Raised: During software initialization Cleared: Only be reset Indication to BTS:
No messages OR
GPSS Status = (3) PPS not Synchronized GPSS Faulty =(1) GPS Receiver Faulty 3 GPS not detected Solid Red The CPU is unable to detect the presence of GPS.
The GPS is assumed to be totally nonfunctional Raised: During software initialization if GPS
fails to respond to initialization procedures Cleared: Only by reset Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (1) GPS Receiver Faulty 4 GPS comm. failure Solid Red The CPU is experiencing difficulty in communicating No. Alarm State vescription with the GPS to the extent that the CPU is unable to control the GPS or to obtain the information necessary for the mandatory BTS reports Raised: The first time that a mandatory BTS report cannot be completed due to GPS communications Cleared: The first time that a mandatory BTS report is successfully accomplished Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty =(1) GPS Receiver Faulty GPS Internal Failure Solid Red The GPS self-test has reported a failure of the GPS
ROM and/or RAM. GPS self-test is run following CPU
reset.
Raised:Based on the self test results Cleared: Only by reset Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (1) GPS Receiver Faulty 6 GPS No satellites Flash Amber There are no satellites available to the GPS
receiver.
(Flash Raised: When the No. of satellites used for positioning Green) of the GPGGA sentence indicates 0 Cleared: When one or more satellites are indicated Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (0) GPS Receiver running 7 GPS PPS Not available Flash Amber The GPS indicates that the PPS will not be output.
(Flash Raised: When the PPS Availability Status field of the Green) GPTps sentence indicates "PPS not output"
Cleared: When PPS output is indicated Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (0) GPS Receiver running 8 GPS No UTC Parameter Flash Amber The UTC parameter has not been obtained and PPS
(Flash accuracy cannot be ensured.
Green) Raised: When the Date/Tirne of UTC Parameter field of No:: Alarm State Description the GPTps sentence indicates "000000000000"
Cleared: When a valid date/time stamp is indicated Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (0) GPS Receiver running 9 GPS Time Not valid Flash Amber The GPS time has not been determined.
(Flash Raised: When the Validity Flag field of the GPgpt Green) sentence indicates "GPS Time not determined yet"
Cleared: When "GPS Time valid" is indicated.
Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty =(0) GPS Receiver running GPS Position Mode, Not Flash Amber The GPS indicates that the position data is not valid.
valid (Flash Raised: When the Position System Mode Indication Green) field of the GPGLL, GPRMC or GPVTG sentence indicates "Data not valid" or The GPS Quality Indication field of the GPGGA sentence indicates "0:
Fix not available or invalid"
Cleared: When "Autonomous mode" or "Differential mode" is indicated Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (0) GPS Receiver running 11 PPS not detected Flash Amber Information from the GPS indicates that the PPS is (Flash being generated, but the PPS is not detected by the Green) CPU.
Raised: When PPS output is expected and PPS is not found for 2 seconds Cleared: When a PPS is detected Indication to BTS:
GPSS Status =(3) PPS not Synchronized GPSS Faulty =(0) GPS Receiver running 12 GPS Navigation receiver Flash Amber The GPS indicates a Navigation receiver warning in the warning (Flash Status field of the GPGLL, GPRMC, or GPRMC
Green) sentences.

No. . Alarm State Description Raised: When "Nav receiver warning" is indicated Cleared: When "Data Valid" is indicated Indication to BTS:
GPSS Status = (3) PPS not Synchronized GPSS Faulty = (0) GPS Receiver running 13 GPS Sats less than 4 Solid Green The No of satellites used for positioning field of the GPGGA or GPGSA sentence indicates that less than 4 satellites are available.
Raised: When 1,2 or 3 satellites are indicated Cleared: When 4 or more satellites are indicated Indication to BTS:
GPSS Status = (0) PPS Loclced GPSS Faulty = (0) GPS Receiver running 14 GPS PPS TRAIM Alarm Solid Green TRAIlV1 is available and indicates a problem with the PPS timing.
Raised: When the PPS Output Result Status field of the GPrrmn sentence is "1"
Cleared: When the a value other than "1" is indicated Indication to BTS:
GPSS Status = (0) PPS Loclced GPSS Faulty = (0) GPS Receiver running 15 GPS Antenna Failure Solid Green The GPS self-test has reported a failure on the GPS
antenna connection. GPS self-test is run following CPU
reset.
Raised:Based on the self-test results Cleared: Only by reset Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty = (0) GPS Receiver running 16 GPS Position Mode, Solid Green The GPS is operating in stand-alone mode.
Autonomous Raised: When the Position System Mode Indication field of the GPGLL, GPRMC or GPVTG sentence indicates "Autonomous mode"
Cleared: When "Autonomous mode" is not indicated Indication to BTS:

No. Alarm State Descriptiori GPSS Status = (0) PPS Locked GPSS Faulty =(0) GPS Receiver running 17 GPS Position Mode, Solid Green The GPS is operating in differential mode.
Differential Raised: When the Position System Mode Indication field of the GPGLL, GPRMC or GPVTG sentence indicates "Differential mode"
Cleared: When "Differential mode" is not indicated Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty =(0) GPS Receiver running 18 GPS DGPS not received Solid Green The GPS is not receiving DGPS data.
Raised: When the DGPS status field of the GPdie sentence is "0"
Cleared: When the DGPS status field of the GPdie sentence is non-zero Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty = (0) GPS Receiver running 19 GPS DGPS base station Solid Green The GPS is receiving DGPS information but the DGPS
unhealthy base station is unhealthy, the GPS will operate stand-alone Raised: When the DGPS Base station's Health Condition field of the GPdie sentence indicates "unhealthy"
Cleared: When "healthy" is indicated Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty = (0) GPS Receiver running 20 GPS DGPS Data Solid Green The GPS is receiving DGPS information but the DGPS
Abnormal data is bad, the GPS will operate stand-alone Raised: When the DGPS Data Status field of the GPDie sentence indicates "Abnormal"
Cleared: When "Normal" is indicated Indication to BTS:
- GPSS Status = (0) PPS Locked No.' Alarm State ' Description GPSS Faulty =(0) GPS Receiver running 21 GPS DGPS Error Solid Green The GPS indicates that an error associated with the DGPS.
Raised: When the DGPS Error Code field of the GPdie sentence is other than "0"
Cleared: When "0" is indicated Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty = (0) GPS Receiver running 22 GPS TRAIM Detection Solid Green TRAIlVI is available but there are only enough satellites Only to detect alarm conditions, deletion of abnormal satellites is not possible Raised: When the TRAIM Status field of the GPrrm sentence is "1"
Cleared: When the value is other than "I"
Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty =(0) GPS Receiver running 23 GPS TRAIM Not Solid Green TRAIM is not available Available Raised: When TRAIM Status field of the GPrrm sentence is "2"
Cleared: When the value is other than "2"
Indication to BTS:
GPSS Status = (0) PPS Locked GPSS Faulty = (0) GPS Receiver running Operational Processes [0049] This section describes the procedures followed by the illustrative TMU software. With the exception of the some of the initial startup processing, all procedures refer to operational mode.
Startup [0050] The startup procedure is performed following any reset of the C51.
The purpose of the startup process is to bring up the platform and establish an operational state. The startup procedure also performs a self-check of the TMU

platform and a software integrity test. If the software integrity test fails, the TMU
enters boot mode.
Establish C51 Control 100511 The first part of the startup procedures establishes the operation of the C51 and configures UO for control of the TMU platform.
1. Check for the presence of a software image.
2. Check the integrity of the software image.
3. Configure the C51 I/O mapping 4. Disable PPS and Serial output to the BTS.
5. Configure the LED drive.
6. Check and switch to the external oscillator.
7. Configure the serial communications ports Establish Control of the GPS
[0052] The second part of the startup procedure establishes control of the GPS. When establishing control of the GPS, the TMU may perform either a warm or a cold restart. A cold restart assumes that the GPS engine must be completely reinitialized and that all previous information is lost. Under these conditions, several minutes may be required before timing can be reestablished. A warm restart attempts to reestablish timing sooner by preserving the information stored in GPS. This is possible because the GPS is an independent subsystem of the TMU. Under some conditions, such as a button reset, the C51 is reset but the GPS is not. In addition, since no power interruption was experienced, the GPS is still operating normally. In these cases, a warm restart reestablishes control of the GPS without disrupting its operation.
[0053] A cold restart of the GPS will be performed if any of the following conditions exist, otherwise, a warm restart will be atteinpted.

= The C51 experienced a power-on-reset.
= A hard self-reset was commanded.

= The GPS does not respond to communications.
= The GPS self test indicates an error.
= The reset button was pushed while the LED state was other than SOLID
GREEN.

Cold Restart [0054] A cold restart of the GPS involves the following steps.
1. Give the GPS a hard reset by asserting its reset signal line.
2. Send the $PFEC,GPclr,l command.
3. Stop all periodic report messages.
4. Perform a Sef-Test.
5. Configure timing for periodic messages.
6. Configure PPS delay due to cable length.
7. PPS Control Mode is set to be output always.
8. Proceed to establishing position.
Warm Restart [0055] A warm restart of the GPS involves the following steps.
1. Give the GPS a hard reset by asserting its reset signal line.
2. Send the $PFEC,GPclr,2 cominand.
3. Stop all periodic report messages.
4. If GPS fails to return response messages, perform Cold Restart.
5. Perform a Self-Test.
6. If Self-Test indicates that backed up data is bad, perform Cold Restart.
7. Configure timing for periodic messages.
8. Configure PPS delay due to cable length.
9. Configure PPS ContNol Mode to be output always.
10. Proceed to establishing position.
Establishing Position [0056] Once the TMU has established control of the GPS, its next objective is to establish its position. The GPS must determine its position before it will be able to produce accurate time information. Following a warm restart, the TMU checks the GPS to determine if the position is already known and fixed (fixed observation mode) by the GPS. If the position is both known and fixed, the TMU reads the location from the GPS and proceeds as normal. If the position is known but not fixed, the TMU
reads the location and proceeds with self-survey as described in the next section. If the position is unknown (or for the case of a cold start), the TMU proceeds with establishing its position.
[0057] The TMU can obtain its position information (latitude, longitude and altitude) from one of three sources, console input, nonvolatile memory or self-survey.

The TMU stores its last known location in its nonvolatile memory. To determine its current position, the TMU sets the GPS to the estimated observation mode and sets the initial position to its last know location. The TNW then proceeds with self-survey.
[0058] A position can be entered manually via console command. If this is done, the location replaces the location data stored in the nonvolatile memory, the GPS is set to the estimated observation mode and the specified location data is written to the GPS as the initial position. The TMU then proceeds with self-survey.
[0059] When position is unknown, there is no last location stored, and there is no console input, the TNW relies completely on the self-survey process. In this case, the GPS is set to the estimated observation mode, and the last known location is used as the initial position. The self-survey process is then allowed to correct the location information. If the last known location is very far from the actual location, it may require an extended amount of time for the TMU to establish its time synchronization.
Self Survey [0060] The TNW utilizes the self-survey process to determine its exact position and, thereby, produce the most accurate timing. To determine location, the TNW places the GPS into the estimated observation mode. h1 this mode, the GPS
will determine its location from the satellites that it can observe. While performing self-survey, the TMU will periodically read the location data from the GPS and compute an average location. Note that self-survey does not prevent the TMLT
from outputting time synchronization information once an initial location has been established by the GPS. The self-survey process will continue for up to12 hours. At the completion of the self-survey period, the GPS will be set to the fixed observation mode and the average location computed will be set. The location determined by self-survey will replace the last known location stored in the TMU nonvolatile memory.
Position Averaging [0061] While performing self-survey, the TNW obtains the estimated location information once each minute in the $GPGGA message. The TMU
iinplements independent averages for longitude, latitude and altitude parameters. The TNW implements a majority-voting algorithm on the integer pontion of each parameter and an averaging of the fi actional portion. The integer portion of latitude and longitude includes degrees and integer minutes. The integer portion of altitude is the whole l 00s of meters. The fractional portion is the fractional minutes of latitude or longitude and altitude modulo 100.
[0062] For integer portions, the majority-voting algorithm observes the current reported value; the two previously reported values and the last known location (LKL) value. If the integer portion of the three reported values agrees with each other but disagree with the LKL, the LKL is discarded and replaced with the agreed integer portion. For example, if the integer portion of the three most recent latitude values agree but disagree with the LKL, the integer portion of the LKL is replaced with the agreed upon value. The fractional portion of the LKL is replaced with the average of the fractional portion of the consenting values.
[0063] If the integer portion of all four values agrees, the fractional portion of the newest value is averaged into the LKL. If all values, except the newest value agree, the fractional portion of the newest value is not averaged into the LKL. The fractional portion is computed by a straight average of all of the contributions since the last time the LKL was replaced.
[0064] The majority-voting algorithm helps to protect the average from the influence of anomalous locations. Additional rules or algoritluns may be employed in determining the stability of the location average and allow a more rapid change to fixed position mode.
Last Known Location [0065] The TMU stores its last known location in its nonvolatile memory.
This location is utilized to hasten the establishment the GPS time output. To minimize the wear on nonvolatile memory, the value will be updated only on one of the following conditions.
= When a manual location is entered via console command.
= On completion of the self-survey processing.
= Whenever, the self-survey average differs from the stored location by more than 1/100 minute of latitude or longitude, or more than 10 meters of altitude.
Antenna Cable Length [0066] The length of the cable to the GPS antenna can affect the accuracy of the PPS. The TMU requires that this value be entered manually during installation.
The POSITION console command is provided for this purpose. The cable length will be stored to non-volatile memory and will be utilized every time the GPS is configured.
Initiating Output to BTS
[0067] The TMU configures the GPS to begin output of timing data immediately. The TMU configures the GPS to begin outputting the PPS signal immediately. If the GPS is in the fixed observation mode, the PPS will be accurate for as long as one satellite is available. If the GPS is in the estimated observation mode, the PPS will become accurate when 4 satellites are available to fix the position, the UTC parameter is available, ephemeris data for satellite is available, and the UTC
computation completes.
[0068] The TMU will begin sending the Periodic Pulse Report (GPppr) and the Position Data Report (GPGGA) to the BTS immediately after initialization.
As soon as the PPS signal is available from the GPS, the TMU will begin sourcing the PPS signal to the BTS as well. However, the GPSS Status field of the GPppr will indicate "PPS Not synchronized" until all alarm conditions in the above table marked Flash Green are clear.
Supporting Greater Timing Accuracy [0069] The TNIU attempts to support the greatest possible timing accuracy by allowing the GPS to utilize its DGPS and TRAIM features. These features are enabled by default.
Synchronization Loss [0070] Once timing output has successfully cominenced, the occurrence of any critical alarm will cause the GPSS Status field of the GPppr to indicate, "PPS Not synchronized" until the condition clears.
BTS Messages Supported [0071] The TMU supports only the messages that are mandatory. lii addition, only the mandatory fields within these messages are supported. These messages are:
1. Periodic PPS Report 2. Position Data Report Periodic PPS Report ($PTP,GPppr) [0072] The GPS TOW Standard Deviation field of the Periodic PPS Report will be populated as follows.

= If 5 or more satellites used for positioning, the field will be set to 50nsec = If 4 or less satellites are used for positioning, the field will be set to lOOnsec = If no satellites cause are available, the GPS Status field will be set to (3) PPS
not synchronized Position Data (&GPGGA) [0073] The optional fields; DGPS Data Time, DGPS Station ID and the checksum will not be provided. The fields; DOP, Geoid ofAltitude, and Un.it of Geoid are set to a blank.

Console Port Operation [0074] The console port allows human interaction and monitoring of the T1VIU through an ASCII terminal or terminal emulation software. Following reset or by entering escape at the command prompt, the console interface enters status display mode. In this mode, alarms and other event driven status strings are sent to the console. The console can collect these strings to monitor the operation and health of the TMU.
[0075] When the enter key is pressed while in status display mode, the console interface changes to command entfy mode and issues the command prompt.
The command prompt reflects the current mode of TMU operation; boot, test or operational. Commands may then be entered and the results will be sent to the console. All spontaneous alarm and status string output will be inhibited while in command entry mode.
[0076] The commands available are limited by the TMU's mode of operation. An operator may change modes to obtain access to the desired cominands.
The operator should be aware of the consequences of invoking any TMU mode of operation.

4. External Interface Unit (EIU) (illustrative embodiment) [0077] As discussed, to enable synchronized GSM operation, a 1 PPS signal may be provided to the BTSs. For sites that already have an LMU deployed with them, the 1 PPS signal may already be available on those existing LMUs (since the L1V1Us include built-in GPS receivers). However, for certain types of BTS
equipment, the following may be true:

= The 1 PPS signal needs to be converted to RS-422 signal levels for this application.
= In addition to the 1 PPS conversion, the timing information related to I PPS
signal also needs to be sent over the RS-422 interface using the proprietary protocol called for by the BTS equipment manufacturer (e.g., Ericsson).
[0078] The protocol conversion hardware unit that performs these two operations is called an EIU and is applicable to those cell sites that already have an LMU deployed there.
Inapact orz GBE and mE-board connectivity = The ERJ will be connected to the 9-pin RS-232 serial port on the LMU. This is the same port that is also used to connect the GBE (ground based electronics) in AOA deployments. Hence, in their present forms the GBE and EIU cannot be co-deployed. Therefore, installation of EIU precludes AOA deployment.
The solution to this problem is to use a TMU instead of an ERJ in cases where AOA is needed.
= Similar to the above problem is the case of using the environmental board (which is sometimes called the mini environmental board, or mE-board). It also uses the same port and cannot be deployed where an ERJ is used.
Architecture [0079] An exemplary architecture for an ERJ is depicted in Figure 6, which shows the internal architecture and external interfaces of the EIU. It connects to the 9 pin serial port and the 1 PPS on the LMU side, and converts both of these interfaces to RS-422 signal levels for connection with the BTS. The 1 PPS and serial data signals are fanned out to 4 four ports that comprise a quad output connector. Each output port provides both a 1 PPS and a serial data output in RS-422 signal levels.

LMU-N Interface [0080] The illustrative EIU receives timing messages from its LMU
interface in RS-232 signal format/levels. The RS-232 signal connection pin outs will be as shown in table 1. The EIU receives the 1 PPS signal from the LMU through its 1 PPS port. The 1 PPS EIU port appears as a 50-ohm load from outside.

Pin Signal Name Description 1,7,8,9 NC
2 RX Port 1 Receive, from the PC to the processor 3 TX Port 1 Transmit, from the processor to PC
4 DTR Data Terminal Ready - from PC
GND Ground 6 DSR Data Set Ready Table: RS-232 Connector Pin Outs BTS Interface [0081] The EIU transmits the LMU synchronization data messages and the 1PPS signal to the BTS in RS-422 signal levels as shown in Figure 4. The synchronization data interface to the BTS is a serial communications link.
[0082] The 1 PPS signal will have a frequency of 1 Hz and an accuracy of lOOns RMS at the 1 PPS EIU output port with respect to UTC time.
[0083] The signal connection pin outs for each port will be as shown in the table below.
Pin Signal Name Description 1 TX+ Transmit 2 TX- Transmit return 3 TX+ Transmit (optional) 4 1PPS Pulse Per Second 5 1PPS- Pulse Per Second Return 6 TX- Transmit return (optional) 7,8 NC
9,10 GND

Table: RS-422 Single Port Pin Outs Serial Communications Link 100841 The serial communications link physical layer is based on a RS-422 UART. Specific characteristics are as follows:
= RS-422 interface with 100 ohm termination in the BTS
= 9600 bits/s = No parity = One start bit = 8 bits data length = One stop bit One PPS
[0085] A RS-422 transmitter in the EIU drives the one PPS signal. The 10 -90 % rise time will be less than 10 ns at the EIU output. The BTS has a built in 100 ohm termination.

5. Conclusion [0086] The true scope the present invention is not limited to the illustrative and presently preferred embodiments disclosed herein. For example, the foregoing disclosure of a Wireless Location System uses explanatory terms, such as LMU, TMU, EIU, BTS, BSC, SMLC, and the like, which should not be construed so as to limit the scope of protection of the following claims, or to otherwise imply that the inventive aspects of the Wireless Location System are limited to the particular methods and apparatus disclosed. Moreover, as will be understood by those skilled in the art, many of the inventive aspects disclosed herein may be applied in location systems that are not based on TDOA techniques. In such non-TDOA systems, the SMLC described above would not be required to perform TDOA calculations.
Similarly, the invention is not limited to systems employing LMUs constructed in a particular manner, or to systems employing specific types of receivers, computers, signal processors, etc. The LMUs, SMLC, etc., are essentially programmable data collection and processing devices that could take a variety of forms without departing from the inventive concepts disclosed herein. Given the rapidly declining cost of

26 digital signal processing and other processing functions, it is easily possible, for example, to transfer the processing for a particular function from one of the functional elements (such as the SMLC) described herein to another functional element (such as the LMU) without changing the inventive operation of the systein. In many cases, the place of implementation (i.e., the functional element) described herein is merely a designer's preference and not a hard requirement. Accordingly, except as they may be expressly so limited, the scope of protection of the following claims is not intended to be limited to the specific embodiments described above.
[0087] In addition, any reference herein to control channels or voice channels shall refer to all types of control or voice channels, whatever the preferred terminology for a particular air interface. Moreover, there are many more types of air interfaces (e.g., IS-95 CDMA, CDMA 2000, and UMTS WCDMA) used throughout the world, and, unless the contrary is indicated, there is no intent to exclude any air interface from the inventive concepts described within this specification.
Indeed, those skilled in the art will recognize other interfaces used elsewhere are derivatives of or similar in class to those described above.

27

Claims (23)

1. In a network overlay wireless location solution for a wireless communications system comprising a network of Base Transceiver Stations (BTSs), a method of improving spectrum, comprising synchronizing a plurality of BTSs with a timing signal, wherein at least one BTS is provided with said timing signal by a Timing Measurement Unit (TMU).

2. A method as recited in claim 1, wherein said wireless communications system comprises a GSM communications network.

3. A method as recited in claim 1, wherein said wireless communications system comprises a UMTS communications network.

4. A method as recited in any of claims 1 - 3, wherein the timing signal is provided to each BTS by either a Location Measurement Unit (LMU) or a Timing Measurement Unit (TMU).

5. A method as recited in claim 4, wherein each LMU and TMU comprises a GPS-based timing reference module and means for generating a periodic timing signal which is synchronized within a pre-specified degree of accuracy with the timing signals generated by each other LMU and TMU.

6. A method as recited in claim 5, wherein the LMUs are used to measure the timing of uplink and/or downlink signals in the cellular network in support of location techniques.

7. A method as recited in claim 6, wherein the LMUs and TMUs distribute timing signals, including a periodic electrical pulse as well as time description information.

8. A method as recited in claim 7, wherein the format of the electrical pulse and time description information are modified through hardware and software to adapt to formats required by various BTS types.

9. A method as recited in claim 8, wherein the BTSs with co-located LMUs receive a synchronization signal with little or no hardware cost, and wherein BTS
sites not equipped with an LMU are equipped with a TMU that has the single function of providing BTS time signals in the same formats as provided by the LMUs, wherein the time signals provided by the TMUs are synchronous to the signals provided by the LMUs and the timing-only TMU has a lower cost than the LMU because it does not support uplink or downlink signal measurement functions.

10. A network overlay wireless location system for use in association with a wireless communications system comprising a network of Base Transceiver Stations (BTSs), comprising a plurality of Location Measurement Units (LMUs) and at least one Timing Measurement Unit (TMU), and a mechanism for synchronizing a plurality of BTSs with a timing signal, wherein at least one BTS is provided with said timing signal by said at least one TMU.

11. A wireless location system as recited in claim 10, wherein said wireless communications system comprises a GSM communications network.

12. A wireless location system as recited in claim 10, wherein said wireless communications system comprises a UMTS communications network.

13. A wireless location system as recited in any of claims 10 - 12, wherein the timing signal is provided to each BTS by either a Location Measurement Unit (LMU) or a Timing Measurement Unit (TMU).

14. A wireless location system as recited in claim 13, wherein each LMU and TMU comprises a GPS-based timing reference module and means for generating a periodic timing signal which is synchronized within a pre-specified degree of accuracy with the timing signals generated by each other LMU and TMU.

15. A wireless location system as recited in claim 14, wherein the LMUs are used to measure the timing of uplink and/or downlink signals in the cellular network in support of location techniques.

16. A wireless location system as recited in claim 15, wherein the LMUs and TMUs distribute timing signals, including a periodic electrical pulse as well as time description information.

17. A wireless location system as recited in claim 16, wherein the format of the electrical pulse and time description information are modified through hardware and software to adapt to formats required by various BTS types.

18. A wireless location system as recited in claim 17, wherein the BTSs with co-located LMUs receive a synchronization signal with little or no hardware cost, and wherein BTS sites not equipped with an LMU are equipped with a TMU that has the single function of providing BTS time signals in the same formats as provided by the LMUs, wherein the time signals provided by the TMUs are synchronous to the signals provided by the LMUs and the timing-only TMU has a lower cost than the LMU
because it does not support uplink or downlink signal measurement functions.

19. A wireless location system for use in association with a wireless communications system comprising a network of Base Transceiver Stations (BTSs), comprising a plurality of Location Measurement Units (LMUs) and at least one Timing Measurement Unit (TMU), wherein said LMUs and at least one TMU are operative for synchronizing a plurality of BTSs with a timing signal, wherein at least one of said BTSs is provided with said timing signal by said at least one TMU;
and wherein each LMU and TMU comprises a GPS-based timing reference module and means for generating time description information and a periodic timing signal that is synchronized with the timing signals generated by each other LMU and TMU.

20. A wireless location system as recited in claim 19, wherein BTSs with co-located LMUs receive a synchronization signal with little or no hardware cost, and wherein BTS sites not equipped with an LMU are equipped with a TMU that has the single function of providing BTS time signals in the same formats as provided by the LMUs, wherein the time signals provided by the TMUs are synchronous to the signals provided by the LMUs and the TMU has a lower cost than the LMU because it does not support uplink or downlink signal measurement functions.

21. A wireless location system as recited in claim 20, wherein said wireless communications system comprises a GSM communications network.

22. A wireless location system as recited in claim 20, wherein said wireless communications system comprises a UMTS communications network.

23. A wireless location system as recited in claim 20, wherein the format of the timing signal and time description information are modified through hardware and software to adapt to formats required by various BTS types.