1. Field of Invention
The present invention pertains generally to communications technology and more particularly to the synchronization of the transmission of data in time slots.
2. Background of the Invention
Currently, there are three common approaches for transferring data over a wireless communication link. The first approach uses a contention system that operates such that any mobile or fixed unit which is transmitting a data message over a wireless communication link can do so at any time. In this type of wireless communication system, if any two or more devices transfer information over the wireless communication link at the same time, the data transmitted from these two or more devices will interfere with each other such that data messages will most likely be lost. This can occur even though busy link detect logic is utilized since both the mobile and fixed units may test the link at the same time and transmit data believing that the link is clear.
The second approach uses a poling system that employs queuing. In a poling system, the base station sends a data message over the wireless communication link which queues a particular mobile unit to transfer data in the next time slot. The fixed unit transmits a data message that contains an identification number of the mobile unit indicating that the mobile unit should communicate data during the next time slot. The mobile unit decodes the data message and determines that it has been queued and then transfers data over the wireless communication link. The fixed unit will delay transmission of a queue data message to another mobile unit for a predetermined time in order to allow the first mobile unit to send messages. After a delay timer has expired, the fixed unit then sends the next queue data message to the next mobile unit. The process repeats until all of the mobile units have been queued. Hence, the poling system prevents units from using the same wireless communication link at the same time so that data throughput is increased. However, for each data message sent by the mobile unit, there must be a queue message sent by the base station, thus doubling the required amount of air time needed for communicating.
The third approach uses internal clock systems and pre-assigned time slots. In accordance with this approach, each mobile unit and base station utilize an internal time clock to determine the time at which that particular unit can transmit data (transmit time). This technique maximizes channel efficiency since collisions do not occur and poling is not required. Using this method, each mobile unit and base station is assigned a time slot of a given duration during which that unit can use to transmit data. However, such systems require complex clock synchronization techniques to insure that each unit is synchronized with the other units in the system and to prevent data collisions.
Various techniques have been used to synchronize the transmission of data in time slots that employ GPS synchronization such as disclosed in U.S. Pat. No. 5,367,524 issued Nov. 22, 1994 entitled xe2x80x9cMethod for Sequential Data Transmission,xe2x80x9d U.S. Pat. No. 5,510,797 issued Apr. 23, 1996 entitled xe2x80x9cProvision of SPS Timing Signals,xe2x80x9d and U.S. Pat. No. 5,875,402 issued Feb. 22, 1999 entitled xe2x80x9cTime-Synchronous Communication Systemxe2x80x9d, all of which are specifically incorporated herein by reference for all that they disclose and teach. Although each of these references discloses the use of GPS synchronization signals to transmit data in assigned time slots, none of these systems discloses the use of empirically derived synchronization signals to remove error skew that may exist within the system so as to ensure proper sampling of data.
Hence, it would be advantageous to have a simple technique for synchronizing the transmission of data between various units and a base station in a system where mobile units are used that are continuously changing location and ensure that proper sampling can occur by using both GPS synchronization and empirically derived synchronization.
The present invention overcomes the disadvantages and limitations of the prior art by providing a system that synchronizes the transmission of data between multiple units and a base station by using a GPS synchronous clock signal. The GPS synchronous clock signal is transmitted to mobile units and a base station and is capable of providing a synchronized universal time clock that is independent of the location of the mobile unit. The internal bit clocks of the mobile units and the base station are synchronized with a GPS sync pulse. Each mobile unit then calculates the transmission delay between the mobile unit and the base station using the GPS location coordinates of the mobile unit and the GPS location coordinates of the base station to determine a transmission distance. The channel delay of the mobile unit, which is the delay of the internal circuitry of the unit is then combined with the distance delay to calculate a transmission time. The mobile unit can be calibrated at the factory and stored in the mobile unit. Alternatively, the channel delay can be calibrated in the field periodically. The channel delay and the calculated distance delay are then combined to determine the time at which date should be transmitted in accordance with the assigned time slot of the mobile unit. A time slot for each mobile unit can be assigned during log-on of the mobile unit to the system or can be pre-assigned.
The present invention also corrects any additional error skew that may be encountered in the system. Error skew is corrected by transmitting a preamble with each data burst that is transmitted from the base station to the mobile unit that comprises a series of ones and zeros. The error skew is then determined by detecting the skew that exists between the delay data pulse and the bit clock that has been synchronized with the GPS sync pulse. In this fashion, synchronization of the bit clock of each of the mobile units with the actual data received from the base station is insured.
The present invention may therefore comprise a method of synchronizing the transmission of information in assigned time slots between mobile transceiver units having GPS receivers and a base station having a GPS receiver comprising: using the GPS receiver in the mobile units for receiving GPS synchronization data; using the GPS receiver in the base station to receive the GPS synchronization data; using a GPS synchronization pulse generated from the GPS synchronization data to synchronize internal bit clocks in the mobile units and the base station to provide GPS synchronized internal bit clocks in the mobile units and the base station; transmitting base station synchronization pulses from the base station to the mobile units that are synchronized with the GPS synchronized internal bit clock of the base station; further synchronizing the GPS synchronized internal bit clocks of the mobile units with the base station synchronization pulses to produce empirically synchronized internal bit clocks of the mobile units; transmitting data from the mobile units in assigned time slots using the empirically synchronized internal bit clocks of the mobile units.
The present invention may further comprise a system for synchronizing the transmission of data in assigned time slots comprising: mobile units that have communication transceivers; a base station having a communication transceiver for communicating with the mobile units; GPS receivers disposed in the mobile units and the base station that receive GPS synchronization data and generate GPS synchronization pulses and GPS coordinate information from the GPS synchronization data; internal bit clocks disposed in the mobile units and the base station that are calibrated to the GPS synchronization pulses to produce GPS synchronized internal bit clocks; logic control devices disposed in the mobile units that are programmed to control the transmission of data from the mobile units to the base station in assigned time slots and that are programmed to further synchronize the GPS synchronized internal bit clocks of the mobile units with base station synchronization pulses, the base station synchronization pulses being synchronized with the GPS synchronized internal bit clock of the base station.
The advantages of the present invention are that the GPS sync pulse is used to synchronize each of the mobile units and the base station. Additionally, each of the mobile units, which may be moving and changing locations, can utilize its GPS coordinates to determine the distance between the mobile unit and the base station. The transmission delay can then be calculated in a simple and easy fashion and as frequently as several times a second. In this manner, each mobile unit can be assured that data transmitted from the mobile units arrive at the base station during the assigned time slots for each mobile unit. Hence, mobile units that contain GPS receivers can utilize those receivers in a very advantageous fashion to synchronize transmissions in a time division multiplexed communication system. Further, empirically detected error skew can be eliminated using synchronization pulses transmitted from the base station to the mobile units.