Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-09-30
2001-10-02
Nguyen, Chau (Department: 2663)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S503000, C375S134000
Reexamination Certificate
active
06298054
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to wireless communication systems, and more specifically to a method and apparatus in a wireless messaging unit for acquiring transmitter or receiver synchronization.
BACKGROUND OF THE INVENTION
Synchronous messaging protocols, such as Motorola's FLEX™ protocol for a messaging system, require the periodic transmission of synchronization signals so that the wireless messaging units used in the messaging system can maintain synchronization with the system. The FLEX™ protocol, for example, requires a synchronization word at the beginning of each frame, i.e., every 1.875 seconds.
Some existing and proposed systems can encounter difficulties in providing sufficient synchronization signals to keep the wireless messaging units synchronized. Examples are: shared systems, high speed Quadrature Amplitude Modulation (QAM) systems, frequency-hopped spread-spectrum (FHSS) systems, and two-way systems. In a shared system, the synchronization signals of one system are interrupted when the other system uses the channel. In QAM systems, for best efficiency the designers prefer to use only a QAM sync, but a QAM sync is more difficult to acquire than a frequency-shift-keyed (FSK) sync. In a FHSS system, if synchronization is lost, the receiver will have great difficulty finding where in the frequency hopping sequence the transmitted signal is, and scanning will be lengthy. In a two-way system, the wireless messaging units' inbound transmissions are synchronized with the outbound transmissions. Maintaining synchronization between widely separated synchronization signals calls for expensive crystals in the wireless messaging units.
Thus, what is needed is a method and apparatus in a wireless messaging unit for establishing transmitter or receiver synchronization. Preferably, the method and apparatus will not add great expense to the wireless messaging unit.
SUMMARY OF THE INVENTION
An aspect of the present invention is a method in a wireless messaging unit for establishing receiver synchronization with a first channel which utilizes a first plurality of time slots for transmission. The method comprises the steps of determining that the first channel does not have a signal suitable for acquiring receiver synchronization; and, in response, attempting to acquire an initial synchronization with a second channel which periodically transmits a synchronization signal in a second plurality of time slots having a predetermined timing relationship with the first plurality of time slots. The method further comprises the step of, after acquiring the initial synchronization, applying the predetermined timing relationship to the initial synchronization to establish a secondary synchronization with the first channel.
Another aspect of the present invention is a wireless messaging unit for establishing receiver synchronization with a first channel which utilizes a first plurality of time slots for transmission. The wireless messaging unit comprises a receiver for receiving a message, and a processing system coupled to the receiver for processing the message. The processing system is programmed to cooperate with the receiver to determine that the first channel does not have a signal suitable for acquiring receiver synchronization; and, in response, to attempt to acquire an initial synchronization with a second channel which periodically transmits a synchronization signal in a second plurality of time slots having a predetermined timing relationship with the first plurality of time slots. The processing system is further programmed to, after acquiring the initial synchronization, apply the predetermined timing relationship to establish a secondary synchronization with the first channel.
Another aspect of the present invention is a method in a wireless messaging unit for establishing transmitter synchronization with a first channel utilizing a first plurality of time slots, wherein a message is to be transmitted as a frequency-hopped, spread-spectrum signal having a predetermined frequency-hopping sequence. The method comprises the step of attempting to acquire an initial synchronization with a second channel which periodically transmits a synchronization signal in a second plurality of time slots having a predetermined timing relationship with the first plurality of time slots, wherein the synchronization signal includes an identification number for uniquely identifying an order of transmission of the synchronization signal within a predetermined repeating cycle of transmissions. The method further comprises the steps of, after acquiring the initial synchronization, applying the predetermined timing relationship to the initial synchronization to establish a secondary synchronization with the first channel, and calculating from the identification number a point in the predetermined frequency-hopping sequence to use for beginning to transmit the message.
Yet another aspect of the present invention is a wireless messaging unit, comprising a transmitter for transmitting a message as a frequency-hopped, spread-spectrum signal on a first channel in a first plurality of time slots having a predetermined frequency-hopping sequence, and a processing system coupled to the transmitter for controlling the transmitter. The processing system is programmed to cooperate with the transmitter to attempt to acquire an initial synchronization with a second channel which periodically transmits a synchronization signal in a second plurality of time slots having a predetermined timing relationship with the first plurality of time slots. The synchronization signal includes an identification number for uniquely identifying an order of transmission of the synchronization signal within a predetermined repeating cycle of transmissions. The processing system is further programmed to, after acquiring the initial synchronization, apply the predetermined timing relationship to the initial synchronization to establish a secondary synchronization with the first channel, and to calculate from the identification number a point in the predetermined frequency-hopping sequence to use for beginning to transmit the message.
REFERENCES:
patent: 5381443 (1995-01-01), Borth et al.
patent: 5425049 (1995-06-01), Dent
patent: 5684794 (1997-11-01), Lopez et al.
Dorenbosch Jheroen Pieter
Souissi Slim
Breeden R. Louis
Mehra Inder Pal
Motorola Inc.
Nguyen Chau
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