Transceiver

Field of the Invention

This invention generally relates to transceivers and more specifically to a method and apparatus of offsetting the frequencies generated in the receiver and transmitter, thereby eliminating self-quieting within the transceiver.

Background of the Invention

Recently in the field of radio frequency (RF) communications there has been an increased demand for digitally encrypted voice and high speed data communications. Since the HF spectrum is inherently limited, one must devise a new system concept in organizational features to accommodate the increased demand. A time division multiple access (TDMA) system is one such system which offers more efficient spectrum utilization and increased capacity.

In its simplest form, a TDMA system is comprised of a transmitting base station, which is capable of time multiplexing messages from at least two users on a single RF channel, and one or more remote receiving stations capable of receiving at least one of the time multiplexed messages.

Typically, the remote receiving station would be a mobile or portable radiotelephone capable of transmitting and receiving TDMA messages to and from the base station.

One implementation of a TDMA system uses a time division duplex (TDD) slot structure. The TDD system uses one RF channel to send and receive control and communication information between the remote receiving station and the base station. One problem which arises in a TDD system is that the transmitter's offset frequency is equal to the receiver's intermediate frequency (IF). If both of the frequency generators remain on during transmission and reception, they will interfere with each other, causing self- quieting of the data transmitted and received. This problem can occur in the remote receiving station as well as in the base station.

One solution which has been developed to cure this problem is to use fast locking frequency synthesizers as the frequency generators in the transmitter and receiver located either in the base station or in the remote receiving station. The fast locking synthesizers can be turned on and off between transmission and receiving time slots and turned back on and quickly locked to their desired frequency within the specification of the TDD system. The fast locking frequency synthesizers system is very expensive, consumes a large amount of power, and has complex implementations requirements.

Therefore, it is desirable to have an inexpensive power efficient TDD transceiver which is not difficult to implement and avoids self-quieting of the transmitted or received data.

Summary of the Invention

The present invention encompasses a method of operating a radio frequency transceiver. The transceiver includes a local oscillator which generates a two local oscillator signals at different times. Each local oscillator signal has a unique frequency. During a first time period, the radio frequency transceiver receives a radio frequency signal. The first radio frequency signal is combined with one of the local oscillator signals, forming an intermediate frequency signal. During a second time period, the transceiver a frequency offset signal. The frequency offset signal is combined with the second local oscillator signal, forming a second radio frequency signal. The first and second radio frequency signals have the same frequency.

Brief Description of the Drawings

FIG. 1 is a block diagram of the radiotelephone communications system which may employ the present invention.

FIG. 2 is an illustration of a TDMA/TDD time frame in accordance with the present invention.

FIG. 3 is a block diagram of a transceiver in accordance with the present invention.

Detailed Description of a Preferred Embodiment

The preferred embodiment encompasses a digital TDD transceiver of a radiotelephone. The digital transceiver includes a local oscillator (LO), an intermediate frequency (IF) synthesizer and a offset frequency synthesizer. The local oscillator is a fast locking frequency synthesizer capable of changing frequency up to 12 Mhz within 1.5 milliseconds as required to change channels within the TDD system. The frequency synthesizers in the transmitter and receiver are offset from each other by 2 Mhz allowing 60 dB of isolation between the two to avoid the problem of self-quieting. The local oscillator alternates the frequency generated between the transmit and receive time slots. These alternating frequencies are mixed with the IF and the offset frequency of the transmitter and receiver. During a receive timeslot, the received radio frequency is mixed-down, using the LO, to the appropriate IF. During a transmit timeslot, the offset frequency is mixed-up, using the LO, to the same radio frequency.

FIG. 1 is a block diagram of a radiotelephone system which may employ the present invention. In the radiotelephone system, the fixed site transceiver 103 sends and receives radio frequency (RF) signals to and from the mobile and portable radiotelephones contained within a fixed geographic area. Radiotelephone 1O1 is one such radiotelephone served by the fixed site transceiver 103.

While receiving signals from these fixed site transceivers 103, the radiotelephone 101 uses the antenna 105 to couple the RF signal and to convert the RF signal into an electrical radio frequency signal 107. The electrical RF signal is received by the radio receiver 109 for use within the radiotelephone 101.

The receiver 109 generates an intermediate frequency signal for use in demodulating the electrical RF signal 107 and recovering the data contained within the electrical radio frequency signal 107. The resulting data signal 121 is output from the receiver into the processing system 117. The processing system 117 formats the data signal 121 into voice and/or data for the user interface 123. The user interface 123 contains a microphone, a speaker and a keypad.

Upon transmission of RF signals from the portable radiotelephone 101 to the fixed site transceiver 103, the voice and/or data signals from the user interface 123 are processed by the processing system 117. The processed signals are input into the transmitter 111. The transmitter 111 generates an offset frequency signal used to modulate the processed signal 119 into electrical RF signals which are then input into the antenna 105. The electrical RF signals 107 are converted into RF signals and output by the antenna 105. The RF signals are received by the fixed site transceiver 103 which interfaces to the landline telephone signals. FIG. 2 is a block diagram illustrating the time division duplex TDMA system of the preferred embodiment. The TDD frame 200 is 5 -milliseconds in duration on a radio frequency channel. Identical TDD frames are repeated every 5 milliseconds. In the portable radiotelephone 101 of FIG. 1, the radiotelephone will transmit information to the fixed site transceiver 103 during time slot 205. The portable radiotelephone 101 will receive radio frequency signals from the fixed site transceiver 103 during receive time slot 207. After the transmit time slot 205 is complete, the portable radiotelephone 101 has 1.875 milliseconds in which to set up

the transceiver to receive data via antenna 105. The specific implementation of the preferred embodiment is illustrated in the detailed block diagram of FIG. 3.

FIG. 3 is an illustration of a radio frequency transceiver in accordance with the present invention, hi the preferred embodiment, antenna 105 receives a radio frequency signal between 1895 to 1907 Mhz. For purposes of illustration we will refer to signal 107 as a 1900. Mhz electrical radio frequency signal 107. Likewise, the antenna 105 transmits a 1900 Mhz electrical radio frequency signal 107 containing information from the processed data signal 119. The local oscillator 303 is set to a frequency between 1762-1775 Mhz. This local oscillator is a fast acquisition frequency synthesizer which allows the oscillator to operate within a 12 Mhz range within 1.5 milliseconds. This fast acquisition frequency synthesizer is necessary in any radiotelephone in order to change channels within the radiotelephone system.

Upon transmitting information during the transmit time slot 205, the transmitter 111 receives the processed data signal 119 from the processing system 117. This processed data signal 119 is modulated and properly formatted in the functional box 307. The output of the functional box 307 is an offset frequency signal 313 having a frequency of 131 Mhz. The offset frequency signal 313 is mixed with the local oscillator frequency 113 at mixer 305 to produce 1900 Mhz electrical radio frequency signal which is output to the antenna 105. During the transmit time slot 205 the frequency of the local oscillator 303 is set to 1769 Mhz. After the data contained in the processed data signal 119 is transmitted, the transceiver 300 has 1.875 milliseconds as illustrated in time duration 203 of

FIG. 2 to change frequencies for the reception during receive time slot 207.

During receive time slot 207 the receiver 109 receives an electrical radio frequency signal 107 from the fixed site transceiver 103 of FIG. 1 having a frequency of 1900 Mhz. The electrical radio frequency signal 107 is then mixed with the local oscillator frequency signal 113 at mixer 301 contained within the receiver 109. The resulting signal 311 is an intermediate frequency signal having a frequency equal to 133 Mhz. In order to generate this intermediate frequency, the local oscillator frequency is set to 1767 Mhz. Thus, the local oscillator frequency signal 113 needs to change during the 1.875 milliseconds of delay from 1769 Mhz to 1767 Mhz. This intermediate frequency signal 311 is input to the functional box 309 which generates the resulting data signal 121.

By requiring change in frequency of the local oscillator 303 during this dead time. The system avoids the self-quieting problem which is caused by the offset frequency signal 313 and the intermediate frequency signal 311 having the same frequency. If they do have the same frequency, the self- quieting occurs resulting in corrupted data and errors during phone calls. Since the fast locking frequency synthesizer of the local oscillator 303 is already required for the radiotelephone system the system is cost, size and power efficient.

What is claimed is: