Abstract:
A transreceiver allows efficient operation in a full or half-duplex TDD or FDD system. The preferred transreceiver includes a common set of filters used for both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) operation in a given range of frequencies thereby reducing circuitry count and implementation costs. Thus, the transreceiver of the present invention can operate in FDD operation or TDD operation.

Description:
FIELD OF THE INVENTION 
     The field of the invention pertains to wireless communication transreceivers including, more particularly, transreceivers capable of FDD and TDD operation. 
     DESCRIPTION OF RELATED ART 
     One method of providing duplex communication is through use of FDD (Frequency Division Duplex) protocols in which frequency allocations in the PCS band is split into a forward sub-band and a reverse sub-band. This split can accommodate FDD where transmission is limited to one of the sub-bands. However, this split presents a problem to coexisting TDD (Time Division Duplex) systems which transmit and receive on the same frequency and can use either of the frequency sub-bands for transmission. 
     Shown in FIG. 1 is a known implementation of a TDMA-FDD system, such as PCS-1900 or IS-136 with a switch placed between the radio and the antenna. The switch, placed before the forward and reverse channel filters, selectively routes the RF signal path either to the receiver or from the transmitter in response to the mode of the transreceiver (either transmit or receive). The receiver subsystem will typically employ a bandpass filter tuned to the forward channel, and the transmitter subsystem will typically employ a filter tuned to the reverse channel. As a result of this switch placement, only the forward channel path or only the reverse channel path may be selected. This configuration precludes transmission and reception in both the forward channel frequency band or the reverse channel frequency band, thereby limiting the available frequency bands for a TDD system. Other known implementations remove the switch entirely to allow simultaneous transmit and receive (e.g., IS-54 and IS-19) but are still limited to different frequencies. 
     While a transreceiver operable in FDD or TDD could employ duplexers, or dual filters, for FDD operation plus an additional filter and switch for TDD operation, since either the TDD or FDD mode uses its own set of filters, one set of components will be under utilized rendering the system cost inefficient. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a transreceiver architecture that allows a common set of filters to be used for either FDD (Frequency Division Duplex) or TDD (Time Division Duplex) operation in a given range of frequencies, thereby reducing circuitry count and implementation costs. Accordingly, the present invention allows operation in a full or half-duplex TDD or FDD system. 
     In a preferred embodiment of the invention, switches are incorporated after the forward and reverse channel filters to create three paths controlled by predetermined logic. This configuration permits use of both the forward and reverse channel filters for a TDD system and combines the forward and reverse channel filters to create a filter that can pass both frequency sub-bands. These single device frequency duplexers are often commercially available, thereby allowing for ease in implementation and cost reduction. 
     Some frequency allocation plans, such as those in the USA, often have a frequency band between the transmit and receive regions. In the USA, this frequency band can be referred to as the “unlicensed frequency band” and can be used for TDD only. An alternative preferred embodiment comprises an additional filter to allow exploitation of multiple frequency bands for transmission and reception. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a known TDMA-FDD transreceiver architecture. 
     FIG. 2 is a schematic diagram of a transreceiver of the present invention. 
     FIG. 3 is a schematic diagram of a second transreceiver of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the known TDMA-FDD capable architecture depicts an antenna  1  electrically coupled to forward channel filters  2  and reverse channel filters  3 , each tuned to a different frequency F 1  and F 2  respectively. Thus, in the known FDD transreceiver the switch  4  will be placed in the Rx position to connect the antenna to channel filter  2  to receive signals and will be placed in the Tx position to connect the antenna  1  to the reverse channel filter  3  to transmit signals. While more than one antenna and more than one set of forward and reverse channels will usually be employed, only one set is described herein for simplicity and efficiency. Alternatively, the antenna  1  can be split into a receive and transmit antenna, each individually connected to a respective filter (not shown). 
     As the preferred dual mode FDD/TDD transreceiver, depicted in FIG. 2, the forward channel filter  11  and the reverse channel filter  12  are tuned to different frequencies F 1  and F 2  respectively, both connected to the antenna  10  without the typical Tx/Rx switch  4  interposed between them. Instead, a TDD-Tx switch  13  is interposed in a receive path  20  between the forward channel filter  11  and the low-noise amplifier  16 . Also, a TDD/Rx switch  14  is interposed in the transmit path  22  between the reverse channel filter  12  and the power amplifier  17 . Additionally, an FDD/TDD switch  15  is interposed in TDD path  24  between the receiver and transmit paths  20  and  22  respectively. When the dual mode FDD/TDD transreceiver is operating in the full or half duplex FDD mode, as dictated by the Boolean equations for the switch states, the TDD/Tx switch  13  will be closed, the FDD-TDD switch  15  will be open and the TDD/Rx switch  14  will be closed. The result is that the diplexer function created by the forward channel filter  11  and the reverse channel filter  12  passes the portion of the signal in the F 1  frequency ranges through the TDD/Tx switch  13  to the low-noise amplifier  16  and into the receive channel. In the transmit mode, the output of the transmitter channel is passed through the TDD/Rx switch  14  to the reverse channel filter  12  and is transmitted by the antenna  10 . This functionality represents classical FDD operation. 
     Although the filters and switches may be described having input and output ends, signals are being propagated in both directions. Thus, although input and output ends may be described, such labels are for references and orientation purposes. 
     If it is desired to operate the dual mode TDD/FDD transreceiver in the TDD mode, the FDD-TDD switch  15  will be closed. In the TDD receive mode, the TDD-Tx switch  13  will be closed and the TDD-Rx switch  14  will be open. In the TDD transmit mode, the TDD-Rx switch  14  will be closed and the TDD-Tx switch  13  will be open. Thus, while transmitting in the TDD-Tx mode, the TDD-Rx switch  14  is closed and the output of the transreceiver is applied to both the reverse channel filter  12  and also the forward channel filter  11  through the FDD-TDD switch  15 . Thus signals in both frequency ranges F 1  and F 2  will be passed to the antenna  10  with minimal loss. When receiving, the portion of the received signal in the forward frequency band F 1  passes through the forward channel filter  11  and the portion of the received signal in the reverse frequency band F 2  passes through the receive channel through the LNA  16 . 
     In the preferred embodiment, the forward channel and reverse channel filters have a total electrical length between their filter outputs that is either very small (e.g. less than π/20 radians) or equal to n*π radians where ‘n’ is as small an integer as possible. The reason for this is that filters are generally reflective outside their passband and, in the case of typical radio filters, behave as open circuits. An open circuit translated through a transmission line whose electrical length is a multiple of π radians will still appear as an open circuit. Thus, a signal that is in the forward passband will pass through the forward channel filters  11  and the reverse channel filters  12  and will appear as an open circuit. Since the transmission line will appear as an open circuit, the signal is effectively rejected by the circuit, i.e. will have no influence on the forward channel filters  11 . Should a filter behave as a short circuit, an additional π/2 may be added to transform the short circuit to an open circuit. 
     An alternative preferred arrangement is disclosed in FIG.  3 . The architecture operates in a similar manner as that shown in FIG. 2 except that an additional bandpass filter  19  permits the system to operate in the TDD mode over an additional frequency band F 3 , such as the “unlicensed frequency band”. 
     As depicted in FIG. 3, the forward channel filter  11 , the reverse channel filter  12 , and the unlicensed band filter  19  are tuned to different frequencies F 1 , F 2 , and F 3  respectively. Each filter can be connected to the antenna  10  without a Tx/Rx switch  4  interposed between them. Instead, a TDD-Tx switch  13  is interposed in a receive path  30  between the forward channel filter  11  and the low-noise amplifier  16 . Also, a TDD/Rx switch  14  is interposed in the transmit path  33  between the reverse channel filter  12  and the power amplifier  17 . Additionally, TDD switch  34  is interposed in TDD path  31  between the forward channel filter  11  and the unlicensed band filter  19 . Finally, TDD switch  35  is interposed in TDD path  32  between the reverse channel filter  12  and the unlicensed band filter  19 . 
     When the dual mode FDD/TDD transreceiver is operating in the FDD mode, as dictated by the Boolean equations for the switch states, the TDD/Tx switch  13  will be closed, both TDD switch  34  and TDD switch  35  will be open and the TDD/Rx switch  14  will be closed. The result is that the diplexer function created by the forward channel filter  11  and the reverse channel filter  12  passes the portion of the signal in the F 1  frequency ranges through the TDD/Tx switch  13  to the low-noise amplifier  16  and into the receive channel. In the transmit mode, the output of the transmitter channel is passed through the TDD/Rx switch  14  to the reverse channel filter  12  and is transmitted by the antenna  10 . This functionality represents classical FDD operation. 
     If it is desired to operate the dual mode TDD/FDD transreceiver in the TDD mode, unlicensed filter  19  can be included with forward channel filter  11  and reverse channel filter  12  to accommodate the “unlicensed” frequency band. In the TDD receive mode, the TDD-Tx switch  13  will be closed and the TDD-Rx switch  14  will be open. With TDD switch  34  closed and TDD switch  35  open, path  31  to the unlicensed band filter  19  is completed. Thus, with the TDD-Tx switch  13  closed, the receive portion of the transreceiver is applied to both the forward channel filter  11  and also the unlicensed filter  19 . As a result, signals in both frequency ranges F 1  and F 3  will be received from the antenna  10 . When TDD switch  35  is closed, path  32  is completed and the frequency range F 2  can pass through the reverse channel  12  from the antenna  10 . When receiving, the portion of the received signal in the forward frequency band F 1  passes through the forward channel filter  11  and the portion of the received signal in the reverse frequency band F 2  passes through the receive channel through the LNA  16  and the frequency band F 3  passes through the unlicensed channel  19 . Thus, with TDD switch  34  closed and TDD switch  35  closed, signals in frequency ranges F 1  ,F 2  and F 3  will be received from the antenna  10 . 
     In the TDD transmit mode, the TDD-Rx switch  14  will be closed and the TDD-Tx switch  13  will be open. With TDD switch  35  closed and TDD switch  34  open, path  32  to the unlicensed band filter  19  is completed. Thus, the output of the transreceiver is applied to both the reverse channel filter  12  and also the unlicensed filter  19  through the TDD switch  35 . As a result, signals in both frequency ranges F 2  and F 3  will be passed to the antenna  10  with minimal loss. When TDD switch  34  is closed, path  31  is completed and the frequency range F 1  can pass through the forward channel  11  to the antenna  10 . Thus, with TDD switch  34  and TDD switch  35  closed, signals in frequency ranges F 1 , F 2  and F 3  will be passed to the antenna  10  with minimal loss. In the preferred embodiment, the forward channel and reverse channel filters have a total electrical length between their filter outputs that is either very small (e.g. less than π/20 radians) or equal to n*π radians where ‘n’is as small an integer as possible.