# Patent

## Title
Filtering method to allow FDD and TDD operations in PCS transreceivers

## 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.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention is a continuation of U.S. application Ser. No. 09/780,948 filed Feb. 9, 2001, now U.S. Pat. No. 6,434,122 which is a continuation of U.S. application Ser. No. 09/075,415 filed May 8, 1998 (now U.S. Pat. No. 6,212,172).


FIELD OF THE INVENTION
The field of the invention pertains to wireless communication transreceivers including, more particularly, transreceivers capable of FDD and TDD operation.
BACKGROUND OF THE INVENTION
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
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 F1 and F2 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 F1 and F2 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 F1 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 F1 and F2 will be passed to the antenna 10 with minimal loss. When receiving, the portion of the received signal in the forward frequency band F1 passes through the forward channel filter 11 and the portion of the received signal in the reverse frequency band F2 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 pi/20 radians) or equal to n*pi 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 pi 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 pi/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 F3, 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 F1, F2, and F3 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. 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 F1 and F3 will be received from the antenna 10. When TDD switch 35 is closed, path 32 is completed and the frequency range F2 can pass through the reverse channel 12 from the antenna 10. When receiving, the portion of the received signal in the forward frequency band F1 passes through the forward channel filter 11 and the portion of the received signal in the reverse frequency band F2 passes through the receive channel through the LNA 16 and the frequency band F3 passes through the unlicensed channel 19. Thus, with TDD switch 34 closed and TDD switch 35 closed, signals in frequency ranges F1, F2 and F3 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 F2 and F3 will be passed to the antenna 10 with minimal loss. When TDD switch 34 is closed, path 31 is completed and the frequency range F1 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 F1, F2 and F3 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 pi/20 radians) or equal to n*pi radians where “n” is as small an integer as possible.

## Claims

1. An apparatus, comprising:
a first filter tuned to a first PCS band;
a second filter tuned to a second PCS band; and
a mode select switch to couple between said first filter and said second filter, the apparatus to operate in a first mode when said mode select switch is open, and the apparatus to operate in a second mode when said mode select switch is closed, the second mode being a TDD mode.

2. An apparatus as claimed in claim 1, the first and second filters to couple to an antenna.

3. An apparatus as claimed in claim 1, one of the first mode and the second mode being a half-duplex mode, and another of the first mode and the second mode being a full-duplex mode.

4. An apparatus as claimed in claim 1, the first mode being an FDD mode.

5. An apparatus as claimed in claim 1, further comprising a transmit switch to couple to said first filter, and a receive switch to couple to said second filter, said transmit switch and said receive switch to select between a transmit and a receive operation.

6. An apparatus, comprising:
a first filter tuned to a first PCS frequency band;
a second filter tuned to a second PCS frequency band;
a third filter tuned to a third frequency band;
a first mode select switch disposed between said first filter and said third filter; and
a second mode select switch disposed between said second filter and said third filter, the apparatus to operate in a first mode when said first mode switch and said second mode switch are open, and the apparatus to operate in a second mode when at least one of said first mode switch and said second mode switch is closed.

7. An apparatus as claimed in claim 6, said first filter, said second filter, and said third filter to couple to an antenna.

8. An apparatus as claimed in claim 6, said first filter, said second filter, and said third filter to couple directly to an antenna.

9. An apparatus as claimed in claim 6, one of the first mode and the second mode being a half-duplex mode, and another of the first mode and the second mode being a full-duplex mode.

10. An apparatus as claimed in claim 6, the first mode being an FDD mode.

11. An apparatus as claimed in claim 6, the second mode being a TDD mode.

12. An apparatus as claimed in claim 6, the first mode being an FDD mode, and the second mode being a TDD mode.

13. An apparatus as claimed in claim 6, further comprising a transmit switch to couple to said first filter, and a receive switch to couple to said second filter, said first and second switches to select between a transmit and a receive operation.

14. An apparatus as claimed in claim 6, the third frequency band being an unlicensed frequency band.

15. An apparatus as claimed in claim 6, the third frequency band being in betweenthe first PCS frequency band and the second PCS frequency band.

16. A method, comprising:
operating an apparatus in a first mode when a first filter tuned to a first frequency band is uncoupled from a second filter tuned to a second frequency band; and
operating the apparatus in a second mode when the first filter and the second filter are coupled, wherein one of the first mode and the second mode is a half-duplex mode, and another of the first mode and the second mode is a full duplex mode.

17. A method as claimed in claim 16, the first mode being an FDD mode.

18. A method as claimed in claim 16, the second mode being a TDD mode.

19. A method as claimed in claim 16, the first mode being an FDD mode, and the second mode being a TDD mode.

20. A method as claimed in claim 16, further comprising coupling a third filter tuned to a third frequency band to at least one of the first filter and the second filter when operating the apparatus in the second mode.

21. A method as claimed in claim 16, further comprising coupling a third filter tuned to a third frequency band to both the first filter and the second filter when operating the apparatus in the second mode.

22. An apparatus as claimed in claim 1, the first and second filters to couple directly to an antenna.

23. The method as claimed in claim 16, the first frequency band being a first PCS frequency band, and the second frequency band being a second PCS frequency band.