WO1997021287A1

WO1997021287A1 - System and method for frequency division duplex/time division duplex radio frequency communications

Info

Publication number: WO1997021287A1
Application number: PCT/US1996/013534
Authority: WO
Inventors: Javier V. Magna
Original assignee: Advanced Micro Devices, Inc.
Priority date: 1995-12-04
Filing date: 1996-08-22
Publication date: 1997-06-12
Also published as: EP0865693A1; US5956326A

Abstract

A method of radio frequency communications between a first unit and a second unit. The method includes the steps of transmitting during a first time interval over a first frequency by the first unit, receiving during the first time interval over the first frequency by the second unit, transmitting during a second time interval over a second frequency by the second unit, receiving during the second time interval over the second frequency by the first unit. The method also includes the step of setting a select frequency shift between the first frequency and the second frequency. In one application of the method, the first unit is a cordless telephone hand set and the second unit is a cordless telephone base station. The method is particularly effective when the first unit and the second unit operate in TDD mode.

Description

SYSTEM AND METHOD FOR FREQUENCY DIVISION DUPLEX/ TIME DIVISION DUPLEX RADIO FREQUENCY COMMUNICATIONS

Background of the Invention

The mvention relates to radio frequency communications and, more particularly, to a frequency division duplex (FDD)/tιme division duplex (TDD) system and method for radio frequency communications over limited spectrum and for low costs

Radio components are some of the most expensive parts of radio frequency (RF) communications equipment This is particularly the case in cordless or wireless telephony In RF communications and particularly, in cordless or wireless telephony, costs and operational requirements can be very important to the success of communications equipment designs In the engineering of those designs, the designer is often presented with design constraints imposed or dictated by costs of components or by operational requirements Such costs and operational requirements are particularly important considerations when communications equipment is intended for lower-end consumers, such as individuals and households

In RF communication technology, various standards established by the industry and other sources often dictate aspects of performance and equipment requu-ements Standards have been established, for example, for cordless or wireless telephony products and other communications devices Certain of the most common standards o the cordless telephone industry include the Cordless Telephony Second Generation (CT2) standard, the European Conference of Postal and Telecommunications Administrations (CEPT) standard, referred to as the Cordless Telephony First Generation (CT1) standard, the Cordless Telephony First Generation Plus (CT1+) standard, and the Digital European Cordless Telecommunications (DECT) standard, among others

The CT2 standard, for example, employs a time division duplex (TDD) system and methodology In TDD, transmit and receive communications occur among two stations, such as, for example, a handset and base set unit ofa cordless telephone, in a burst manner at distinct intervals of time In the past, devices conforming to CT2 have transmitted and received over an identical earner frequency withm the bandwidth dictated by the standard Communications have been possible in TDD units because different tune intervals are employed for transmissions and receptions by each station Dunng an interval that one station is transmitting, the other is receiving, and vice versa, all over the same bandwidth Devices built accordmg to the CT2 standard have been considered lower-end devices, that is, the devices are typically low-cost to consumers This low cost is, in part, attπbutable to the use in those devices of only a smgle radio front end That is possible in CT2 devices because communications occur over the same carrier frequency m the TDD manner The prior TDD devices, however, at least those devices conforming to the CT2 standard, have implemented the TDD methodology using a smgle earner frequency It has previously been thought that use of limited bandwidth through implementation of TDD methodology over a smgle earner frequency provides the greatest advantages This has not necessarily resulted, however, in lowest cost for the prior TDD devices, as later detailed

Other cordless telephone standards, such as the CT1 standard, at times have employed a frequency division duplex (FDD) concept In typical FDD, transmit and receive communications occur over two distinct separate earner channels Thus, two FDD communications stations, such as, for example, a handset unit and a base set unit of a cordless telephone, each transmit and receive over different earner channels While a first unit is transmitting over a particular channel, the second unit is receiving on that same channel Then when the second unit transmits, it does so on an entirely different channel, and the first unit receives on that different channel FDD systems have tended to be more expensive than TDD systems because additional radio front end components have been required in prior FDD systems in order to accomplish the transmissions and receptions over the separate channels

Beyond TDD and FDD, other communications methodologies are continually bemg developed Certain promising methodologies include use of a dual channel synthesizer and spread spectrum approaches A dual channel synthesizer may be operable in the case of dual channel communications Prior to employing dual channel synthesizers, communications across different channels required generation of an external local oscillator (LO) A drawback of that approach was that additional filters are required to attenuate the frequency image (sometimes referred to herein as "spurs') The use of dual channel synthesizers overcomes that drawback Such pnor use incidentally increases costs, however, because ofthe expense for components of two synthesizers, rather than just one

In spread spectrum technology, a sequential noise-like signal structure is employed to spread normally narrow band mformation signals over a relatively wide band of frequencies The receiver in such systems correlates the signals to retneve the oπginal information signal Spread spectrum technology can provide certain benefits A disadvantage of spread spectrum technology, however, is increased expense for devices operating accordmg to the technology because significant processing capacity is necessary

As is apparent, there are numerous ideas and approaches to radio frequency communications and, in particular, to such communications in cordless telephony On the one hand, TDD methods can be advantageous because ofthe minimal spectrum necessary for such communications On the other hand, FDD methods provide advantages of continuous transmission and reception The limitations of both such methods, though, are apparent, as previously discussed The newer communications methods also provide certain advantages, but they do so only at greater expense or by use of more spectrum It would, therefore, be a significant improvement in the art and technology to provide an improved system and method for radio frequency communication which is low cost and requires less spectrum

The present invention employs a unique and novel system and method for FDD/TDD radio frequency communications over a limited spectrum and for limited costs in order to provide those advantages, as well as others Though the background ofthe invention has been described, in part, with respect to cordless telephone applications and, m particular, to applications under the vanous standards and emerging technologies described above, the invention has other and varied applications which will be hereinafter more fully understood The invention is, thus, a significant improvement and advance in the technology and art

Summary of the Invention

An embodiment of the invention is a method of radio frequency communications The method comprises fhe steps of transmitting in a first time interval, over a first frequency, and receiving in a second tune interval, over a second frequency

In another aspect, the first time interval and the second time interval differ and the first frequency and the second frequency differ In yet another aspect, the method further compnses the step of setting a select frequency shift between the first frequency and the second frequency

In even another aspect, the method further compnses the steps of receiving in the first tune interval over the first frequency and transmittmg the second time interval over the second frequency

Another embodiment ofthe invention is a method of radio frequency communications between a first unit and a second unit The method comprises the steps of transmitting dunng a first time interval, over a first frequency, by the first unit, receiving durmg the first time interval, over the first frequency, by the second unit, transmitting dunng a second time mterval, over a second frequency, by the second unit, and receiving durmg the second time interval, over the second frequency, by the first unit

In another aspect, the first unit is a cordless telephone hand set and the second unit is a cordless telephone base set

In yet another aspect, the first unit and the second unit operate in TDD mode

Yet another embodiment ofthe invention is a system for radio frequency communications over a first frequency and a second frequency The system transmits and receives communications in bursts dunng a first time mterval and a second time interval The system comprises circuitry for transmitting during the first time interval, over the first frequency, circuitry for transmitting during the second time interval, over the second frequency, circuitry for receivmg dunng the first time mterval, over the first frequency, and circuitry for receiving durmg the second tune interval, over the second frequency

In another aspect, the system further comprises circuitry for selecting the first frequency and circuitry for providing a select frequency shift between the first frequency and the second frequency to synchronize more than one communications unit

Another embodiment ofthe invention is a system for radio frequency communications between a first unit and a second unit The communications are conducted over a first frequency and a second frequency Each of the first unit and the second unit transmit and receive communications m bursts over select tune intervals, including a first time interval and a second time interval The system compnses circuitry for transmitting durmg the first time interval, over the first frequency, by the first unit, circuitry for receiving during the first time interval, over the first frequency, by the second unit, circuitry for transmitting durmg the second time interval, over the second frequency, by the second unit, and circuitry for receiving during the second time interval, over the second frequency, by the first unit

In another aspect, the circuitry for transmittmg during the first time interval and the circuitry for receivmg duπng the second time mterval are each contained within the first unit and the circuitry for transmittmg durmg the second time mterval and the circuitry for receiving dunng the first tune interval are each contamed within the second unit

In yet another aspect, the first unit is a cordless telephone hand set and the second unit is a cordless telephone base set

In a further aspect, the first unit and the second unit operate m a TDD mode

Yet another embodiment ofthe invention is a method of radio frequency communications The method comprises the steps of transmitting over a carrier channel m a time division duplex manner at a first frequency within a bandwidth ofthe carrier channel and receiving over the carrier channel in a time division duplex manner at a second frequency within the bandwidth ofthe carrier channel.

In another aspect, the method further comprises the step of setting the first frequency according to a select frequency shift from the second frequency.

Another embodiment ofthe invention is a method of radio frequency communications. The method comprises the steps of first transmitting by a first unit, first receiving by a second unit simultaneously with the step of first transmitting, second transmitting by the second unit non-simultaneously with the step of first transmitting, and second receiving by the first unit simultaneously with the step of second transmitting.

In another aspect, the method further comprises the steps of setting a first frequency for performance of the steps of first transmitting and first receiving and setting a second frequency, by a select frequency shift from the first frequency, for performance ofthe steps of second receiving and second transmitting.

In yet another aspect, the steps of first transmitting and second receiving are performed by a first communications unit and the steps of first receiving and second transmitting are performed by a second communications unit.

In even another aspect, the steps of setting the first frequency and setting the second frequency cause the first frequency and the second frequency to differ by a select frequency shift.

Brief Description of the Drawings

FIG. 1 is an illustration of transmit and receive signals by a RF communications unit employing a FDD methodology, wherein transmit and receive signals pass over different frequency channels;

FIG. 2 is an illustration of transmit and receive signals ofa RF communications unit employing a TDD approach, wherein both transmission and reception occur over the same frequency channel;

FIG. 3 is an illustration of transmissions and receptions by a unit employing the TDD FDD RF communications approach ofthe present invention, wherein two carrier channels are employed for frequency division duplexed transmissions and receptions and such transmissions and receptions are burst in a time division manner;

FIG. 4 is an example diagram ofa handset unit for performing TDD radio frequency communications according to the CT2 standard; and

FIG. 5 is an embodiment of an example device for performing the FDD/TDD RF communications according to the present invention, which is an adaptation of the example TDD device of FIG. 4 according to the CT2 standard.

Detailed Description

The following detailed description addresses certain embodiments of a novel system and method for FDD/TDD RF communications. The mvention is explained by reference to prior TDD and FDD technology. It is also presented through an explanation of various cordless telephony standards and, in particular, through a discussion of adaptation ofa prior RF communications device operating according to the CT2 standard. Those skilled in the art will readily recognize that the invention may be employed in varied manners and under varied conditions, as well as in other applications. All other embodiments and applications are intended to be included in the description herein. Referring first to FIG 1, transmit (T_x)12 and receive (R 14 communications of a device operating accordmg to FDD prmciples are illustrated FDD signal transmissions 12 are accomplished over a first carrier frequency and signal receptions 14 are accomplished over a second earner frequency In the illustration of FIG 1, the transmission 12 over time is depicted by the top box and the reception 14 over time is depicted by the bottom box The vertical displacement of the two boxes is employed to indicate that two separate carrier frequencies serve for transmission 12 and reception 14, respectively The same displacement representation is used m FIG 3 for the same reason In the presentation of FIG 1, as well as FIGS 2 and 3, time progresses in passmg from left to right across the page

Still referring to FIG 1, this FDD methodology is like that employed in cordless telephones operating according to the CT1 standard Accordmg to that standard, analog signals are passed between communication devices as transmissions 12 and receptions 14 Operation over separate frequencies for transmit 12 and receive 14 communications has previously required more devices or components, such as, for example, radio components, in order to perform such communications, and so FDD methodology devices can be relatively expensive Also, because FDD operations occur over the separate frequencies for transmission 12 and receptions 14, more spectrum may be used up in FDD communications, at least in comparison to typical TDD communications

Referring now to FIG 2, transmit 4, 6, 7 and receive 5, 8, 9 signal sequences ofa TDD communications device are depicted In TDD communications, the communications are digitized by converting the communications, for example, voice or data, mto a binary pattem The digital binary pattem is then buffered and transmitted at a high rate in bursts at distinct intervals of time Only a smgle carrier frequency may be necessary for TDD communications Tune division of transmissions 4, 6, 7 and receptions 5, 8, 9 into distinct time intervals allows both receive 5, 8, 9 and transmit 4, 6, 7 signals to be accomplished over the smgle frequency

Continumg to refer to FIG 2, both the left and right boxes represent digitized communications being transmitted and received throughout periods of tune The box on the left represents a transmission (TJ 4, 6, 7 The transmission 4, 6, 7 may include certain beginning transmit control bits 4 and certain transmitted information bits 6 The information bits 6 may, for example, be digitized voice or data signals The transmission 4, 6, 7 may also mclude end control bits 7 The transmission 4, 6, 7 occurs on a particular carrier frequency and is burst over distinct intervals of time

Further still referring to FIG 2, reception (RJ 5, 8, 9 of communicated mformation occurs over the same carrier frequency as the transmission 4, 6, 7, but at different intervals of time This mterval of time for reception 5, 8, 9 is different from the mterval of the transmission 4, 6, 7 The reception 5, 8, 9 may include beginning control bits 5, received mformation bits 8 and end g control bits 9 Because distinct intervals of time are set for transmissions 4, 6, 7 and receptions 5, 8, 9, the same carrier frequency can support both transmissions 4, 6, 7 and receptions 5, 8, 9, albeit at those different intervals of time

Still referring to FIG 2, the TDD communication sequence illustrated here is the type performed by the prior communications units which conform to the CT2 standard for cordless telephones To conform to that standard, both transmissions 4, 6, 7 and receptions 5, 8, 9 occur over the same carrier in the typical TDD manner The mterval dunng which both transmission 4, 6, 7 and reception 5, 8, 9 occurs is two milliseconds long In each one millisecond of time, the standard provides that 64 bits can be transmitted 4, 6, 7 or received 5, 8, 9 by a CT2 device A disadvantage of prior communications devices conforming to the CT2 standard has been cost ofthe devices

Now referring to FIG 3, it may be understood how the present mvention employs a new FDD/TDD approach to obtam a low cost implementation ofa digital cordless telephone Transmission 4, 6, 7 and reception 28, 26, 30 according to the FDD/TDD approach of the present invention are illustrated In the FDD/TDD approach, a RF communications unit that is, the "radio front end" of a communications device is designed and configured to employ FDD m a TDD manner Such an RF communications unit (e g , 120 in FIG 5), may be employed with a telephone (e g , 52 in FIGS 4 and 5) that operates according to TDD methodology, for example, such as one conforming to the CT2 standard In effect, the FDD/TDD approach employs a dual duplex design, ; e , a first carrier channel for transmissions and a different, second earner channel for receptions Over each earner channel, communications are passed in bursts of distinct time intervals in a tune division manner In accordance with the prmciples ofthe present mvention, such an FDD/TDD approach can be implemented using fewer filters and other components than required by the prior TDD devices, such as those conforming to CT2 Furthermore, the FDD/TDD approach may be implemented usmg a single radio front end, rather than two as typically required in FDD unplementations, in each RF communications unit 120 (FIG 5) Because fewer components are necessary in the embodiments of the invention implementing the FDD/TDD approach, the embodiments provide lower cost RF communication devices relative to costs of prior devices

Still referring to FIG 3, m an embodiment ofthe present invention, transmission (TJ 4, 6, 7 occurs over a first earner channel in a distinct tune interval Over a different, second earner channel, reception (RJ 28, 26, 30 occurs at a different, dist ct time mterval In this manner, communications by transmissions 4, 6, 7 and receptions 28, 26, 30 occur in distinct and different time mtervals, over different carrier frequencies for transmissions 4, 6, 7 and receptions 28, 26, 30

Still referring to FIG 3, the FDD/TDD methodology is particularly suited in the case of CT2 spec RF communications units, although the methodology is not limited to that application CT2 defines a burst structure for transmissions of D channel bits 4, 7 which are meant for system control and 64 bits of voice data 6, and also the same burst structure for receptions, t e , control bits 28, 30 and voice data 26 CT2 also defines a mux 1 2 or 1 4 In the 1 2 mux, CT2 provides for two bits per burst In the 1 4 mux, CT2 provides for four bits per burst CT2 further provides for tune division duplexed burst communications In the past, CT2 spec devices have been designed for transmissions and receptions over the same earner frequency This was possible because ofthe time division of the communicated bursts Presumably, the single carrier frequency was employed in order to conserve spectrum use Significant cost advantages are realized, however, if, instead, the FDD/TDD methodology ofthe present invention is employed

Now referring to FIG 4, an example CT2 device 48 ofthe prior technology is shown in order to better understand implementation ofthe FDD/TDD methodology The CT2 device 48 shown in FIG 4 is employable in a cordless telephone hand or base unit The telephone portion 52 ofthe device 48 is generally shown to the πght side ofthe figure, and the RF communications portion 50 is generally shown to the left side of the figure Components of the telephone portion 52 are not described in detail or particularly identified in the detailed description because the focus here is on the RF communications portion 50

Still referring to FIG 4, the RF communications portion 50 mcludes an antenna 60 for receivmg signals communicated to the device 48 and for transmitting signals by the device 48 Incorporated with the antenna 60 is a band-pass filter 62 Connecting with the antenna 60 and filter 62 is an antenna switch 64, which serves to allow switchmg between transmission and reception of RF signals by the device 48 On the reception side, the RF communications portion 50 mcludes a low noise RF amplifier 66 for amplifying RF signals received by the device 48 The amplifier 66 passes the amplified signal to an image filter 68, and the signal after filtering is mixed 70 with a buffered signal from a voltage controlled oscillator 98 The mixer 70 converts the amplified 66 and filtered 68 signal to an intermediate frequency 71 signal

Continumg to refer to FIG 4, the intermediate frequency 71 signal is passed to a SAW filter 72 The signal is then demodulated by a down converter 74 The down converter 74 receives the signal and amplifies it, filters it, limits it, and generates receive baseband ( ^.^) 84 and receive signal strength indicator (RSSI) 86 outputs The input ofthe down converter 74 may be a mixer that is converted to an amplifier by generating a DC imbalance in the Gilbert Cell by altermg the biasing of a crystal oscillator 94 through a resistor connected to ground The output of the mixer may then be filtered 76 The output of the filter 76 may connect to an intermediate frequency amplifier section ofthe converter 74 The output ofthe amplifier may be limited for noise reduction and AM rejection Output ofthe limiter may be coupled to an mput of a multiplier A phase shifted version of the limiter may be connected to the other input of the multiplier The phase shift may be provided by a parallel tuned circuit which may be adjusted so that the two inputs to the multiplier are in quadrature The converted output signal ofthe multiplier passes on to a low pass filter 80 and a sheer 82, and then is fed to the telephone portion 52 as the received baseband signal 84

Still referring to FIG 4, it is apparent that the oscillator 94 signal is bumped up by multiplying 92, and then buffered 90 before passing to the demodulator 74 In effect, the multiplying 92 and buffenng 90, followed by mixing ofthe oscillator 94 signal with the intermediate frequency 71 signal at the converter 74, serves to down convert the received signal to a select recovered baseband signal 84 for input to the telephone portion 52 The baseband signal 84 input to the telephone portion 52 is passed to post detection processing circuitry of the portion 52 The RSSI signal 86 is passed to a baseband processor ofthe telephone portion 52

Further referring to FIG 4, a signal from the local oscillator 94 also passes to a buffer 96 and may be used by the telephone portion 52 The oscillator 94 signal furthermore is mput to a synthesizer/voltage controlled oscillator (VCO) 98 as a reference On the transmit side, a Non-Return to Zero (NRZ) signal 97 is generated by the telephone portion 52 The NRZ signal 97 is passed through a low pass filter 104 and supplied to the synthesizer/VCO 98 The synthesizer/VCO 98 may be a phase-locked loop that takes the oscillator 94 signal and drives a divide-by-P prescaler and divide-by-N counter The output of the divide-by-N counter may be connected to an mput ofa phase/frequency detector The oscillator 94 signal may be connected to a divide-by-R counter The output ofthe divide-by-R counter may connect to a reference mput ofthe phase/frequency detector The detector generates an error signal that is used to drive the synthesizer/VCO 98 onto a programmed frequency The synthesizer/VCO 98 may connect with a voltage regulator 100 Still referring to FIG 4, the signal from the voltage controlled oscillator is the carrier frequency for the transmissions 103 The transmit signal 103 then passes to a buffer 102 and is supplied to the reception side mixer 70 and is also supplied to a transmit side mixer 1 10 At the mixer 1 10, a buffered 108 signal from a tπpler 106 is mixed with the transmit signal 103 The resulting signal is passed to a filter 112 and amplified 1 14 The amplified signal passes to the antenna switch 64 which is toggled for transmission, and the signal is transmitted The mixing 1 10 encodes the transmit signal 103 onto the carrier frequency so that the signal 103 may be transmitted

Because the prior device 48 operates in a TDD manner, both transmissions and receptions mav, as they do the particular arrangement shown m Fig 4, occur in the RF over a single earner frequency As previously mentioned, the use of a single communication frequency by TDD units has been perceived as an advantage of TDD Although perceived as an advantage, use ofthe smgle frequency for TDD transmissions and receptions requires mixmg 110 Mixmg 1 10, however, mcreases the numbers of components required because frequency _ spurs are generated and must be attenuated To attenuate the frequency spurs, costly filters, for example, such as filter 112 in the device 48, or other components are necessary in the units The mixing 110 and filtering 112 which have previously been necessary the pnor device 48 and other TDD communication systems, are avoided by the FDD/TDD approach ofthe present invention By avoiding the mixmg 1 10, fewer components, such as filters 1 12, are necessary and costs ofthe RF communications portion 50 is reduced

Now referring to FIG 5, an FDD/TDD RF communications portion 120 that implements the FDD/TDD device 118 approach ofthe present mvention is shown The FDD/TDD RF communications portion 120 may be operable in conjunction with a telephone portion 52 ofthe type ofthe prior TDD technology The FDD/TDD RF communications portion 120, in companson with the prior technology (shown in FIG 4), in effect operates to set a given frequency shift, coordmated at both units involved in a communication, in order to transmit and receive, respectively, over two distmct earner frequencies rather than over a single carrier frequency luce in the pnor TDD technology (FIG 4) By coordinating the frequency shift between the units, TDD transmissions and receptions are accomplished m a FDD manner at significant cost savings for the unit 118

Still referring to FIG 5, the FDD TDD communications unit 118 may have the following charactenstics ai a communications system compnsed ofa handset and a base station The handset receiver may, for example, cover the range of about 914 8 MHz to about 916 8 MHz, and the base station receiver may, for example, cover the range of about 925 5 MHz to about 927 5 MHz, both m 100 KHz increments The first local oscillator in that example may operate at about 107 MHz above the receive frequency in the base station In the embodiment, the 10 7 MHz is the frequency shift coordinated at the transmit and receive units, to achieve the two distmct carrier frequencies for transmissions and receptions by the handset and the base station

Continuing to refer to FIG 5 and the embodiment therein shown, on the reception side the portion 120 includes an antenna 122 for receivmg communicated signals The antenna 122 is connected with a low pass filter 123, such as, for example, a 2-pole LC, lowpass filter, and an antenna switch 124, for example, a SPDT, PIN diode switch The switch 124 operates to distinguish receptions and transmissions of the portion 120 and to switch between them as communication signals are received and transmitted by the device 1 18 Received signals are thereafter amplified, filtered, detected and converted to received data (R_xDl J 1 4 for use by the baseband board ofthe telephone portion 52 The received signals, first, pass to a first band-pass filter 126, for example, a 2-pole dielectric resonator, bandpass device, then to a low noise RF amplifier 128, such as, for example, a common emitter design If desired, the amplifier 128 may be powered only durmg periods in which receptions are being received to conserve power The output of the amplifier 128 connects with a second band-pass filter 130, for example, identical to filter 130 The filter 130 serves to improve the overall image rejection and to filter the image noise caused by the amplifier 128

Still referring to FIG 5, the output ofthe second band-pass filter 130 connects to a down converter 131 The down converter 131 may include a mixer 132, a voltage controlled oscillator 168, and an intermediate frequency amplifier 133 The mixer 132 converts the amplified 128 and filtered 130 received signal, for example, a signal the range of about 914 8 MHz to about 916 8 MHz to the handset or in the range of about 925 5 MHz to about 927 5 MHz to the base station, to an intermediate frequency, using the internal voltage controlled oscillator 168 as the first local oscillator The voltage controlled oscillator 168 may use an extemal tank circuit 169, for example, compnsmg a chip varactor diode and a printed inductor Some ofthe signal from the voltage controlled oscillator 168 is coupled from the ductor and buffered, then split for use by a synthesizer 154 and other transmit side circuitry The output from the mixer 132, for example, a 10 7 MHz output, is amplified 135, such as, for example, by a common emitter amplifier If desired, the amplifier 135 may be powered only dunng penods in which receptions are being received A first intermediate frequency filter 134, for example, a ceramic device which may have a bandwidth of approximately 1 10 KHz, receives the amplifier 135 output and emits a filtered intermediate frequency signal.

Further referring to FIG 5, the intermediate frequency signal is then mput to an intermediate frequency (LF) subsystem 136 The subsystem 136 takes the intermediate frequency signal and amplifies, filters and limits it, and generates receive baseband (R_^j^ 144 and receive signal strength indicator (RSSI) 146 outputs The first stage of the subsystem 136 may be a mixer The mixer converts the mput intermediate frequency signal to a second intermediate frequency signal Thereafter, the second intermediate frequency signal may be amplified The mrxer may be converted to provide the amplification by generating a DC imbalance in a Gilbert Cell 139, such as by altenng the bias g ofa crystal oscillator through a resistor connected to ground The amplified signal may then be input to a second intermediate frequency filter 137 The output of that filter connects to an intermediate frequency amplifier that drives a final intermediate frequency filter 138 that connects to a limiter The limiter adds gain and reduces FM noise and rejects AM

Continuing to refer to FIG 5, the output ofthe limiter may be coupled to an input of a multiplier A phase-shifted version of that output from the limiter is connected to another input ofthe multiplier The phase- shift may be provided, for example, by a parallel tuned circuit which may be adjusted so that such output and such phase-shifted version ofthe output are m quadrature The output ofthe multiplier is the recovered receive baseband signal pnor to bemg amplified and restored The receive signal strength indicator (RSSI) 146 signal from the subsystem 136 may be passed to the baseband processor of the telephone portion 52 The RSSI 146 signai may be produced by the subsystem 136 by monitormg the current drawn in the amplifier and limiter stages

Still referring to FIG 5, the subsystem 136 may pass the receive baseband signal to various post detection processmg circuitry Thereafter, the amplified receive baseband signal may be passed to a DC restorer circuit, for example, to clamp the bottom edge ofthe signal The DC restorer circuit may include for example, a filter 140 and sheer 142 The sheer 142 converts the baseband signal to a digital signal, for example, using a I bit quantizer The digital signal is passed on to the telephone portion 52 as the received data (R, Data) signal 145

Still referring to FIG 5, in sum, the reception side circuitry, in effect, serves to receive the received signal at a select first RF frequency ofthe earner channel and render the received signal useable by the telephone portion 52 as a digital received signal 145 The select first RF frequency ofthe received signal is dictated at the transmitter ofthe other ofthe communication units The select first RF frequency ofthe received signal ofthe communications unit (which, of course, is transmitted by the other communications unit) is set at a select frequency shift from the frequency of transmitted signals ofthe communications unit, as hereinafter described

Continuing to refer to FIG 5, on the transmit side ofthe RF communications portion 120, transmit side circuitry may accept the signal 175 from the voltage controlled oscillator 168 An amplifier 170 may then amplify the signal 175 The amplifier 170 may be powered by the same supply voltage supplied to the synthesizer 154 The amplified signal from the amplifier 170 may then be passed through a pad 171, for example, a lOdB pad The pad 171 serves to minimize dynamic pulling ofthe voltage controlled oscillator 168 The output ofthe pad 171 may then proceed to one or more transmitter power amplifiers 172, 174 If desired, the transmit stage may be powered only dunng transmit mtervals The stage may, for example, deliver about +2 dBm to the antenna switch 124 In that example, the transmit output level at the antenna 122 may be about +1 dBm

Still referring to FIG 5, the transmit side circuitry also includes the synthesizer 154 The synthesizer 154 may, for example, be a smgle chip phase-locked loop utilizing an oscillator 152, such as, for example, a 12 8 MHz crystal, for reference generation A digital baseband transmit 150 signal is generated in a TDD manner by the baseband chip, for example, by a burst mode controller, ofthe telephone portion 52 The split signal 173 from the voltage controlled oscillator 168 may be mput to the synthesizer 154 and may drive a divide-by-P prescaler and divide-by-N counter An output ofthe divide-by-N counter may be connected to an input of a phase/frequency detector Such a phase/frequency detector may generate an error signal, which error signal may be used to dπve the voltage controlled oscillator 168 onto a programmed frequency The divide-by-P, divide-by- N, and divide-by-R counters may be under serial control from the baseband board

Continuing to refer to FIG 5, the components and sizes indicated on FIG 5 are examples ofa possible configuration ofthe FDD/TDD RF communications portion 120, for example, such a portion 120 to be used m a CT2 cordless telephone device In the example, the frequency shift between transmissions 4, 6, 7, on the one hand, and receptions 28, 26, 30, on the other hand, is 21 MHz and the receive signal frequency is at about 904 MHz or about 925 MHz and the transmit signal is at the other of those frequencies Of course, the particular frequency shift and receive and transmit signal frequencies are only an example of possible configurations employing the FDD/TDD approach ofthe present invention

In sum regarding the embodiments, the circuitry of the transmit side, in effect, serves to set the frequency ofthe transmitted signal at a select frequency ofa select shift from the received signal but nonetheless, withm the limits of the desired frequency range The effect of setting the receive and transmit frequencies accordmg to a select frequency shift so that receive and transmit frequencies differ is to transmit and receive m a FDD manner Although transmissions and receptions occur in a FDD manner in the embodiment, only a smgle radio front-end is necessary m each communications unit This is so because the select frequency shift dictates the transmit frequency accordmg to the receive frequency The select frequency shift, rather than another radio front-end. thus, provides for the FDD Furthermore, employing FDD in this manner m conjunction with TDD allows for a reduction in the numbers of components required The reduction in components and use of only a smgle radio front-end provide a particularly low cost unit

As those skilled in the art will readily appreciate, the FDD/TDD system and method described herein provide significant improvements and advantages over the prior technology Those skilled m the art will readily recognize the numerous variations and substitutions that may be made in the system and method and their use and configuration to achieve substantially the same results as achieved by the embodiment and, in particular, the preferred embodiments expressly descπbed herein Each of those vaπations and substitutions, as well as all other applications, ofthe system and method are mtended to be included in the invention The foregoing detailed description is, thus, to be clearly understood as being given by way of illustration and example only, the spirit and scope ofthe present invention bemg limited soiely by the appended ciauns

Claims

What is Claimed is

1 A method of radio frequency communications, comprising the steps of transmittmg in a first time mterval, over a first frequency, and receiving m a second tune interval, over a second frequency

2 The method of claim 1, wherein said first time mterval and said second time mterval differ and said first frequency and said second frequency differ

3 The method of claim 1 further compnsmg the step of setting a select frequency shift between said first frequency and said second frequency

4 The method of claim 3, further compnsmg the steps of receiving in said first time interval over said first frequency and transmitting in said second time mterval over said second frequency

5 A method of radio frequency communications between a first unit and a second unit, comprising the steps of transmitting durmg a first time interval, over a first frequency, by said first unit, receiving durmg said first time mterval, over said first frequency, by said second unit, transmittmg duπng a second time mterval, over a second frequency, by said second unit, and receivmg durmg said second time mterval, over said second frequency, by said first unit

6 The method of claim 5, wherem said first unit is a cordless telephone hand set and said second unit is a cordless telephone base set.

7 The method of claim 6, wherein said first unit and said second unit operate in TDD mode

8 A system for radio frequency communications over a first frequency and a second frequency, said system transmittmg and receivmg communications in bursts durmg a first time interval and a second time mterval, compnsmg circuitry for transmitting durmg said first time interval, over said first frequency, circuitry for transmittmg durmg said second time interval, over said second frequency, circuitry for receiving during said first time interval, over said first frequency, and circuitry for receiving during said second time interval, over said second frequency

9 The system of claim 8, further compnsmg circuitry for selecting said first frequency, and circuitry for providmg a select frequency shift between said first frequency and said second frequency to synchronize more than one communications unit

10 A system for radio frequency communications between a first unit and a second unit, said communications conducted over a first frequency and a second frequency, each of said first unit and said second umt transmittmg and receivmg communications m bursts over select time mtervals, including a first time mterval and a second time interval, comprising circuitry for transmittmg durmg said first time interval, over said first frequency, by said first unit, circuitry for receivmg duπng said first tune mterval, over said first frequency, by said second unit, circuitry for transmittmg durmg said second time interval, over said second frequency, by said second unit, and circuitry for receivmg during said second time interval, over said second frequency, by said first unit

1 1 The system of claim 10, wherein said circuitry for transmitting dunng said first time interval and said circuitry for receivmg dunng said second time mterval are each contamed withm said first unit and said circuitry for transmittmg durmg said second tune interval and said circuitry for receiving dunng said first tune mterval are each contamed within said second unit

12 The system of claim 10, wherem said first unit is a cordless telephone hand set and said second unit is a cordless telephone base set

13 The system of claim 12, wherem said first unit and said second unit operate in TDD mode

14 A method of radio frequency communications, comprising the steps of transmittmg over a earner channel in a tune division duplex manner at a first frequency within a bandwidth of said carrier channel, and receiving over said carrier channel m a time division duplex manner at a second frequency withm said bandwidth of said carrier channel

15 The method of claim 14, further compnsmg the step of setting said first frequency according to a select frequency shift from said second frequency

16 A method of radio frequency communications, compnsmg the steps of first transmitting by a first unit, first receiving by a second unit simultaneously with said step of first transmitting, second transmittmg by said second unit non-simultaneously with said step of first transmittmg, and second receivmg by said first unit simultaneously with said step of second transmitting

17 The method of claim 16, further compnsmg the steps of setting a first frequency for performance of said steps of first transmitting and first receiving and setting a second frequency, by a select frequency shift from said first frequency, for performance of said steps of second receiving and second transmittmg

18 The method of claim 17, wherem said steps of first transmitting and second receiving are performed by a first communications unit and said steps of first receiving and second transmitting are performed by a second communications unit

19 The method of claim 17, wherem said steps of setting said first frequency and setting said second frequency cause said first frequency and said second frequency to differ by a select frequency shift