Abstract:
A cordless telephone provides extended operating range and battery life. In one embodiment, a time division duplex communications protocol is used when the measured signal strength or the signal quality between the handset and a corresponding base unit does not satisfy a known criteria, such as a predetermined threshold. The telephone is also configured to operate using a frequency division duplex communications protocol when the signal strength or signal quality between the handset and the base unit satisfies the known criteria. Other aspects of the invention are directed to measuring the received signal strength of the communications channel after initial communications is established, and selecting the communications protocol that is most advantageous in light of the measured signal strength and/or signal conditions.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method of operation of a wireless telephone system. 
     BACKGROUND OF THE INVENTION 
     Radio components are some of the most expensive parts of radio frequency (RF) communications equipment. This is particularly the case in cordless or wireless telephone. In RF communications, and particularly, in cordless or wireless telephone, costs and operational requirements can be very important to the success of communications equipment designs. Engineering such equipment often involves 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 consumer-level uses and household-type markets. 
     Various standards, established by the RF communication technology, industry and other sources, often dictate aspects of performance and equipment requirements. Standards have been established, for example, for cordless or wireless telephone products and other communications devices. Certain of the most common standards of the cordless telephone industry include the Cordless Telephone Second Generation (CT2) standard, the European Conference of Postal and Telecommunications Administrations (CEPT) standard which is also referred to as the Cordless Telephone First Generation (CT1) standard, the Cordless Telephone 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 a handset and base set unit of a 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 carrier frequency within the bandwidth dictated by the standard. Communications have been possible in TDD units because different time intervals are employed for transmissions and receptions by each station. During an interval that one station is transmitting, the other is receiving, and vice versa, all over the same bandwidth. Devices built according to the CT2 standard have been considered lower-end devices, that is, the devices are typically low-cost to consumers. This low cost is partly attributable to the use in those devices of only a single radio front end. That is possible in CT2 devices because communications occur over the same carrier frequency in the TDD manner. The prior TDD devices, however, at least those devices conforming to the CT2 standard, have implemented the TDD methodology using a single carrier frequency. It has previously been thought that use of limited bandwidth through implementation of TDD methodology over a single carrier frequency provides the greatest advantages. This has not necessarily resulted, however, in the lowest cost for the prior TDD devices. 
     Other cordless telephone standards, such as the CT 1  standard, at times have employed a frequency division duplex (FDD) concept. In typical FDD, transmit and receive communications occur over two distinct, separate carrier 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 carrier channels. While a first unit is transmitting over a particular channel, the second unit is receiving on that same channel. The second unit transmits on a 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. 
     In addition, FDD systems use more power to operate the additional circuitry necessary to utilize multiple carrier frequencies as compared to TDD systems. This additional disadvantage for FDD systems contributes to a reduction in the useful battery life for the handset in FDD systems as compared to TDD systems. Providing a cordless communication system with the longest possible useful battery life is a desirable design goal of all cordless communication systems. 
     FDD systems, however, possess an advantage over TDD systems in that the addition of the additional communication channel in the FDD system allows the communications data rate for each of the channels to be one-half of the rate used by the TDD system that provides the same total communication capacity. The halving of the data rate used by each channel increases the energy per bit provided by the FDD system. This in turn permits the link quality to be more robust than a TDD system operating in the same environment. This reduction in the bit error rate for the FDD channel increases the effective usable range of the cordless communications system by providing an improved signal-to-noise ratio. 
     There are certain advantages in selecting a particular type of radio frequency communications and, in particular, for communications involving cordless telephone. For example, TDD methods can be advantageous because of the minimal spectrum necessary for such communications and the reduced power consumption for these systems. FDD methods provide advantages of continuous transmission and reception while permitting lower data rates which in turn provide lower bit error rates and correspondingly greater operating range. Traditionally, system requirements lead to selection of one type, along with its advantages, and not the advantages of the other type. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a cordless telephone apparatus and method of use thereof In one particular embodiment, a cordless telephone apparatus includes a battery-powered handset and a base unit. The battery-powered handset is configured and arranged to operate using a TDD communications protocol when the measured signal strength or the signal quality between the handset and the base unit is strong, and is configured and arranged to operate using a FDD communications protocol when the measured signal strength or the signal quality between the handset and the base unit is not strong, for example, is degrading. 
     According to another aspect of the present invention, a method of operation of a cordless telephone apparatus capable of operating using both TDD and FDD communications protocol comprises establishing communication between the handset and base unit of the cordless telephone, measuring a value for the received signal strength for the communications channel used to communicate between the handset and base unit, selecting and using the FDD protocol if the received signal strength is less than a threshold value, and selecting and using the TDD protocol if the received signal strength is greater than or equal to a threshold value. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the subsequently presented detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 is an illustration of transmit and receive signals by an RF communications unit employing a conventional FDD methodology, wherein transmit and receive signals pass over different frequency channels; 
     FIG. 2 is an illustration of transmit and receive signals of a RE communications unit employing a conventional 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 a TDD/FDD RF communications approach, according to one example embodiment of the present invention; 
     FIGS. 4 a  and  4   b  are block diagrams of example embodiments of cordless telephone systems, according to the present invention; and 
     FIGS. 5 a  and  5   b  are block diagrams of example embodiments of cordless telephone handset and base unit apparatus, also according to the present invention. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention may be applicable to a variety of systems and arrangements which communicate between devices using various RF communications protocols. The invention has been found to be particularly advantageous in application environments where cordless telephone systems utilize one or more RF communications protocols to perform data and continue communications between handset and base units. An appreciation of various aspects of the invention is best gained through a discussion of application examples operating in such an environment. 
     FIG. 1 is a diagram of transmit (T x ,) and receive (R x ) communications sequences  12 , 14  of a device operating according to a conventional FDD protocol. FDD signal transmissions are provided over a first carrier frequency and signal receptions are provided over a second carrier frequency. In the illustration of FIG. 1, the transmission over time is depicted by the sequence  12  and the reception over time is depicted by the sequence  14 . The vertical displacement of the two sequences is employed to indicate that two separate carrier frequencies serve for the transmission and for the reception. The same displacement representation is used in FIG. 3 for the same reason. In the presentation of FIG. 1, as well as FIGS. 2 and 3, time progresses in passing from left to right across the page. 
     Still referring to FIG. 1, this particular example FDD methodology is similar to cordless telephones operating according to the CT1 standard in which modulated analog data sequences  12 ,  14  are passed between communication devices. Operation over separate frequencies has previously required more devices or components, such as radio components leading to relatively expensive implementations. Also, because FDD operations occur over the separate frequencies for transmission and reception, more spectrum may be used up in FDD communications, at least in comparison to typical TDD communications. 
     FIG. 2 is a diagram of the transmit signal and receive signal  5  sequences of a conventional TDD communications using the CT2 standard for cordless phones. In TDD communications, the communications are digitized by converting the communications, for example, voice or data, into a sequence of binary patterns. The sequence of digital binary patterns is then buffered and transmitted at a high rate in bursts at distinct intervals of time. Only a single carrier frequency may be necessary for TDD communications. Time division of transmissions and receptions into distinct time intervals allows both receive and transmit signals to be accomplished over the single frequency. 
     Continuing to refer to FIG. 2, both the left and right sequences represent digitized communications being transmitted and received throughout periods of time. The sequence on the left represents a transmission (T x ) signal having bit groupings  4 ,  6 ,  7 . The transmission 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 may also include end control bits  7 . The transmission occurs on a particular carrier frequency and is burst over distinct intervals of time. 
     Further still referring to FIG. 2, communicated information is transmitted -and received over the same carrier frequency, at different intervals of time, for example, at two-millisecond time intervals as set forth in the CT2 standard. The interval of time for reception () is different from the interval of the transmission (T x ). The reception may include beginning control bits  5 , received information bits  8  and ending control bits  9 . 
     In one embodiment the CT2 standard provides that 66 bits can be transmitted  4 ,  6 ,  7  or received  5 ,  8 ,  9  by a CT2 device each in one millisecond of time. A disadvantage of such communications devices has been their cost due to the need for additional components. The components are required for additional filters used to attenuate frequency spurs created in mixing the transmit and receive channels onto a single carrier frequency. 
     FIG. 3 is a diagram of the transmission ( 4 ,  6 ,  7 ) and reception ( 28 ,  26 ,  30 ) signal sequences according to an example embodiment of the present invention. In this FDD/TDD approach, the radio front end of an RF communications device is designed and configured to employ both FDD and TDD. Such an RF communications unit, or front end, may be employed with a telephone that operates according to TDD methodology, such as one conforming to the CT2 standard. The FDD/TDD approach can be viewed as employing a dual duplex design, involving a first carrier channel for transmissions and a different, second carrier channel for receptions. Over each carrier channel, communications are passed in bursts of distinct time intervals in a time division manner. Transmission occurs over a first carrier channel in a distinct time interval. Over a different, second carrier channel, reception occurs at a different, distinct time interval. In this manner, communications by transmissions and receptions occur in distinct and different time intervals, over different carrier frequencies. For further information concerning a cordless telephone system utilizing an FDD/TDD communication protocol, reference may be made to U.S. Patent application 08/567,133, filed Dec. 4, 1995, entitled “System and Method for Frequency Division Duplex/Time Division Duplex Radio Frequency Communications,” incorporated herein by reference. 
     Referring to FIG. 4 a , an example embodiment of a cordless telephone arrangement involves a base unit and a handset which communicate with each other using one of any number of communications protocol. In one application, these protocols include the CT2 specification using a TDD protocol. In another application, this communications protocol uses the CT1 standard with the FDD communications protocol. According to the present invention, useful battery life of the telephone handset is extended by selectively utilizing both the TDD and the FDD communications protocols, when appropriate, based upon the measured signal strength between the two units. 
     Referring to FIG. 4 b , an example embodiment of the present invention is disclosed. In an extended-range operation, occurring when the measured signal strength between the base unit  410  and the handset  411  is less than a predetermined threshold, these two devices use the FDD protocol in which the base unit and handset communicate with each other using two separate communications channels operating at a lower data rate than possible in TDD/FDD communication mode, such as 36 kbps, 48 kbps, etc. In this mode, communications between the base unit can occur simultaneously with the communications from the handset back to the base unit over these two separate channels. Conventional circuits, such as receive signal strength indicators (RSSI), can be used to provide the threshold criteria. 
     In alternative embodiments of the present invention, determining whether the received signal is acceptable (for example, sufficiently strong to maintain a conversation without audibly detectable interference) to select FDD strong is accomplished using one of a combination of the following: comparing the received signal strength to a threshold level; monitoring the received signal strength over a period of time and comparing the gradient of the monitored received signal strength to a known criteria; and detecting a substantial degradation (such as jitter or missing data bits) in the received signal. 
     In the normal range mode of operations, base unit  410  communicates with handset  412  using a TDD communications protocol in which the communications between the base unit and the handset operate, for example, at an aggregate bit rate of 72 kbps. However, the transmitter and receiver operate at a fifty-percent duty cycle such that each is on, or active, only one-half of the time. The effect of this arrangement is that the total communications capacity of the communications channel between the base unit and handset is the same as the capacity found in the extended range mode operating at one-half of the bit transfer rate. The lower bit transfer rate found in the extended range operation provides more energy per bit being transferred, thus allowing a lower bit error rate for a given range, and allowing the handset to operate at an extended range while permitting effective communications between the handset and the base station. 
     Referring to FIG. 5 a , the cordless telephone handset is operated under the control of a programmable controller  501 . Depending on the particular application, the controller is implemented using programmable processing devices. The AMD 79C413, 79C433 and 79C434 processing devices are examples of such devices. For further information concerning such devices, reference may be made to the related technical literature, including the Technical Manuals entitled, “Am79C413 CT2 PhoX ™ Controller for Digital Cordless Telephones” and “Am79C433 ISM PhoX™ Controller for Digital Cordless Telephones”, filed herewith and incorporated herein by reference. The controller interacts directly with the telephone ringer  505 , the telephone handset speaker  504 , the microphone for the telephone  503  and keypad used to dial the telephone numbers  502 . This controller device  501  transmits digital data streams  511  directly to RF transmit and receive circuitry  509  for ultimate transmission using the antenna  510 . The controller device  501  also provides control signals  512  and  515  to the transmit and receive power supply control circuitry  507  and  508  respectfully which control the operation of the RF transmit and receive circuitry. These power supplies are turned on and off periodically to conserve power of this battery operated device. 
     The controller device  501  also accepts measurement data from received signal strength measurement circuitry  506  to obtain an indication of the receive signal strength from the RF circuitry. In one particular embodiment, the received signal strength measurement circuitry  506  generates its signal measurement by monitoring the current draw in the amplifier limiter stages of the RF circuitry. 
     The controller device, implemented as a programmable microcontroller, is programmed with the necessary software to perform the communications protocol switching of the claimed invention as described previously. This processing device can be implemented in any number of implementations which accomplish the same described function. 
     Referring to FIG. 5 b , a base unit for the cordless telephone apparatus once again consists of a programmable controller  550  which interacts with a telephone ringer  553  and received signal strength measurement circuitry  551  and the RF transmit and receive circuitry  552  with its corresponding antenna  554  its operation as described in the handset. The handset also has a telephone line interface  555  which provides the necessary connection to an outside telephone system, a central office, a private PBX or its equivalent. 
     As those skilled in the art will readily appreciate, the FDD/TDD embodiments described herein provide significant improvements and advantages over the prior technology. Those skilled in 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 example embodiments expressly described herein. The foregoing detailed description is, thus, to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.