Methods and apparatus for control of frequency hopping in an analog cordless telephone

[0002] The present invention relates generally to improvements to cordless telephony. More particularly, the invention relates to advantageous apparatus and methods for providing a frequency hopping cordless telephone.

[0003] A typical cordless telephone system includes a base station and a handset. The base station is typically connected by wire to a central office. The handset communicates with the base station by cordless means, such as through radio frequency signals, without a direct physical connection to the base station. Cordless telephones allow considerable freedom and mobility to the user, allowing the user to move about while engaged in a call, and to conduct a call at a considerable distance from the base station. Presently, two preferred cordless systems exist, digital and analog. Analog systems provide the advantages of simplicity, low cost, and high sound quality, but are vulnerable to in-band interference or jamming. Moreover, analog systems may interfere with other telephones. For this reason, analog systems are restricted in power, resulting in a reduced range.

[0004] Digital systems exist, which have both advantages and disadvantages with respect to analog systems. Digital systems provide a digital processing gain, reducing susceptibility to inband interference, with the resistance to interference increasing with processing gain. Digital systems are less likely to interfere with other telephone systems, and are permitted greater transmit power than are analog systems. The combination of greater transmit power and digital processing gain provides digital systems with a significantly greater range than is possessed by analog systems. Disadvantages of digital systems are a much higher cost due to the need to have powerful processors to perform voice coding and decoding, as well as digital signal processing (DSP) to achieve processing gain. Many parts of a digital system transmitter and receiver, such as a modulator and demodulator, are subject to more stringent requirements than are the corresponding parts of analog systems, and are therefore more complicated. Moreover, digital voice coding, interleaving, and spreading take time to perform, delaying signal transmission. Delay is especially prevalent in systems employing time domain duplexing, in which transmitter and receiver work in turns, half time each. This delay is added to the delay caused by digital processing, and may create a problem called echo. Echo in digital systems produces a significant degradation of sound quality, with the amount and nature of the degradation being influenced by the amount of delay and the spectrum content of the reflection. Echo cancellation in digital systems of the prior art represents a considerable challenge and increases the cost of the system.

[0005] An important aspect of transmission in cordless telephone systems is spreading. Systems of the prior art commonly use one of two techniques. These techniques are direct sequence and frequency hopping. In direct sequence systems the information signal is multiplied by a pseudorandom sequence of bits, widening the spectrum of the resulting signal. The received signal is multiplied with the same sequence once again to despread it. Many well-known techniques exist for performing spreading and despreading.

[0006] In frequency hopping systems, information bits are grouped into frames. Each frame is transmitted on a different carrier frequency. Over a period of time, therefore, the energy of the signal is spread throughout the entire bandwidth. Frequency hopping systems are appropriate for a time division duplexed system (“TDD”), since the transmitter and receiver of a TDD must be rapidly switched on and off. The transmitter and receiver of a TDD employing a frequency hopping system repeat an on-off cycle, with each repetition of the cycle employing a different frequency. A TDD employing frequency hopping is particularly vulnerable to echo.

[0007] It is possible to combine direct sequence and frequency hopping spreading techniques. A system using these combined techniques would have a large processing gain and a high interference immunity, but would be complex and expensive. Due to an extensive processing and hopping delay, it would also be susceptible to echo.

[0008] A need therefore exists in the art for a cordless telephone system with a low cost, high processing gain with resulting high interference immunity, and resistance to echo.

[0009] A cordless telephone according to the present invention includes a handset and a base station wherein frequency hopping is controlled in at least one of the handset and the base station by a signal received from the other of the handset and the base station. Preferably the base station and the handset each have a transmitter and a receiver operating simultaneously on different frequencies. The handset performs frequency hopping according to information contained in a signal received from the base station and the base station performs frequency-hopping according to information in a signal received from the handset. Preferably, the frequency-hopping information is transmitted in a single signal together with user information, such as a voice communication to be transmitted by the handset.

[0010] In a preferred embodiment of the invention, the handset transmits to the base station a modulated carrier signal comprising a digital portion and an analog portion. The base station receiver receives the signal and demodulates it to recover the digital portion and the analog portion. The analog portion is employed to provide user information, while the digital portion is employed to generate a hopping control signal used to control frequency hopping by the base station transmitter. The base station transmitter also employs the hopping control signal to produce a local oscillator signal used to control frequency conversion in the base station receiver. Similarly, the base station transmits to the handset a modulated carrier signal containing a digital portion and an analog portion. The handset recovers the digital portion and the analog portion and employs the analog portion to provide user information, while employing the digital portion to control frequency hopping in the handset transmitter and receiver. The handset and base station employ the digital portions of the signals to provide for frequency hopping without an interruption of communication.

[0011] A cordless telephone according to the present invention includes advantages of an analog cordless telephone, together with processing gain and spread spectrum capabilities, having resistance to echo and a cost significantly below that of prior-art digital techniques. The combination of analog modulation and digital data provides frequency hopping without an interruption of communication.

[0012] A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.

[0013] FIG. 1 illustrates a prior art cordless telephone system;

[0014] FIG. 2 illustrates a handset and base station for use in a cordless telephone system according to the present invention;

[0015] FIG. 3 illustrates a modulated carrier signal preferably employed by a cordless telephone system according to the present invention;

[0016] FIG. 4 illustrates a transmitter for use in a cordless telephone system according to the present invention;

[0017] FIG. 5 is a frequency plan preferably used by a handset of a cordless telephone system according to the present invention;

[0018] FIG. 6 is a frequency plan which may be used by a base station of a cordless telephone system according to the present invention;

[0019] FIG. 7 is a flowchart illustrating the steps of a method for frequency-hopping cordless telephony according to the present invention; and

[0020] FIG. 8 is a flowchart illustrating the steps of an alternative method for frequency-hopping cordless telephony according to the present invention.

[0021] FIG. 1 illustrates a cordless telephone system 100 according to the prior art. The telephone system 100 includes a handset 102 and a base station 104. The handset 102 includes a transmission section 106 and a reception section 108. The transmission section 106 includes a microphone 110, which receives voice inputs from the user and produces a voice signal which is then passed to a voice coder 112. The voice coder 112 produces a coded signal which is then passed to an interleaver 114. The interleaver 114 produces an interleaved signal which is passed to a spreader 116. The spreader 116 spreads the interleaved signal to produce a spread signal, and passes the spread signal to a pulse shaper 118. The pulse shaper 118 produces a pulse-shaped signal which is then passed to a transmitter 120 for transmission to the base station.

[0022] The reception section 108 includes a receiver 124. The receiver 124 receives a transmission from the base station 104 to produce a received signal which is then passed to a despreader 126 to produce a despread signal. The despread signal is passed to a deinterleaver 128, which produces a deinterleaved signal and passes the deinterleaved signal to a voice decoder 130. The voice decoder 130 produces a voice decoded signal which is then passed to a loudspeaker 132, which produces audible sounds which can be heard by a user.

[0023] The base station 104 includes a transmission section 122 similar to the transmission section 106, which produces a signal for reception by the reception section 108 of the handset. The transmission section 122 includes a voice coder 134, an interleaver 136, a spreader 138, a pulse shaper 140, and a transmitter 142. The base station 104 also includes a reception section 143, which includes a receiver 144, despreader 146, deinterleaver 148, and voice decoder 150. The transmission section 122 operates similarly to the transmission section 106 of the handset 102, and the reception section 142 operates similarly to the reception section 108 of the handset 102. The transmission section 122 does not include a microphone, and the reception section 142 does not include a loudspeaker. Instead, the base station includes a line interface 152 which Epasses signals to and from a central telephone office (not shown). When the base station 104 receives a signal from the handset 102, the signal is processed by the reception section 142 (being subject to normal processing delays) and passed to the line interface 152. A portion of the signal is reflected from the telephone line (not shown) to the line interface 152 and passed along the transmission section 122, being subjected to similar processing delays. The delays result in an echo which is heard in the loudspeaker 132.

[0024] Analog telephones are subject to a reflection from the telephone line, but as analog telephones are not subject to processing delays, this reflection is much less harmful to analog telephone operation than to digital telephone operation.

[0025] FIG. 2 illustrates a cordless telephone system 200 according to the present invention. The telephone system 200 includes a handset 201 and a base station 202. The handset 201 includes a microphone 203, a first control data unit 204, and a first signal combiner 206. The first microphone 203 provides a first analog signal to the first signal combiner 206, which also receives a first digital signal, used as a control data signal, from the control data unit 204. The first signal combiner 206 combines the first analog signal and the first control data signal to produce a first combined signal. The handset 200 further includes a first transmitter 208, a first control unit 210, a first signal separator 212, a first receiver 214, a first duplexer 216, a first antenna 218 and a loudspeaker 220. The first transmitter 208 receives the first combined signal from the first signal combiner 206, employing the first combined signal as a first modulation input signal. The first transmitter 208 receives a first hopping control signal for control of frequency hopping. The first transmitter 208 produces a first local oscillator signal, the first local oscillator signal preferably being an unmodulated signal, and being employed as a first local oscillator output, and a first modulated carrier signal, employed as a modulated carrier output. Modulation of the first modulated carrier signal is accomplished using the first modulation input signal received from the first signal combiner 206. The first modulated carrier signal thus includes a digital information and analog information. The first modulated carrier signal is received by the first duplexer 216, which passes the first modulated carrier signal to the first antenna 218 for transmission to the base station 202, while allowing the simultaneous reception of a second modulated carrier signal, similar to the first modulated carrier signal, transmitted from the base station 202 and received at the first antenna 218. The first receiver 214 employs the second modulated carrier signal received from the base station 202 and demodulates the second modulated carrier signal to produce a first demodulated signal, used as a first demodulated output. The first receiver 214 employs the first local oscillator signal from the first transmitter 208 as a local oscillator input, and employs the local oscillator input for frequency conversion in the first receiver 214. The first demodulated signal is provided to the first signal separator 212, which produces a first analog output signal which is provided to the loudspeaker 220, and a first digital output signal, employed as a data signal, which is supplied to the first control unit 210. The first control unit 210 produces the first hopping control signal which is provided to the transmitter 208.

[0026] The base station 202 includes a line interface 222, which receives a line input and produces a line output. The base station further includes a second control data unit 224 and a second signal combiner 226. The line output of the line interface 222 is provided as an input signal to the second signal combiner 226. The base station 202 further includes a second transmitter 228, a second control unit 230, a second signal separator 232, a second receiver 234, a second duplexer 236, and a second antenna 238. The second signal separator 232 provides a second analog output signal as the line input to the line interface 222. The base station 202 operates similarly to the handset 201 described above, except that the microphone 203 of the handset 201 is replaced in the base station 202 by the output of the line interface 222, and the loudspeaker 220 of the handset 201 is replaced in the base station 202 by the input to the line interface 222. The base station 202 employs the output from the line interface 222 as a second analog signal, and a second digital signal, to produce a second combined signal employed as a second modulation input signal which is provided to the second transmitter 228 for use in producing the second modulated carrier signal. The second transmitter 228 also produces a second local oscillator signal, which is preferably an unmodulated signal, and which is provided to the second receiver 234 as a second local oscillator input to enable frequency conversion in the second receiver 234. The second receiver 234 receives the first modulated carrier signal from the handset 201 and demodulates it to produce a second demodulated signal. The second demodulated signal is provided to the second signal separator 232, which produces a second analog output signal which is provided to the line interface 222, and a second digital output signal, employed as a second data signal, which is supplied to the second control unit 230. The second control unit 230 produces a second hopping control signal which is provided to the second transmitter 228.

[0027] Once communication is established between the handset 201 and the base station 202, the first transmitter 208 and the first receiver 214 operate simultaneously on different carrier frequencies. The first transmitter 208 receives the first combined signal as the first modulation input signal, as noted above. The first modulation input signal contains the first analog signal and the first data signal. The operation of the first transmitter 208 is controlled by the first control unit 210, which provides the first hopping control signal to the first transmitter 208. The first hopping control signal is based on digital information contained in the digital portion of the second modulated carrier signal received from the base station 202. The first control unit 210 may also be employed to provide data to the first signal combiner 206, in which case the first control data unit 204 is unnecessary. The first transmitter 208 generates the first modulated carrier signal, which is modulated through the use of the combined signal produced by the first signal combiner 206, provided to the first transmitter 208 as the first modulation input signal. The first transmitter 208 also generates the first local oscillator signal to enable a frequency conversion in the first receiver 214. Typically, frequency conversion is a down conversion, but frequency conversion may be accomplished in any of a number of ways commonly known in the art. As noted above, the first receiver 214 receives the second modulated carrier signal from the base station 202 and demodulates it. The first demodulated signal produced by the first receiver 214 by demodulating the second modulated carrier signal from the base station 202, has the same structure as the first modulation input signal described above, containing an analog portion containing user information and a digital portion containing frequency control data. The base station 202 operates simultaneously with, and in a similar fashion to, the handset 201.

[0028] The principle of a telephone system according to the present invention, such as the telephone system 200 illustrated in FIG. 2, is that in the handset the first modulated carrier signal and the first local oscillator signal hop simultaneously or very close to simultaneously, for example within a few microseconds. Similarly in the base station, the second modulated carrier signal and the second local oscillator signal also hop simultaneously or very close to simultaneously. The hopping interrupts the audio signal, but so briefly that the interruption is not noticed. Synchronization between base station and handset must be as close as possible, and the digital data transmitted together with the analog signal provides the control necessary to provide such close synchronization. Since the base station 202 and the handset 201 can send data simultaneously, it is well known to those skilled in the art how to use the data transferred between the base station 202 and the handset 201, as taught in the present invention, to synchronize the moment of frequency change. It is important to note that generation of the new frequency must be as quick as the synchronization itself, and for this reason traditional methods of generating a frequency signal (for example, phase locked loop frequency synthesizers with voltage controlled oscillators) are not acceptable. A typical phase locked loop takes a relatively long time to retune, on the order of hundreds of microseconds. Additionally, returning of a phase locked loop will create a frequency sweep resulting in strong pulses at receiver outputs and will also cause interference to all frequencies through which it sweeps. The first and second modulated carrier signals and the first and second local oscillator signals may preferably be generated using direct digital synthesis (“DDS”) or a similar approach which allows sufficiently rapid frequency generation. DDS and other presently known techniques allow synchronization of 1 microsecond or better. Also, a combination of techniques could be used, such as using DDS for generating a hopping sequence and mixing it with another frequency source to obtain the final frequency value. Alternatively, any harmonic of any of the sources could be used. Modulation could be applied to any of the sources.

[0029] FIG. 3 illustrates the structure of a signal spectrum 300 suitably used as a first modulation input signal or a second modulated input signal employed by a telephone system according to the present invention. The base-band signal spectrum 300 is formed from an analog signal spectrum 302 and a digital data spectrum 304. The digital data spectrum 304 is multiplied by a spreading signal spectrum 306, and also by a multiplier 308, to produce a spread data spectrum 310 above the highest frequency of the analog signal spectrum 302. The spread data spectrum 310 is combined with the analog signal spectrum 302 to produce the base-band signal spectrum 300. The simplest implementation of spreading is Manchester coding, but more complex spreading schemes are known in the art and may also be suitably implemented. Alternatively, the data may be arranged to have the needed spectrum without requiring spreading. Moreover, if a different method is developed which allows the spectra to overlap but still be successfully combined and separated, such a method could be used to create the combined analog-digital signal. The analog signal spectrum may theoretically have a low frequency as low as 0 Hz, although in present-day practical telephony systems such a spectrum is not used.

[0030] FIG. 4 is a functional diagram of a transmitter section 400 which may suitably be used as the first transmitter 208 in the handset of FIG. 2. The transmitter section 400 may similarly be used as the second transmitter 228 in the base station of FIG. 2, but to avoid duplication only its use as the first transmitter 208 will be discussed. The transmitter section 400 produces the first modulated carrier signal discussed above in connection with FIG. 2, and the second modulated carrier signal produced by the base station 202 is used to control frequency hopping by the transmitter section 400. The first modulated carrier signal has a variable frequency designated here by TX_h. The second modulated carrier signal has a variable frequency designated here by RX_h. The transmitter section 400 includes a first oscillator 402, which receives a first modulation input signal and produces a first oscillator signal having a variable frequency designated here by F1. The transmitter section 400 further includes a second oscillator 404 which produces a second oscillator signal having a variable frequency designated here by F2. The transmitter section 400 also includes a DDS generator 406, which receives the hopping control signal and produces a hopping signal having a variable frequency designated here by H. The frequency range and number of steps of the DDS generator 406 are dictated by overall system requirements and the needed frequency range which must be covered for the frequency TX_hof the first modulated carrier signal and the frequency RX_hof the second modulated carrier signal produced by the base station 202 and used to control frequency hopping by the handset 202, as well as cost considerations. The transmitter section 400 further includes a second oscillator 404, which produces a second oscillator signal having a variable frequency designated here by F2, a first mixer 408 which mixes the second oscillator signal and the hopping control signal while providing image suppression, and an n-times multiplier 410, which produces a first local oscillator signal having a variable frequency designated here by LO. The frequency F2 of the second oscillator signal is chosen so that (F2+H) x n is equal to the desired value of LO.

[0031] This is the frequency of the first local oscillator signal produced by the transmitter section 400 and used to control frequency hopping by the first receiver. Also, the range of H frequencies needs to be such that when H goes from its lowest to highest value, the nth harmonic of (F2+H) covers the whole range of local oscillator frequencies. The transmitter section 400 further includes a second mixer 412. The second mixer 412 receives the first oscillator signal and produces the first modulated carrier signal. Also shown, but not part of the transmitter section 400, are a duplexer 414, a third mixer 416 and an antenna 418. The antenna 418 transmits the first modulated carrier signal from the handset and receives the second modulated carrier signal from the base station. The duplexer 414 receives the second modulated carrier signal from the antenna 418 and passes it to the third mixer 416, while simultaneously passing the first modulated carrier signal from the second mixer 412 to the antenna 418. The third mixer 416 receives the first local oscillator signal from the n-times multiplier 410 and combines it with the second modulated carrier signal to produce an IF output signal having a variable frequency designated here by IF. The IF output signal may be passed to a demodulator (not shown), or alternatively may be passed to another mixer (not shown) to perform a second frequency conversion before demodulation. Either alternative may be suitably employed in accordance with the present invention, with the choice of alternatives being dictated by particular design requirements.

[0032] FIG. 5 is a graph 500 illustrating an overall frequency plan for a handset according to the present invention, with relative values shown for H, F2, F2+H, LO, TX_h, RX_h, F1, and IF. The frequency TX_his the frequency of the first modulated carrier signal, which is transmitted by the handset to the base station. The frequency RX_h, is the frequency of the second modulated carrier signal, which is received by the handset from the base station. The graph 500 illustrates a configuration employing a low side injection for TX_hand RX_h. The frequency range of TX_his higher than the frequency range for RX_h. The frequency F1 is the difference between LO and TX_h, while the frequency IF is the difference between LO and RX_h. However, a high side injection could be used for both RX_hand TX_h, as well as any combination such as low side for RX_hand high side for TX_h. The range for H frequencies is typically relatively low, on the order of hundreds of kHz. F2 is typically on the order of tens of MHz, as are F1 and IF. Frequency ranges for LO, TX_hand RX_hare equal. In the unlicensed 900 MHz band typically employed for cordless telephone systems, the ranges for LO, TX_hand RX_hare typically 10-12 MHz.

[0033] FIG. 6 is a graph 600 illustrating an overall frequency plan for a base station according to the present invention, with relative values shown for H, F2, F2 +H, LO, TX_b, RX_b, F1, and IF. In the case of the base station, RX_bis the frequency of the first modulated carrier signal received by the handset from the base station, and has the same value as TX_hdescribed in FIG. 5. The frequency TX_bis the frequency of the second modulated carrier signal transmitted by the base station to the handset, and has the same value as RX_hdescribed above. Because TX_band RX_brepresent the same frequencies as do RX_hand TX_h, respectively, the frequency ranges for TX_band RX_bare reversed with respect to TX_hand TX_h. The frequency range of RX_bis higher than the frequency range for TX_b. As with the frequency plan of FIG. 5, the frequency F1 is the difference between LO and TX_b, while the frequency IF is the difference between LO and RX_b. However, in the frequency plan of FIG. 6, the frequency IF is greater than the frequency F1.

[0034] FIG. 7 is a flowchart illustrating the steps of a method 700 for cordless telephony according to the present invention. At step 702, communication is established between a handset and a base station, such as the handset 201 and the base station 202 of FIG. 2. The handset and the base station transmit analog signals for communication of user information. At step 704, the handset produces a first control signal for control of frequency hopping in the base station. The first control signal may suitably be combined with a first communications signal for transmitting user information to the base station to form a first combined signal. The first communications signal an analog signal, while the first control signal may be an analog or a digital signal. At step 706, the handset transmits the first control signal to the base station, either together with the first communications signal as part of a first combined signal, or separately from the first communications signal, depending on design choice. At step 708, the base station employs the first control signal to control frequency hopping by the base station. At step 710, the base station produces a second control signal for control of frequency hopping in the handset. The second control signal may suitably be combined with a second communications signal for transmitting user information to the handset to form a second combined signal. The second communications signal is preferably an analog signal, while the second control signal may be an analog or a digital signal. At step 712, the base station transmits the second control signal to the handset, either together with the second communications signal as part of a second combined signal, or separately from the second communications signal, depending on design choice. At step 714, the handset employs the second control signal to control frequency hopping by the handset.

[0035] Steps 704-714 of the method 700 need not and typically may not be executed sequentially. For example, steps 704-708 are executed when the handset transmits a message to the base station, and steps 710-714 are executed when the base station transmits a message to the handset.

[0036] FIG. 8 is a flowchart illustrating the steps of an alternative method 800 for cordless telephony according to the present invention. At step 802, communication is established between a handset and a base station, such as handset 201 and base station 202 of FIG. 2. At step 804, the handset combines a first analog signal with a first digital signal to form a combined signal. At step 806, the handset transmits the combined signal to the base station. At step 808, the base station receives the combined signal and separates the combined signal to recover the first analog signal and the first digital signal. At step 810, the base station processes the first analog signal to produce an output for a user. At step 812, the base station employs the first digital signal to control frequency hopping by the base station. At step 814, the base station combines a second analog signal with a second digital signal to form a second combined signal. At step 816, the base station transmits the second combined signal to the handset. At step 818, the handset receives the second combined signal and separates the second combined signal to recover the second analog signal and the second digital signal. At step 820, the handset processes the second analog signal to produce output for a user. At step 822, the handset employs the second digital signal to control frequency hopping by the handset.

[0037] Steps 804-822 of the method 800 need not and typically may not be executed sequentially. For example, steps 804-812 are executed when the handset transmits a message to the base station, and steps 814-822 are executed when the base station transmits a message to the handset.

[0038] While the present invention is disclosed in the context of a presently preferred embodiment, it will be recognized that a wide variety of implementations may be employed by persons of ordinary skill in the art consistent with the above discussion and the claims which follow below. For example, although the preferred embodiment employs a signal having a digital component to control frequency hopping and an analog component to transmit user information, it will be noted that a telephone system according to the invention may be constructed which employs a signal using an analog component for the control of frequency hopping.