Abstract:
The present invention provides a method and system to improving voice transmission quality within an environment containing multiple off-hook cordless telephones all residing on the same subscriber line. This is achieved by coordinating the sending, receiving, and acknowledging (i.e., handshaking) of control and telephone signals between the multiple cordless telephones and the base. The communication between cordless telephones takes place through a single household telephone line and the RF link existing between each cordless handset and its accompanying base station. When a single handset of a multiple handset cordless telephone system is registered as off-hook, the handshaking between that handset and base results in the transmission of voice signals between the off-hook handset and the base which are digitized using Adaptive Differential Pulse Code Modulation (ADPCM). When one or more handsets become active and are registered as off-hook, the transmission of voice signals between the off-hook handsets and base is accomplished through the use of speech coding at a lower rate than that of ADPCM coding. The successful switching between ADPCM and speech coding within a multiple handset environment allows for speech quality to be maximized at all times.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to cordless telephones and, more particularly, to a cordless telephone having a speech encoding/decoding switching system for use within a multiple handset environment. 
     2. Description of the Related Art 
     Cordless telephones have proven to be popular in domestic, business and industrial environments due to the unrestricted freedom of movement they offer users. In fact, in 1997, for the first time ever, sales of cordless telephones exceeded sales of corded telephones with total cordless units sold being in excess of 28 million. Furthermore, total sales for 1998 are expected to have increased over 1997 sales by at least 25 percent. 
     Standard cordless telephones route incoming and outgoing telephone signals through a base station which is hard wire connected to a telephone line. The base station communicates with the battery-operated handset using a wireless signal transmitted over a distance. That is, the physical hard wire connection between a conventional handset and telephone base is replaced by a radio frequency (RF) link, which can range from the 46 and 49 MHz bands to the more recent 900 MHz bands. The spoken voice is usually communicated between the base and handset by first converting the user&#39;s voice into an analog electrical signal and modulating the signal using an RF carrier for radio transmission to the receiver, typically through the use of a Narrow-Band Frequency Modulation (NBFM) technique. At the receiver, the modulated analog voice signal is demodulated and directed to a speaker through which the voice is heard. Outgoing telephone signals follow a reverse direction through generation at the handset, transmission to the base, and then routing to the outgoing phone line. The wireless transmission of the telephone signals between the handset and the base can occur over a range of wavelengths and to varying distances. 
     The quality of the wireless signal is of paramount importance to the user of a cordless telephone. The quality of a transmission between handset and base is limited by the size of the components and frequency of the signal. A more powerful transmitter results in a more powerful signal which can travel longer distances. However, such power comes with the attendant negative factors of bulky handsets and bases and shortened battery life. In addition, the bandwidth within which cordless telephones are limited is subject to interference from numerous electromagnetic sources. 
     Cordless telephone systems must, therefore, meet a basic standard of speech quality, often called “toll quality” speech. Toll quality speech transmission standards comprise the minimum bandwidth needed to assure recognition of the speaker by the receiver at the other end of the link in combination with at least 98% understandability of the speech in context. Originally, in telephone signals, the minimum bandwidth was 300 Hz to 3400 Hz, which resulted in 4 kHz frequency spacing for single sideband (SSB) cable and radio transmission. These standards have been preserved in digital transmission, using pulse code modulation (PCM), and are perpetuated in the increasingly common ISDN standard. 
     To provide toll quality speech transmission between the handset and the base, some cordless telephone systems rely on digital transmission of the analog telephone signal. This requires that a digital to analog coder/decoder chip (“codec”) be placed in both the handset and the base. According to information theory, when PCM was discovered the sampling rate of an analog signal was set at 2 W for perfect recovery of signals having a bandwidth of less than W. In order to prevent foldover intermodulation distortion, the speech spectrum had to be strictly limited to less than 4 kHz. Thus, the sampling rate for voice telecommunications was set at 8 K samples/sec. 
     CCITT Recommendation G.726-1990 specifies how a digital telephone signal is to be compressed before transmission and how a received digital signal is to be expanded after reception using ADPCM. ADPCM is a technique for converting sound or analog information to binary information by taking frequent samples of the sound and expressing the value of the sampled sound modulation in binary terms. The G.726 standard specifics the functionality that is required for the receive (ADPCM decoder) and transmit (ADPCM encoder) signal processing functions. G.726 allows for the conversion of a 64 kilobit-per-second (kbps) pulse code modulation (PCM) channel to and from a 40, 32, 24, or 16 kbps ADPCM channel. G.726 incorporates the previously-existing G.721 (32 kbps) and G.723 (24 kbps) standards. In addition to cordless handsets, ADPCM is used to encode data on CD-ROMS and data transmitted over fiber-optic transmission lines. 
     While 8 Kb/s transmission rates meet a minimum level for speech comprehension, it is by no means an ideal transmission rate. An 8 Kb/s transmission rate transmits all vowels very well. However, transmission of consonants, which have main speech energies concentrated between 7 kHz to 8 kHz, is rudimentary at best. Generally, speech taken in context provides sufficient clues for good understandability, although unexpected words and names typically must be spelled in order to circumvent the lack of bandwidth in toll quality telephone connections. Thus, in general, telephone systems having a higher-fidelity transmission became desirable. 
     Current cordless telephone systems use an improved ADPCM which is capable of much higher quality transmissions. The ADPCM signal conversion device is conventionally known which compresses data and converts that data into PCM signals and further converts from the PCM signals into ADPCM signals. Transmitter side voice signals are compressed and coded in the form of ADPCM signals and then transmitted, and in which on the receiver side the ADPCM signals are expanded and demodulated into voice signals. ADPCM allows analog voice conversation to be carried within a 32 Kb/s digital channel. The sampling rate is 8,000 times per second and three or four bits are used to described each sample. At current transmission values, ADPCM provides a high quality transmission signal between a cordless telephone&#39;s handset and base. 
     Traditionally, to provide multiple handset use in telephone systems utilizing a single subscriber line, multiple sets of bases and handsets must be plugged into that line. Other solutions have involved using multiple hardwired telephones as described in U.S. Pat. No. 5,367,570 (Hector D. Figueroa). To reduce the amount of equipment necessary in such a case, multiple handset cordless telephones are currently available. Such systems allow multiple handsets to transmit and receive signals from a single base station. Such systems are highly desirable in cases in which there is a limited number of incoming lines or there is limited space for the base stations. Multiple handset cordless telephones operate in one of two ways when in use. Either a single handset is used and the other handsets are rendered inactive or any handset may be used to allow multiple participants to use the system. The latter case is preferable in that it mimics a standard telephone system in which multiple persons can speak on the same subscriber line using different telephones. For the concept of a multiple handset telephone to be commercially and practically useful, the system must support simultaneous transmission of signals from the multiple handsets. 
     Current multiple handset cordless systems which support the use of simultaneous off-hook handsets transmit speech between the handsets and base by using speech coding instead of ADPCM. The use of speech coding at a lower bit rate than ADPCM allows multiple communications signals to share the same frequency bandwidth. Speech coding is normally accomplished by a speech codec chip. A speech codec allows analog voice conversation to be carried within a 4, 8 or 16 Kb/s digital channel. This reduced transmission rate allows for multiple transmissions within a limited bandwidth, for example allowing 8, 4, or 2 handset units where only one existed previously on a 32 Kbit/s channel. However, transmission rates of 4, 8 or 16 Kb/s carry a significant penalty in term s of voice quality. Such a transmission is readily discernable from the 32 Kb/s experienced by single handset cordless systems using ADPCM. 
     One attempt to operate multiple handsets in communication with a single base station is described in U.S. Pat. No. 5,689,549 (Bertocci, et al.). A telephone system is described in which a time-sharing method is used to allow multiple, independent conversations to be carried on over each handset. While this allows for the use of ADPCM encoding in a multiple handset environment, it does not allow the multiple handsets to be used in the same conversation. 
     Currently, if multiple handsets are used with a single base station, speech coding or time-sharing techniques are used to transmit cordless telephone signals. Consequently, when such a system is used, reduced speech quality or reduced functionality is experienced in comparison to single-handset cordless telephone systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for improving voice transmission quality within an environment containing multiple off-hook cordless handsets coupled to the same base station. This is achieved by coordinating the sending, receiving, and acknowledging (i.e., handshaking) of control and telephone signals between the multiple handsets and the base station. The communication between the handsets and the base station takes place through an RF link existing between each handset and the base station. 
     In accordance with the present invention, when a single handset of a multiple handset cordless telephone system is registered as off-hook, the handshaking between that handset and base station results in the transmission of voice signals between the off-hook handset and the base station which are digitized using Adaptive Differential Pulse Code Modulation (ADPCM). When one or more handsets become active and are registered as off-hook, the transmission of signals between the off-hook handsets and base station is accomplished through the use of speech coding and a time division duplex or frequency division duplex system. The successful switching between ADPCM and speech coding within a multiple handset environment allows for speech quality to be maximized at all times. 
     A cordless telephone system incorporating the present invention may be implemented without requiring additional wiring or a special connection to the existing telephone subscriber line. Furthermore, the invention may be incorporated into any cordless telephone that uses an RF link between its handset and its base station, allowing multiple handsets to be purchased at later date. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of preferred embodiments given below with reference to the accompanying drawings, in which: 
     FIG. 1 illustrates a block diagram of a typical multiple handset cordless telephone system; 
     FIG. 2 a block diagram of circuitry located within a base station of the invention; 
     FIG. 3 illustrates a block diagram of circuitry located within a handset unit of the invention; and 
     FIG. 4 illustrates a flowchart describing the handshaking required between the handsets and the base station of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described with reference to FIGS. 1-4. Other embodiments may be realized and structural, or logical changes may be made to and equivalents used for the disclosed embodiment without departing from the spirit or scope of the present invention. 
     FIG. 1 depicts a typical multiple handset cordless telephone environment. The FIG. 1 environment consists of a subscriber telephone base station  10  plus a plurality of handsets, e.g. two handsets  20 ,  22 , all of which are operating on the same subscriber telephone line  100  and within the same household. Communication between the base station  10  and the handsets  20 ,  22  is achieved via wireless transmission of signals. In FIG. 1, each of the handsets  20 ,  22  are off-hook and currently engaged in an open line communication with the base station  10 . Each handset  20 ,  22  and the base station are depicted as being coupled together by an RF link  250 ,  252 . 
     Also, in accordance with the disclosed embodiment, each of the handsets  20 ,  22  may independently communicate with each other as well as the base station  10 , as described in detail later herein. 
     For establishing communications between the base station  10  and multiple active handsets  20 ,  22 , the present invention may employ a frequency division duplex system or a time division duplex system, such as that described in U.S. Pat. No. 5,809,417 (Nealon et al.) and incorporated herein by reference. In a frequency division duplex system, for example, each one of the handsets  20 ,  22  is configured with a different starting channel from a plurality of communication channels available in a frequency hopping system. Frequency division duplexing is a multiplexing scheme in which the available transmission frequency range is divided into narrower bands. Each of these bands is used to carry a separate channel. For providing initial communications with the handsets  20 ,  22 , the base station  10  transmits a broadcast signal sequentially over a set of communication channels while monitoring each one of the starting channels of each handset  20 ,  22 . A user-initiated response from the handsets  20 ,  22  receiving the broadcast signal from the base station  10  causes a response signal to be transmitted from the handsets  20 ,  22  to the base station  10  over the respective starting channel of each handset  20 ,  22 . The base station  10  and responding handset  20 ,  22  then establish communications over a set of communication channels assigned to the respective responding handset  20 ,  22 . 
     A general overview of spread spectrum technology including frequency hopping systems is provided by R. C. Dixon,  Spread Spectrum Systems , New York: John Wiley &amp; Sons, 1984, incorporated herein by reference. The specific requirements for the frequency hopping system in which this cordless telephone is designed to operate arc set forth in a Report and Order in General Docket No. 89-354, this Report and Order being adopted by the Federal Communications Commission on Jun. 14, 1990 and released on Jul. 9, 1990, incorporated herein by reference. 
     Turning now to FIG. 2, a block diagram of circuitry located within base station  10  is depicted. Included in the base station  10  are a control unit  110 , a router  114 , a clock  120  for providing synchronization to: 1) the control unit  110 , 2) a time domain duplexer (TDD)  124  and 3) the speech codec  116  and ADPCM module  118 . Also included in the base station  10  are a radio frequency (RF) transceiver  122 , a signal strength monitor circuit  130 , an antenna  102  and a frequency synthesizer  128 . A line interface  112  in the base station  10  connects this unit to a central office or other appropriate switch through tip and ring lines  100  and  101 . An interface unit and display  126  contains the switches and a visual display for configuring the base station in its various modes for communicating with one or more of the handsets  20 ,  22 . The transceiver  122  comprises both an RF transmitter and an RF receiver. The transceiver  122  demodulates voice signals transmitted by the handsets  20 ,  22  and couples these signals via the router  114 , preferably an analog switch controlled by the controller  110 , and either the speech codec  116  or the ADPCM module  118  to the line interface  112 . The transceiver  122  also has as its input speech and other control signals from the line interface  112  which are first coupled through the router  114  and either the speech codec  116  or the ADPCM module  118  before being transmitted to the handsets  20 ,  22  by the transceiver  122 . The line interface  112  serves as a “plain old telephone service” (POTS) interface for signals on the tip-ring lines  100  and  101  and for those signals received from the handsets  20 ,  22  by the RF transceiver  122 . The controller  110  advantageously provides a number of control functions and may be implemented through the use of a microcomputer containing read-only-memory (ROM), random-access-memory (RAM) and through use of the proper coding. 
     The controller  110  controls and configures the TDD  124 . The controller  110  generates a pseudo-random data list and transmits the list to the TDD  124  where it is stored therein. The TDD  124 , in turn, controls the frequencies selected in the frequency hopping cycle of the base station  10  by inputting into the frequency synthesizer  128  at the appropriate time the values stored in the data list generated by the controller  110 . The TDD  124  also refreshes the frequency synthesizer  128  as the synthesizer progresses though the frequency hopping cycle. 
     In order for the base station  10  to achieve effective coverage throughout its operating range, the signal strength monitor circuit  135  continually monitors the strength of the received signal from the handsets  20 ,  22  during ongoing communications with the handsets  20 ,  22 . 
     This signal strength monitor circuit  135  may be, for example, a received signal strength indicator (RSSI) circuit. This RSSI circuit produces an output voltage that is proportional to the strength of the received signal from the handsets  20 ,  22 . 
     Responsive to the strength of the received signal from the handset  20 , as determined by the signal strength monitor circuit  135 , the controller  110  is capable of recalculating the amount of power transmitted by the transmitter in the RF transceiver  122  to the handsets  20 ,  22 . Thus, when the handsets  20 ,  22  are in close proximity to the base station  10 , the level of power radiated by the RF transceiver  122  is reduced to a minimum acceptable level. Similarly, when the handsets  20 ,  22  are determined to be located near the edge of the telephone set&#39;s operating range, the level of power radiated by RF transceiver  122  can be increased to its maximum permitted level. 
     In one embodiment, the functions of the router  114 , speech codec  116 , and ADPCM module  118  are combined within a single digital signal processing (DSP) chip. 
     FIG. 3 depicts a block diagram of circuitry located within a handset  20 . Included within the handset  20  are a controller  158 , a router  152 , a clock  160  for providing synchronization to: 1) the controller  158 , 2) a time domain duplexer (TDD)  162  and 3) the speech codec  154  and ADPCM module  156 . Also included in the handset  20  are an RF transceiver  150 , a signal strength monitor circuit  170 , an antenna  172  and a frequency synthesizer  168 . An interface unit and display  164  contains switches and a visual display for configuring the handset  20  in an appropriate mode for communicating with the base station  10  as well as permits dialing telephone digits and selecting such functions as talk, intercom and page modes for the handset  20  to communicate with the base station  10 . Handsets  20  and  22  contain the same components and are operationally identical. 
     The transceiver  150  comprises both an RF transmitter and an RF receiver. This transceiver  150  demodulates voice signals transmitted by the base station  10  and couples these signals via the router  152  and either the speech codec  154  or ADPCM module  156  to a speaker  176  on line  252 . The transceiver  150  also has as its input digital speech signals which have been transmitted from a microphone  174  in analog form through router  152 , speech codec  154  or ADPCM module  156 , TDD  162 , and frequency synthesizer  168 . Either the ADPCM module  156  or speech codec  154  is used to convert the analog signal to a digital signal which is then provided to the RF transceiver  150 . The signal strength monitor  170  monitors the received signal level from the base station  10  and accordingly varies the level of the output power radiated by the  1 I transceiver  150  in proportion to this received signal level. 
     In one embodiment, the functions of the router  152 , speech codec  154 , and ADPCM module  156  are combined within a single digital signal processing (DSP) chip. 
     Each of the handsets  20 ,  22  must be provided with a security code from the base station  10  during a registration process in order for subsequent radio frequency communications to take place between the base station  10  and a handset  20 ,  22  or between handsets  20 ,  22 . When an RF signal is received from a base station  10 , the control unit  158  enables the TDD  162  to establish synchronization with the RF signal being received from the base station  10 . This may be accomplished by tie base station  10  transmitting a unique identification code which must be registered by the individual handset  20 ,  22 . The security code data is generated in the base station in accordance with the teachings of U.S. Pat. No. 4,736,404 (R. E. Anglikoivski et al.), incorporated herein by reference. The starting channel data is a pseudo-random number seed. This seed, used for generating the starting channel and also random subsequent channels, is generated by the control unit in the base station. The handset unit automatically acknowledges to the base station when it has received the security code data and the starting channel data. Once this data has been received and acknowledged, both the base station and the handset unit begin frequency hopping in the manner described in U.S. Pat. No. 5,353,341 (M. E. Gillis et al.), incorporated herein by reference. 
     Turning now to FIG. 4, a flowchart depicts, in more detail, the operation of the present invention. Starting with step S 300 , a third party calls a subscriber. The incoming telephone signal is received by the base station  10  in step S 302  and routed through the line interface  112  to the router  114  on line  200 . Upon detection of an incoming telephone signal, the controller  110  signals a user that an incoming call has been received in step S 304 . Such signaling may occur, for example, by the use of an audio alarm contained in the base station  10  or by a signal generated within the base and transmitted to the handsets  20 ,  22 . The controller  110  monitors the input of router  114  received from RF transceiver  122  until an off-hook signal has been received from at least one handset  20 ,  22  in step S 306 . 
     Controller  110  controls the router  114  such that the incoming remote signal is routed through the ADPCM module  118 . The ADPCM module  118  encodes the incoming remote signal using ADPCM encoding techniques known in the art such as, for example, G.726 compliant ADPCM techniques. The encoded signal is routed through the TDD  124 , frequency synthesizer  128 , and RF transceiver  122  for RF transmission to the handsets  20 ,  22  in step S 308 . 
     Once transmission of an incoming telephone call has begun outgoing transmissions originating from the handsets  20 ,  22  are now possible. Upon going off-hook through, for example, depression of a specified key by the user in step S 306 , a handset  20 ,  22  generates an off-hook control signal which is transmitted to the base station  10  to be detected by the controller  110 . Dependant upon the number of off-hook handsets  20 ,  22  the controller  11  has detected in step S 310 , being one or more than one, the controller  110  generates a control signal transmitted to the handsets  20 ,  22  to encode outgoing signals using either the ADPCM module  118  in step S 314  or the speech codec  116  in step S 312 , respectively. The speech codec  116  is preferably circuitry designed to encode and decode speech on 4, 8 or 16 Kb/s digital channels through techniques such as, for example, waveform coding, linear coding (PCM), differential coding (DPCM), frequency-domain coding, or parametric coding (a vocoder such as format vocoder (synthesizer) or an LPC vocoder). The speech codecs  116  and  154  may be permanently configured to encode and decode speech based upon 4, 8 or 16 Kb/s digital channels or may be dynamically configurable to process on 4, 8, and 16 Kb/s channels the choice of which is determined by the number of off-hook handsets the controller  110  detects. Any change in off-hook status will force the controller  110  to recalculate the current number of off-hook handsets  20 ,  22  and send control signals to the handsets  20 ,  22  instructing them to route the outgoing signals through either the speech codec  116  if more than handset  20 , 22  is off-hook or the ADPCM module  118  if only one handset  20  or  22  is off-hook. Multiple simultaneous signals received by the base station  10  from the handsets  20  share the speech codec  116  through a time division system controlled by the controller  110  and the router  114 . Alternatively, one speech codec unit  154  may be present for each handset  20 ,  22 . The continuous monitoring of the off-hook status of the handsets  20 ,  22  by the base station  10  in step S 310  continues until the telephone call has been terminated. 
     In one embodiment of the present invention, the handsets  20 ,  22  are capable of communication with each other through the use of control/identification signals such as those described in U.S. Pat. No. 5,809,417 (Nealon et al.) and incorporated herein by reference. In the case in which multiple handsets  20 ,  22  are currently off-hook, outgoing communications signals are encoded using the speech codec  154  and may be accompanied by a control signal also generated by the speech codec  154 . Upon receipt of a speech coded RF signal, the controller  158  recognizes the signal as being speech coded through the use of tie accompanying control signal. The controller  158  routes the signals through router  152  to the speech codec  154  for decoding before transmission to the speaker  176  on line  252 . Incoming signals which are not accompanied by a control signal or have a control signal corresponding to ADPCM encoding are presumed to come from the base  10  and have been ADPCM encoded. The controller  158  routes such signals to the ADPCM module  156 . 
     In another embodiment of the present invention, the handsets  20 ,  22  communicate to one another through the base station  10 . An outgoing signal generated by handset  20  is speech coded by speech codec  154  and transmitted to base station  10 . Base station  10  may configured to either automatically rebroadcast the speech coded signal using the router  114  or to decode the incoming signal using speech codec  116 . If the signal is decoded, it must be encoded again, preferably using the ADPCM module  118  to preserve voice quality transmission. The signal is preferably immediately rebroadcast due to the signal loss experienced by double encoded signals. 
     It should be readily apparent that although only two cordless telephone handsets  20 ,  22  have been depicted in the multi-cordless environment for purposes of simplicity, any number of cordless telephone handsets  20 ,  22  with a single base station  10  may be used in successfully practicing the invention. Furthermore, the invention may be successfully implemented within any cordless telephone environment employing an RF link between a handset and a base station. It should also be noted that the invention&#39;s usefulness is not limited to an RF link of any one specific frequency. Also, although the invention is described using frequency division duplexing techniques, other multiplexing techniques such as time division multiplexing can also be used. 
     In addition, while a preferred embodiment of the invention&#39;s implementation within a multi-cordless environment has been described, it should be readily apparent that any configuration and/or combination of hardware may be used to perform the same, or similar operations as those performed by the FIG.  2  and FIG. 3 block diagrams. While specific circuitry has been depicted as being located within a base station, design modifications may be made such that the circuitry is located within a handset, and vice versa. For example, while the control signals are depicted as being generated within the base station  10 , alternatively, some of those signals may be generated within the handsets  20 ,  22 . Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.