A cordless phone refers to a phone handset in which electromagnetic radiation is utilized to carry communications to and from the handset. Cordless telephone devices have become increasingly popular among individuals both at home and in the workplace. Modern cordless telephones offer a wide range of features. The primary benefit of the cordless phone is that the handset need not be at a fixed location and need not be located close to the base unit. The cordless telephone allows a user to free himself from a wired connection to his local telephone line. Cordless telephones offer the user the freedom to move about and simultaneously place or receive a telephone call without being tethered by a wire to a conventional telephone set. They are easy to keep nearby for use in answering incoming calls. Cordless telephones do not require the user to remain at one given location while engaged in telephony activities. Instead, the user is able to roam within range of the base unit, and as long as the handset is within range, the user can engage in telephony activities such as initiating outgoing telephone calls and receiving incoming telephone calls. As cordless telephone technology advances, the range of the RF link between the base and handset continues to increase. This is partly due to the use of higher powered RF frequencies which are allowed due to the use of digital, as opposed to analog, communication techniques, enabling more advanced digital signal processing features, such as frequency hopping, spread spectrum, forward error correction, etc. Cordless telephones are similar to conventional corded telephones in that they are connected by wire to a local central office. However, they differ in that there is no cord between the base unit of the cordless telephone and the handset; thus, they are called cordless phones. Cordless telephones add the convenience of mobility within the range of coverage of radio transceivers in the cordless telephone base unit and handset. This cordless telephone system offers the function of intercommunication between portable units. That is, voice communication between a calling portable unit and a remotely located called portable unit.

Cordless phones come in a variety of configurations and features. A typical cordless telephone is comprised of a base unit or base station and a portable handset unit which both have antennas and transceivers for communicating with each other. The cordless telephone base unit is the heart of cordless telephone systems. The base unit couples to the public switched telephone network (PSTN) to provide telephonic services to a user of the remote unit via the RF link. The base unit typically determines which features are provided by the cordless telephone system. Such features typically include the number of telephone lines to which the system can be connected, the number of handsets that can be used, and whether ISDN telephone lines can be accessed. For cordless phones, the connection to the PSTN has essentially become standardized. The base unit connects to the public switched telephone network typically using a standard RJ-11 connector. The base unit provides a wireless connection to a handset. The handset is capable of receiving and transmitting signals over a wireless link to the base unit. The handset can be removed from the base unit and used within a predetermined range, which is limited by the RF signal strength and not cord length. The handset typically includes batteries which are recharged by placing the handset in the base station. Cordless telephone handsets typically incorporate a user input mechanism, such as a keypad, through which a user inputs information. The processor in the handset typically processes the user input and communicates the input telephone number to the base unit via the uplink. Cordless telephone handsets also typically include a microphone and a speaker respectively positioned internal to a mouthpiece and an earpiece. When communicating during a telephone call, a user speaks into the microphone in the mouthpiece and listens to audible sounds produced by the speaker in the earpiece. Communication is established between the base station and the handset either upon initiation of a call by the handset user, or an incoming telephone call received over the telephone line by the base unit.

A cordless telephone system commonly includes a programmable embedded processor that controls the operation of the telephone. The processor, such as a microprocessor, digital signal processor (DSP), application specific integrated circuit (ASIC), or logic within the handset interprets the user input and communicates the user input via the uplink to the base unit. While conventional cordless telephones provide a base unit which communicates with a single cordless telephone handset, there can be multiple handsets that may be utilized with a cordless telephone base unit. A multiple handset system typically employs a base unit and multiple handsets. Handsets are able to communicate between each other as an intercom, in addition to communicating with the base unit for interconnectivity with the public switch telephone network. The handset and base station exchange unique security IDs so that they communicate only with the correct partner. Cordless phones typically have other features such as automatic answering circuitry that respond to an incoming call. Cordless phones usually have additional circuitry that is used to provide the aforementioned features and also allow the base to communicate with the headset when the phone is being used by a user. Unlike conventional consumer corded telephones, which typically operate solely from power supplied by the public switched telephone network (PSTN), cordless telephones require an external power source to enable their operation. Cordless phones require a source of power to operate the needed circuitry. The telephone base units are typically supplied with a DC power adapter which is connected to a wall power outlet. The DC power supply converts the 120 volt AC power signal into a DC signal that powers the electronic circuitry of the cordless phone. The handset unit operates on its battery. When the handset is removed from the base unit, a battery in the handset powers the transceiver and other electronics in the handset to allow the portable operation of the telephone. The battery on the handset has a limited charge and must be periodically recharged by returning the handset to the base unit. The base unit detects the battery in the handset and begins a trickle charge operation which takes an amount of time which varies according to the strength of the battery.

Cordless telephones typically operate over a radio frequency (RF) portion of the spectrum set aside for general public use. The frequency band is broken up into a number of channels that may be used by the handset and base station. Only a relatively small portion of the radio spectrum has been allocated to cordless telephones and these telephones operate on a selected band of a relatively few channels. The handset and base station may both be sharing the same channel (time division duplex) or transmitting on one channel and receiving on another (frequency division duplex). Typically, the base unit and the handset are permanently set to the same frequency. Many cordless telephones operate as a time division duplex (TDD) system. In time division duplex, the base unit and the handset alternately transmit such that the units do not transmit at the same time. By using time division duplex, the transmit and receive circuitry within each unit can share common components. Cordless telephones typically use direct sequence spread spectrum (DSSS) modulation in conjunction with TDD. Spread spectrum signals used for the transmission of digital information are distinguished by the characteristic that their bandwidth is much greater than their information rate in bits per second. The large redundancy introduced by spread spectrum operation can be used to compensate for severe levels of interference. To prevent unauthorized use, it is common to employ a security code in cordless telephone systems. Spread spectrum can be used to introduce pseudo-randomness into the signal. Transmission signals spread with a pseudo-random code are difficult to demodulate by receivers other than the intended receiver. In this way, the system is less vulnerable to accidental or deliberate reception by a third party, and the privacy in the communications are protected. The RF link either transmits the voice signals between the base unit and the remote handset using analog techniques (frequency modulation or FM), or using digital techniques such as quadrature amplitude modulation (QAM) or frequency shift keying (FSK). The RF link between the handset and the base unit is typically divided into two portions, such as an uplink from the handset to the base unit at a first frequency, and a downlink from the base unit to the handset at a second frequency.

Presently, two cordless systems exist, including digital and analog systems. 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. Sounds produced by the user are typically converted into electrical signals by the microphone. In an analog system, the electrical signals are then typically transmitted via the uplink to the base unit. In a digital system, on the other hand, the electrical signals are digitized by an analog-to-digital (A/D) converter, processed by the processor in the handset, and transmitted as digital signals via the uplink to the base unit. During an telephone call, analog audio signals are communicated between the base unit and the network via the telephone line interface. In a digital cordless telephone, received analog signals from a far end party to a telephone call are conventionally converted in the base unit to digitally encoded speech samples for digital communication over the RF link between the base unit and the handset. The digital speech samples transmitted between the base unit and the handset are encoded by an encoding unit (vocoder) to reduce the bandwidth required to transmit the samples over the RF link. The vocoder may employ the coding technique known as adaptive differential pulse code modulation (ADPCM) to provide a better voice quality via a low transmission rate or low transmission band, and uses a modem in transmitting encoded voice data. The digital cordless phone system provides diverse advantages. A digital wireless communication system provides better communication quality than that provided by a conventional analog wireless communication system. The digital cordless phone system provides tapping prevention function using encryption and an excellent speech quality within a preset speech communication distance.