Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-06-30
2002-06-25
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S140000, C375S303000
Reexamination Certificate
active
06411646
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates generally to communications. More specifically, the invention relates to wireless telecommunications.
II. Description of the Related Art
The cordless telephone has become a popular consumer good. The cordless telephone allows a user to untether himself from a wired connection to his local telephone line. Typically, a cordless telephone is comprised of two units: a base unit and a handset. 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 use of the wireless link allows the handset to communicate with the base unit.
The consumer cordless telephone market is a competitive market. Cordless telephone products on the market compete with one another based on audio quality, reliability over various operating conditions, privacy, battery life, the physical range over which the device operates, and cost. In order to achieve favorable measures of these characteristics, the wireless interface which connects the handset to the base unit must operate efficiently.
One efficient way which cordless telephones operate is called time division duplex (TDD). In time division duplex, the base unit and the handset alternately transmit such that the units do not transmit at the same time. In a time division duplex system, the same frequency band can be used for both transmission and reception. By using time division duplex, the transmit and receive circuitry within each unit can share common components. In addition, each unit requires less internal isolation between the transmit and receive circuitry. For these reasons, a cordless telephone which operates using time division duplex can be cheaper, more reliable and yet produce higher quality audio signals than a full duplex unit. Even though the wireless link operates using time division duplex, audio compression techniques are used to provide concurrent bi-directional audio communication to the user. Therefore, even though the wireless link signals are time division duplex, the end user perceives simultaneous bi-directional audio communication.
Another technique used in cordless telephones is direct sequence spread spectrum (DSSS). 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 overcome severe levels of interference. In addition, spread spectrum can be used to introduce pseudo-randomness into the signal. Transmission signals spread with a pseudo-random code appear to be random noise and are difficult to demodulate by receivers other than the intended receiver. In this way, a system which uses direct sequence spread spectrum is less vulnerable to accidental or deliberate reception by a third party. In this way, direct sequence spread spectrum, in conjunction with a scrambling scheme, provides a significant element of privacy in the communications channel.
In a direct sequence spread spectrum system, data bits are modulated with a spreading sequence before transmission. Each bit of information is modulated with a series of chips from the spreading sequence. The ratio of the number of chips per bit defines the coding gain. A greater number of chips per bit creates a greater immunity to noise and other interference. For example, in one common cordless telephone spreading technique, each information bit is modulated with a 12 bit spreading code. Because a cordless telephone using direct sequence spread spectrum has some immunity to noise and other interference, the cordless telephone handset may transmit a very low output power. By decreasing the transmit power, the battery life of the handset is increased.
In a typical embodiment, the spreading code contains an even number of one's and zero's. In this way, the energy of the spread spectrum signal is minimized at and close to 0 Hz. For this reason, a baseband spread signal may be subjected to highpass or bandpass filtering with little effect on the information content. In a system in which each information bit is modulated with a 12 bit spreading code, a preferred spreading code can be chosen by examination of the spectral content of each possible 12 bit sequence which is comprised of six 0's and six 1's.
Prior to application of the spreading code to the information bit stream, the information bits may undergo a series of digital operations which further increase the performance of the system. For example, the information bits may undergo differential encoding in order to be more intolerant to an incorrect phase lock in the receiving unit phase locked loop (PLL). The information bits may be scrambled using a long scrambling sequence in order to further decrease the vulnerability of the system to interception.
Conventional cordless telephones utilizing direct sequence spread spectrum coding also use binary phase shift keying (BPSK). In a phase shift keyed system, information is carried in the phase of the signal. For example, in
FIG. 1A
, the binary sequence 1 0 1 1 0 is represented as a series of positive and negative voltage levels. In
FIG. 1B
, the same sequence has been phase shift keyed modulated. In
FIG. 1B
, two different phases are used to denote the two different digital values. Note that whenever the sequence transitions from a “1” to a “0” or from a “0” to a “1”, the phase of the signal in
FIG. 1B
transitions. Such a system is referred to as a BPSK system.
FIG. 2
is a block diagram showing a prior art BPSK architecture. This architecture may be used by both the base unit and handset. The baseband spread spectrum signal is produced by a digital portion of the architecture which is not shown in FIG.
2
. The digital spread spectrum waveform is converted to a baseband analog signal by a one bit digital-to-analog converter (DAC)
62
. The baseband signal is then amplified by a baseband amplifier
60
. After amplification, the signal is passed through bandpass filter
58
. The bandpass filter
58
is necessary because the baseband spread spectrum signal contains higher order harmonics which are removed before transmission in order to avoid transmitting out of band energy. In addition, the bandpass filter
58
attenuates signal energy at frequencies at or near 0 Hz. Attenuation of the low frequency components of the baseband signal aids in suppression of the radio frequency (RF) carrier frequency component of the radio output.
The filtered output of the bandpass filter
58
is modulated with an RF carrier by a mixer
56
. The RF carrier is generated by a phase lock loop comprised of a voltage control oscillator (VCO)
44
, a lowpass filter
46
and a frequency mixer/phase detector
48
. During operation, the mixer/phase detector
48
is programmed by the digital architecture to control the VCO
44
to generate an RF sinusoidal signal at the selected wireless link center frequency. The signal produced by the VCO
44
is applied to the mixer
56
such that the output of the mixer
56
is a BPSK waveform at the desired RF transmit frequency.
The RF BPSK waveform is amplified by an amplifier
54
. In addition, the BPSK waveform is amplified by a variable gain power amplifier
50
. The gain of the power amplifier
50
is set based upon a transmit power level indication received from the digital architecture and converted to usable form by a power amplifier level control unit
52
. The gain of the power amplifier
50
at the transmitter may be decreased as the path loss between the handset and base unit is decreased in order to conserve power. During a transmission period of the time division duplex operation, an RF switch
22
connects the output of the power amplifier
50
to a radio frequency lowpass filter
20
. The output of the lowpass filter
20
is transmit
Beamish Norman J.
Laub D. Vincent
Walley John S.
Conexant Systems Inc.
Knobbe Martens & Olson Bear LLP.
Pham Chi
Phu Phuong
LandOfFree
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