Wireless frequency-division-multiple-access direct sequence spread spectrum telephone system

• US 5,511,090 A
• Filed: 03/17/1994
• Issued: 04/23/1996
• Est. Priority Date: 03/17/1994
• Status: Expired due to Fees

• Current Assignee: Tatung Telecom Corporation (Tatung Company)
• Original Assignee: Tatung Telecom Corporation (Tatung Company)
• Inventors: Denton, D. Darryl, Lau, Douglas
• Primary Examiner(s): Cain, David C.
• Application Number:
  US08/210,187
• Time in Patent Office: 768 Days
• Field of Search: 380/34, 379/61, 379/62, 455/89, 375/205, 375/206, 375/207
• US Class Current: 370/342
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04M 1/72502 with one base station connected to a single line
• CPC Class Codes: Y02D 30/70 in wireless communication networks

Abstract

A cordless, battery-operated telephone of relatively low power consumption suitable to operate in license-exempt spectrum and which may employ an established communication protocol, such as the CT-2 communication protocol, is realized by providing a direct sequence spread spectrum (DSSS) modulator/spreader and demodulator/despreader in a battery-operated handset in connection with an intelligent CT-2 controller to which is added additional circuitry and functionality in the handset for acquiring, pulling-in and tracking, in both operational and low-power-consumption standby modes, a received DSSS signal from a base station unit. The additional functionality in the handset includes a PN phase discriminator with intelligent oversampling to deal with the time division duplex signals during synchronization, a second order delay lock loop for in-service mode tracking at a moderate loop iteration rate, and an extended-range discriminator and very narrow-band third-order delay lock loop for stand-by mode tracking at a very low loop iteration rate.

Claims

1. In a digital cordless telephone system, a battery-operated handset for use in conjunction with a compatible base station unit, said handset comprising:

a modular controller means, said controller means including memory, a central processor unit, a transmit signal output for a transmit signal, a receive signal input for receiving a receive signal, a receive signal strength indicator (RSSI) input for receiving an RSSI signal, a control output for controlling a pseudo-random number generator/decoder means, an audio signal input and an audio signal output;

a pseudo-random number generator means coupled to said controller means, for generating a transmit pseudo-random-noise (PN) bit sequence and a receive pseudo-random-noise (PN) bit sequence for use in spread spectrum coding and decoding;

first digital signal modulator means coupled to receive said transmit pseudo-random-noise bit sequence and said transmit signal to produce a direct sequence spread-spectrum-modulated signal;

upconverting and transmitting means for producing an output spread spectrum analog r.f. signal in a first preselected spread spectrum frequency channel;

receiving means for receiving an input spread spectrum analog r.f. signal in a second preselected spread spectrum frequency channel;

downconverting means for said input spread spectrum analog r.f. signal to produce an intermediate spread spectrum signal;

second digital signal modulator means coupled to receive said receive pseudo-random-noise bit sequence and a fixed oscillator signal to produce a spread-spectrum-demodulated signal;

third digital modulator means coupled to receive said spread-spectrum-demodulated signal and said intermediate spread spectrum signal operative as a despreader to produce a despread receive signal and to produce an RSSI signal;

means coupling said RSSI signal to said controller means;

means for integrating said RSSI signal over a preselected time window of integration to produce a time-integrated RSSI signal;

means for converting said time-integrated RSSI signal to a digital time-integrated RSSI signal; and

signal processing means coupled to receive said time-integrated RSSI signal, to control said integrator means and to control said second digital modulator means for regulating relative phase of said time window of integration for acquiring, pulling-in and tracking said receive spread-spectrum signal.

2. The apparatus according to claim 1 wherein said first spread spectrum frequency channel and said second spread spectrum frequency channel are of the same frequency and wherein said upconverting and transmitting means operates in time-division duplex (TDD) with said receiving means and said downconverting means.

3. The apparatus according to claim 1 wherein said transmit signal and said receive signal are modulated for time-division duplex (TDD) access, and wherein said signal processing means comprises a delay-lock-loop discriminator.

4. The apparatus according to claim 1 wherein said signal processing means includes means for acquiring a TDD signal from said base station unit and upon acquiring of said TDD signal invoking a pull-in mode and thereafter a normal tracking mode of said TDD signal.

5. The apparatus according to claim 4 wherein said TDD signal has frame of less than 100% duty cycle and wherein said signal processing means in said pull-in mode is operative to sample said TDD signal in three pairs at intervals substantially evenly spaced over a frame, said pairs being of not more than ±

1/2 chip separation relative to a nominal phase position.

6. The apparatus according to claim 5 wherein sample pairs are spaced not more than ±

1/8 chip separation.

7. The apparatus according to claim 4 wherein said TDD signal has frame of less than 100% duty cycle and wherein said signal processing means in said normal tracking mode is operative to sample said TDD signal in three pairs at intervals substantially evenly spaced over a frame, said pairs being of not more than 1/2 chip separation relative to a nominal phase position.

8. The apparatus according to claim 5 wherein sample pairs are spaced not more than ±

1/8 chip separation.

9. The apparatus according to claim 4 wherein said signal processing means includes means for invoking a standby tracking mode to save power, said standby tracking mode employing a third-order delay lock loop operating with a low operating duty cycle.

10. In a digital cordless telephone system having a battery-operated handset for use in conjunction with a compatible base station unit, the handset including a modular controller means, a pseudo-random number generator/decoder means coupled to said controller means for generating a transmit pseudo-random-noise (PN) bit sequence and a receive pseudo-random-noise (PN) bit sequence for use in spread spectrum coding and decoding, a method in said handset for receiving signals from said base station unit comprising:

b) in a signal pull-in mode following an indication of a valid signal on a signal channel, said valid signal of less than 100% duty cycle in a time frame,

b-i) sampling in three pairs at intervals substantially evenly spaced over a frame, said pairs being of a PN code phase offset which is a fraction of a chip separation relative to a nominal phase position;

b-ii) selecting among early samples the early sample having maximum value and among late samples the late sample having maximum value;

b-iii) determining as an error value from a delay-lock-loop discriminator the difference between the maximum value early sample and the maximum value late sample;

b-iv) iterating a tracking loop and adjusting phase offset of a local PN bit sequence relative to a matching PN bit sequence on the signal channel as steered by output of said tracking loop; and

b-v) repeating steps b-i) through b-iv) to maintain tracking.

11. The method according to claim 10 wherein said delay-lock-loop discriminator is employed with second-order loop tracking.

12. The method according to claim 10 wherein said PN code phase offset is not more than ±

1/2 chip relative to nominal phase.

13. In a digital cordless telephone system having a battery-operated handset for use in conjunction with a compatible base station unit, the handset including a modular controller means, a pseudo-random number generator/decoder means coupled to said controller means for generating a transmit pseudo-random-noise (PN) bit sequence and a receive pseudo-random-noise (PN) bit sequence for use in spread spectrum coding and decoding, a method in said handset for receiving signals from said base station unit comprising:

c) in a normal signal tracking mode following an indication of a valid signal on a signal channel, said valid signal having less than a 100% duty cycle in a time frame,

c-i) sampling, at a fractional chip of PN code phase offset, signal values for an early value and a late value relative to nominal phase of said valid signal in the time frame;

c-ii) determining as an error value from a delay-lock-loop discriminator the difference between the early value and late value;

c-iii) iterating a tracking loop and adjusting said PN code phase offset relative to a matching PN bit sequence on the signal channel as steered by output of said tracking loop; and

c-iv) repeating steps c-i through c-iii) to track the signals.

14. The method according to claim 13 wherein said delay-lock-loop discriminator is employed with second-order loop tracking.

15. The method according to claim 13 wherein said PN code phase offset is not more than ±

1/2 chip relative to nominal phase.

16. In a digital cordless telephone system having a battery-operated handset for use in conjunction with a compatible base station unit, the handset including a modular controller means, a pseudo-random number generator/decoder means coupled to said controller means for generating a transmit pseudo-random-noise (PN) bit sequence and a receive pseudo-random-noise (PN) bit sequence for use in spread spectrum coding and decoding, a method in said handset for receiving signals from said base station unit comprising:

d) in a standby signal tracking mode following an indication of a valid signal on a signal channel, said valid signal having less than a 100% duty cycle in a time frame,

d-i) comparing a local PN bit sequence with reception on a signal channel for presence of a matching PN bit sequence about a nominal phase to yield a current received signal strength indication (RSSI) value;

d-ii) comparing said current RSSI value with a current maximum RSSI value;

d-iii) upon said current RSSI value exceeding said current maximum RSSI value, substituting said current RSSI value for said current maximum RSSI value, saving an index of location of said then current maximum RSSI value;

d-iv) incrementing phase of said local PN bit sequence relative to phase of said received signal by a fraction of a chip in phase;

thereupon

d-v) repeating steps d-i) through d-iv) for a number of intervals not substantially exceeding a PN code phase uncertainty width of a low-operating duty-cycle discriminator;

d-vi) determining an output of a low-duty cycle, extended-range delay-lock-loop discriminator;

d-vii) iterating said delay-lock loop discriminator and adjusting phase offset of a local PN bit sequence relative to a matching PN bit sequence on the signal channel as steered by said error value;

d-viii) repeating steps d-i) through d-vii) until an incoming call is detected or a call is originated.

17. The method according to claim 16, wherein said determining step d-vi) comprises adding a weighted combination of:

the difference between the sample values on either side of the maximum RSSI value; and

the difference of said index of location of the maximum RSSI value from the nominal phase.

18. The method according to claim 16 wherein said delay-lock-loop discriminator is employed with a third-order loop.