Method and apparatus for despreading a spread spectrum signal at baseband

• US 4,484,335 A
• Filed: 10/14/1982
• Issued: 11/20/1984
• Est. Priority Date: 10/14/1982
• Status: Expired due to Fees

• Current Assignee: E-Systems Incorporated (Rtx Corporation)
• Original Assignee: E-Systems Incorporated (Rtx Corporation)
• Inventors: Sanders, David E., Mosley, William H.
• Primary Examiner(s): Safourek, Benedict V.
• Application Number:
  US06/434,530
• Time in Patent Office: 768 Days
• Field of Search: 343/5 PN, 375/1, 375/39, 375/96, 375/77, 375/97, 364/604, 364/728, 364/819, 329/104, 329/112, 329/110
• US Class Current: 375/150
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/709 Correlator structure

Abstract

A baseband correlation circuit (10) receives an IF spread spectrum input signal which is phase compared with a local oscillator signal and a phase shifted local oscillator signal to produce I channel and Q channel spread baseband signals. Each of these signals is input to four sample and hold circuits (34-48). A local PN code signal is converted from two-phase to four-phase to produce I and Q channel local PN signals. These signals together with the logical inverses thereof are provided as the control inputs for the sample and hold circuits (34-48). For each pair of sample and hold circuits there is a respective summer (86-92). The pair of sample and hold circuits together with the corresponding summer comprises a double balanced multiplier. There are four double balanced multipliers for producing four respective product signals. The product signals are input as pairs into summers (106) and (108) to produce despread, narrowband baseband I and Q channel output signals at output lines (110, 112). The baseband correlator circuit (10) of the present invention serves to despread the spread spectrum IF signal at baseband, thereby eliminating the need for costly and bulky components which are required for despreading at RF frequencies.

Claims

1. A baseband correlator circuit for despreading a spread spectrum signal, comprising;

means for translating said spread spectrum signal to baseband to produce an I (in-phase) channel baseband signal and a Q (quadrature) channel baseband signal, said spread spectrum signal utilizing a pseudonoise (PN) code sequence,

means for converting a two-phase local PN signal comprising said PN code sequence into four-phase signals comprising two in-phase PN signals and two quadrature phase PN signals,

means comprising four double balanced multipliers for multiplying said I and Q channel baseband signals by each of said four PN signals to produce first, second, third and fourth product signals,

means for summing said first and second product signals to produce a despread I channel output signal, and

means for summing said third and fourth product signals to produce a despread Q channel output signal.

2. A baseband correlator circuit as recited in claim 1 wherein said means for converting produces the logical inverse signals of said in-phase PN signal and said quadrature phase PN signal.

3. A baseband correlator circuit for despreading a spread IF signal, comprising:

a phase comparator connected to receive said spread IF signal having a pseudonoise (PN) code and a local oscillator signal for producing an I channel (in-phase) spread baseband signal and a Q channel (quadrature phase) spread baseband signal,

means for converting a two-phase local PN signal, having said PN code therein, into four-phase signals comprising two in-phase PN signals and two quadrature phase PN signals,

a first double balanced multiplier having said I channel spread baseband signal and said in-phase PN signals input thereto and producing a first product signal,

a second double balanced multiplier having said I channel spread baseband signal and said quadrature phase PN signals input thereto and producing a second product signal,

a third double balanced multiplier having said Q channel spread baseband signal and said in-phase PN signals input thereto and producing a third product signal,

a fourth double balanced multiplier having said Q channel spread baseband signal and said quadrature phase PN signals input thereto and producing a fourth product signal,

a first summation circuit connected to receive said first product signal and said third product signal for producing a despread, baseband I channel output signal, and

a second summation circuit connected to receive said second product signal and said fourth product signal for producing a despread, baseband Q channel output signal.

4. A circuit as recited in claim 3 wherein each of said double balanced multipliers comprises:

first and second sample and hold circuits having one of said spread baseband signals input thereto and having the true and logical inverse of one of said PN signals provided respectively to the control inputs thereof, and

a summation circuit connected to receive the outputs of said first and second sample and hold circuits for producing one of said product signals.

5. A method for despreading a spread spectrum signal, comprising the steps of:

translating said spread spectrum signal to baseband to produce an I (in-phase) channel baseband signal and a Q (quadrature) channel baseband signal, said spread spectrum signal utilizing a pseudonoise (PN) code sequence,

converting a local two-phase PN signal comprising said PN code sequence into four-phase signals comprising two in-phase PN signals and two quadrature phase PN signals,

multiplying in four double balanced multipliers each of said I and Q channel baseband signals by each of said PN signals to produce first, second, third and fourth product signals,

summing said first and second product signals to produce a despread I channel output signal, and

summing said third and fourth product signals to produce a despread Q channel output signal.

6. A method for despreading a spread spectrum signal as recited in claim 5 wherein the step of converting includes producing the logical inverse signals for said in-phase PN signal and said quadrature phase PN signal.

7. A method for despreading a spread IF signal having a pseudonoise (PN) code, comprising the steps of:

comparing said spread IF signal with a local oscillator signal to produce an I channel (in-phase) spread baseband signal and a Q channel (quadrature phase) spread baseband signal,

converting a two-phase local PN signal having said PN code therein, into four-phase signals comprising two in-phase PN signals and two quadrature phase PN signals,

double balance multiplying said I channel spread baseband signal and said in-phase PN signals to produce a first product signal,

double balance multiplying said I channel spread baseband signal and said quadrature phase PN signal to produce second product signals,

double balance multiplying said Q channel spread baseband signal and said in-phase PN signal to produce third product signals,

double balance multiplying said Q channel spread baseband signal and said quadrature phase PN signal to produce fourth product signals,

summing said first product signal and said third product signal to produce a despread, baseband I channel output signal, and

summing said second product signal and said fourth product signal to produce a despread, baseband Q channel output signal.

8. A method as recited in claim 7 wherein each of the steps of double balance multiplying comprises:

sample and holding one of said spread baseband signals in response to one of said PN signal and the logical inverse of the one PN signal to produce first and second outputs, and

summing said first and second outputs to produce one of said product signals.