Method and apparatus for continuous phase modulation preamble encoding and decoding
A communication method comprises generating a Golay code having a zero-DC level when differentially encoded and continuously rotated. A preamble of a data stream is spread with the Golay code, and the data stream is modulated with continuous phase modulation (CPM), such as constant envelope 2-CPM, before being transmitted. A method for acquiring a received signal having a spread sync field modulated with 2-CPM comprises performing chip-level differential detection of the received signal, correlating the received signal with a Golay sequence; and accumulating the correlator outputs.
1. A method for communication, comprising:
generating a Golay code having a zero-DC level when differentially encoded and continuously rotated;
spreading at least a preamble of a data stream with the Golay code;
modulating the data stream using continuous phase modulation (CPM); and
transmitting the modulated data stream,
wherein the DC level is defined as:
D
C
=
∑
n
=
0
N
-
1
c
(
n
)
ⅇ
j
n
π
/
2
=
∑
n
=
0
N
-
1
c
(
n
)
j
n
wherein π is the constant Pi, j is the complex square root of −1, and sequence {c(n)} of length N is obtained by differentially encoding a Golay sequence {a(n)} of length N.
2. The method of claim 1 , wherein the Golay code is a generalized Golay code comprising a concatenation of a plurality of Golay codes.
3. The method of claim 1 , wherein the data stream further comprises a header and a payload, and wherein the data stream is modulated with binary 2-CPM modulation having a modulation index of h 2 =½.
4. The method of claim 1 , wherein the data stream further comprises a header, a training sequence, and a payload, and wherein the preamble and the header are modulated with binary 2-CPM modulation with a modulation index of h 2 =½, and wherein the training sequence and the payload are modulated with 4-CPM modulation with a modulation index of h 4 =¼.
5. The method of claim 1 , wherein generating the Golay code comprises performing at least one of preferred Golay generation and preferred generalized Golay generation.
6. The method of claim 1 , wherein the Golay code is of length 16.
7. The method of claim 1 , wherein the CPM modulation is a Gaussian frequency shift keying (GFSK) modulation.
8. A method for communication, comprising:
generating a Golay code having a zero-DC level when differentially encoded and continuously rotated;
spreading at least a preamble of a data stream with the Golay code;
modulating the data stream using continuous phase modulation (CPM); and
transmitting the modulated data stream,
wherein the preamble comprises a sync field and a start frame delimiter field, wherein the sync field comprises a sequence of ones and the start frame delimiter field comprises a minus one.
9. A method for acquiring a received signal having a spread sync field modulated with 2-CPM modulation, comprising:
performing chip-level differential detection of the received signal;
correlating the received signal with a Golay sequence; and
accumulating the correlator outputs,
wherein the spread sync field comprises a sequence of ones spread with one of a Golay sequence and a generalized Golay sequence.
10. The method of claim 9 , wherein the Golay sequence comprises a Generalized Golay sequence comprising a concatenation of a plurality of Golay codes.
11. The method in claim 9 , further comprising continuously de-rotating the received signal by πh 2 prior to performing chip-level differential detection, wherein h 2 denotes a modulation index used for 2-CPM modulation, π is the constant Pi, and wherein the de-rotated signal y(k) at time t=kT is calculated from
y ( k )= r ( k ) e −jkπh 2 ,
wherein r(k) denotes the received signal at time t=kT, k is an integer-valued index, and T denotes chip duration.
12. The method in claim 9 , wherein correlating comprises performing one of preferred Golay correlation and preferred generalized Golay correlation.
13. The method in claim 9 , wherein accumulating comprises employing a memory component of N memory cells for storing accumulated correlator outputs, where N is the length of the Golay sequence.
14. The method of claim 13 , further comprising deriving a timing estimate from the location within the memory component where the accumulated correlator output has a maximum magnitude.
15. The method of claim 14 , further comprising deriving a frequency offset estimate from the angle of the accumulated correlator output's complex value at the timing estimate.
16. The method of claim 9 , wherein accumulating comprises employing an Infinite Impulse Response filter.
17. The method of claim 9 , further comprising comparing accumulated correlator output magnitudes to a threshold value.
18. An apparatus for communication, comprising:
a spreading-code generator configured for generating a Golay code having a zero-DC level when differentially encoded and continuously rotated;
a spreader for spreading at least a preamble of a data stream with the Golay code;
a continuous-phase modulator for modulating the data stream with continuous phase modulation (CPM); and
a transmitter for transmitting the modulated data stream,
wherein the DC level is computed according to
D
C
=
∑
n
=
0
N
-
1
c
(
n
)
ⅇ
j
n
π
/
2
=
∑
n
=
0
N
-
1
c
(
n
)
j
n
wherein π is the constant Pi, j is the complex square root of −1, and wherein sequence {c(n)} of length N is obtained by differentially encoding a Golay sequence {a(n)} of length N.
19. The apparatus of claim 18 , wherein the spreading-code generator comprises at least one of an efficient Golay code generator and a generalized Golay code generator.
20. The apparatus of claim 18 , wherein the continuous-phase modulator is a Gaussian frequency shift keying (GFSK) modulator.
21. An apparatus for acquiring a received signal having a 2-CPM modulated sync field, comprising:
a differential detector for performing chip-level differential detection of the received signal;
a correlator for correlating the received signal with a Golay sequence;
an accumulator for accumulating the correlator outputs; and
a de-rotator for continuously de-rotating the received signal by πh 2 prior to performing chip-level differential detection, wherein h 2 is the modulation index of the 2-CPM modulation and π is Pi.
22. The apparatus of claim 21 , further comprising at least one of an efficient Golay correlator and an efficient generalized Golay correlator for correlating the received signal with a Golay sequence.
23. The apparatus of claim 22 , wherein the accumulator comprises a memory component of N memory cells for storing accumulated correlator outputs, where N is the length of the Golay sequence.
24. The apparatus of claim 22 , further comprising a hypothesis testing device for comparing accumulated detector output magnitudes to a threshold value, determining a maximum magnitude, and deriving a timing estimate from the location within the memory component of the maximum magnitude.
25. The apparatus of claim 24 , further comprising a frequency estimator for deriving a frequency offset estimate from the angle of the accumulator output's complex value corresponding to the timing estimate.