High-density multi-channel QAM modulator using an IDFT/IFFT and polyphase filter bank
Methods and apparatuses are provided for increasing the frequency resolution of a multi-channel QAM modulator and using a novel IDFT/IFFT and polyphase filter bank architecture to provide a more computationally efficient and high density multi-channel QAM modulator. The implementations of the improved multi-channel QAM modulator modulate QAM symbol streams onto respective carrier signals where the frequency of each carrier signal is an integer multiple of a fraction of the input sample rate of the respective QAM symbol stream. The modulated carriers are then combined using a novel IDFT/IFFT and polyphase filter bank architecture.
1. A method comprising:
modulating a plurality of quadrature amplitude modulated (QAM) symbol streams x k (m) onto a plurality of carrier signals respectively to produce a plurality of modulated carrier signals x′ k (m), wherein the plurality of QAM symbol streams have a sample rate equal to f s and wherein the frequency f k of each of the plurality of carrier signals is an unique integer multiple of a fraction of the sample rate of the plurality of QAM symbol stream;
producing a multi-point inverse Fourier transform (IDFT/IFFT) X l (m) of the plurality of modulated carrier signals x′ k (m); and
processing the multi-point IDFT/IFFT X l (m) using a plurality of polyphase subfilters h p (m) of an anti-imaging filter h(m).
2. The method of claim 1 further comprising serializing the outputs of the plurality of polyphase subfilters h p (m).
3. The method of claim 1 :
wherein modulating a plurality of QAM symbol streams x k (m) onto a plurality of carrier signals respectively to produce a plurality of modulated carrier signals x′ k (m) comprises modulating a plurality of QAM symbol streams x k (m), k=0, . . . , MN−1, onto a plurality of carrier signals to produce a plurality of modulated carrier signals x′ k (m), k=0, . . . , MN−1, respectively; and
wherein the frequency f k of each of the plurality of carrier signal is
k
f
s
N
and where N is a nonzero integer.
4. The method of claim 3 wherein producing a multi-point IDFT/IFFT X l (m) of the plurality of modulated carrier signals x′ k (m) comprising producing a MN-point IDFT/IFFT X l (m) of the plurality of modulated carrier signals x′ k (m).
5. The method of claim 4 wherein processing the multi-point IDFT/IFFT X l (m) using a plurality of polyphase subfilters h p (m) of an anti-imaging filter h(m) comprises producing M outputs y p (n), p=0, . . . , M−1, where
y
p
(
n
)
=
∑
m
=
0
∞
X
p
+
Mm
(
n
-
m
)
h
p
(
m
)
where X p+Mm (n−m) is the (p+Mm) mod MN IDFT/IFFT of the plurality of modulated carrier signals x′ k (m) at time (n−m).
6. The method of claim 5 wherein producing M outputs y p (n), p=0, . . . , M−1, where
y
p
(
n
)
=
∑
m
=
0
∞
X
p
+
Mm
(
n
-
m
)
h
p
(
m
)
comprises filtering M groups of N outputs of the MN IDFT/IFFT [X p , X p+M , X p+2M , . . . , X p+(N-1)M ] for each p=0, . . . , M−1 by the corresponding polyphase subfilter h p (n).
7. A multi-channel modulator comprising:
means for modulating a plurality of quadrature amplitude modulated (QAM) symbol streams x k (m) onto a plurality of carrier signals respectively to produce a plurality of modulated carrier signals x′ k (m), wherein the plurality of QAM symbol streams have a sample rate equal to f s and wherein the frequency f k of each of the plurality of carrier signals is an unique integer multiple of a fraction of the sample rate of the plurality of QAM symbol stream;
means for producing a multi-point inverse Fourier transform (IDFT/IFFT) X l (m) of the plurality of modulated carrier signals x′ k (m); and
means for processing the multi-point IDFT/IFFT X l (m) using a plurality of polyphase subfilters h p (m) of an anti-imaging filter h(m).
8. The multi-channel modulator of claim 7 further comprising means for serializing the outputs of the plurality of polyphase subfilters h p (m).