An Efficient Circulant MIMO Equalizer for CDMA Downlink: Algorithm and VLSI Architecture



Yuanbin Guo et al.

Figure 2: The block diagram of the VLSI architecture of the FFT-based MIMO equalizer.


(M × N ) MIMO-FIR block for filtering. To reflect the cor-
rect timing, the correlation and channel estimation mod-
ules at the front end will work in a throughput mode on
the streaming input samples. The FFT-inverse-IFFT modules
in the dotted line block construct the postprocessing of the
tap solver. They are suitable to work in a block mode us-
ing dual-port RAM blocks as interface between blocks. The
MIMO-FIR filtering will also work in throughput mode on
the bu
ffered streaming input data.

4. VLSI-LEVEL COMPLEXITY OPTIMIZATION

4.1. Hermitian optimization

In this section, more emphasis is given to the VLSI-oriented
implementation aspects. For QPSK and QAM modulation
schemes, all the numerical computations in the algorithm are
associated with complex numbers. However, the complexity
in the hardware is reflected by the number of real multipli-
cations, additions, and divisions, and so forth. It is more ac-
curate to clarify the complexity for di
fferent types of com-
putations. For example, a general “
complex (a) × complex
(b)” numerical computation has 4 real multiplications and 2
real additions, but a “
complex (a) × conjugate (a)” reduces
to only 2 real multiplications and 1 real addition. By defin-
ing
Fn1,n2 [0 : LF - 1] as the element-wise FFT vector of the
covariance block-vector for
n1, n2 [1,N], we show that the
element-wise FFT of the circulant covariance vectors admits
a Hermitian structure. This leads to the following lemmas for
complexity reduction.

Lemma 1 (Hermitian). Fn1,n2 = conj(Fn2,n1), where the vector
is formed from the covariance element vector between antennas
n
1 and n2. Fn2,n1 is redundant for n2 <n1.

Lemma 2 (Hermitian complexity). The computation of
Fn1,n1 can be reduced to only L/LF of the full DFT module.

Proof. For the Rx antennas n1, n2, it can be shown that the el-
ements in the circulant column have the following relations,
where
NB is the covariance time-average window length:

NB-1

e(^n2(0) = (Egn 1(0))* = Σ rn 1(i)rn2(i)*,

i=0

NB-1

=        rn1(i)rn2(i+l)*,

i=0                      (12)

NB-1

=        rn2(i)rn1(i+l)*,

i=0

Eg n 2( l ) = (En2), n 1 ( LF l )) *

Egn2 (LF - l) = (En?,n 1(l))*


Eg n 2 ( l ) = ( E⅛). n 1 ( LF - l )) * = 0 otherwise

Using the features of the FFT, it can be proven that
the element-wise FFT results have the relation that
Fn1,n2 =
(Fn2,n1 )* . The submatrix formed by the ith entry of Fn1,n2
is an (N × N) Hermitian symmetric matrix as F(i) =
(Fn1,n2(i))N×N = F(i)H.

Thus, instead of having N × N complex FFT computa-
tions, we only need to compute the element-wise FFT for the
lower triangle matrix. The number of FFTs in the element-
wise FFT is reduced from
N2 to (N2 + N)/2. Moreover, the
element-wise FFT coe
fficients of the diagonal elements are
all real numbers. This leads to the design of the reduced-state
MIMO-FFT blocks.                              □

4.2. Reduced-state FFT

Because the FFT algorithm applies the features of the rota-
tion coe
fficients, the application of the Hermitian feature to
FFT is not straightforward. Here, we derive the VLSI-level
optimization for the reduced-state FFT with pruning op-
erations based on the standard radix-2 decimation-in-time
(DIT) FFT algorithm. Notice that in the standard butterfly



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