## Microsoft word - limassol_revised.docx

**Recovery of a Lattice Generator Matrix from its Gram Matrix for **
**Feedback and Precoding in MIMO **
Francisco A. Monteiro,

*Student Member, IEEE*, and Ian J. Wassell

*Abstract***— Either in communication or in control applications, **
coding, MIMO spatial multiplexing, and even OFDM)

**multiple-input multiple-output systems often assume the **
unified from a lattice perspective as a general equalization

**knowledge of a matrix that relates the input and output vectors. **
problem (e.g., [14]). Advances in lattice theory are therefore

**For discrete inputs, this linear transformation generates a **
**multidimensional lattice. The same lattice may be described by **
**an infinite number of generator matrixes, even if the rotated **
There are several ways of describing a lattice (e.g., via

**versions of a lattice are not considered. While obtaining the **
modular equations [15] or trellis structures [16]), however,

**Gram matrix from a given generator matrix is a trivial **
the two most popular ones in engineering applications are

*i)*
**operation, the converse is not obvious for non-square matrixes **
the generator matrix and

*ii)* the Gram matrix. The

**and is a research topic in algorithmic number theory. This **
computation of the latter given the former is trivial. The

**paper proposes a method to execute such a conversion and **
reverse is not, and an efficient algorithm for this conversion

**applies it in a novel MIMO system implementation where some **
remains an open problem in the theory of lattices.

**of the complexity is taken from the receiver to the transmitter. **
It should be noticed that an efficient algorithm for this

**Additionally, given the symmetry of the Gram matrix, the **
reverse operation can allow a lattice to be described using

**number of elements required in the feedback channel is nearly **
only about half the number of elements usually required
when the dimensionality of the space is sufficiently high,
provided that the Gram matrix is always symmetric. For
Any multiple-input multiple-output (MIMO) system is example, in MIMO communications with channel state
traditionally described by a generator matrix. In the wireless information at the transmitter (CSIT), this means that about
(and recently also in wired [1],[2]) communications systems half the number of coefficients would need to be sent to the
context, the matrix storing the fading coefficients between transmitter (Tx) when compared with that when using
transmit and receive antennas is known as the

*channel *traditional feedback [17]. Using the traditional example in

*matrix,* however in other contexts which operate with [17] , while in a single-input single-output configuration
vectorial spaces, the matrix receives other names. (with BPSK modulation) the channel state information is
Considering that the inputs are restricted to a set of discrete conveyed by one coefficient only, in a 4×4 antenna system
inputs isomorphic to , these systems can be framed in the one has 16 complex variables describing the channel, or
equivalently 32 real coefficients, that need to be periodically
The regularity of a lattice lends itself to the representation fedback to the transmitter. In fact, the number of coefficients
of problems where different signals are interpreted as a point to be fedback is the product of the number of antennas at the
in a multidimensional space. They appear in many areas of transmitter, at the receiver (Rx), the delay spread and, in
signal processing such as quantization[3][4][5] or image multi-user environments, also proportional to the product
processing [6]. Recently, lattices have also become a central with the number of users.
tool in cryptography [7] [8]; they are also used in numerical
This paper shows how an approximate solution to an open
integration (i.e.,

*quadrature*) of multi-dimensional functions problem in algorithmic number theory may lead to a more
constituting

*lattice rules* [9][10], and have a long history in efficient CSIT mechanism. The paper proposes an algorithm
the fields of geometry of numbers, algorithmic number to obtain a close approximation for a generator matrix given
theory [11], multidimensional sphere packing (important in a Gram matrix of a lattice. The algorithm is based on an
coding theory) [12] and also in integer programming [13].
exact technique recently proposed by Lenstra [11] (an
The communication theory community has recently seen historical figure in the fields of algorithms for lattices). This
topics that were thought to be distinct (such as the multiple paper uses the proposed algorithm as a constituent block in a
access channel, the broadcast channel, precoding, space-time novel strategy for closed-loop MIMO communication.
Manuscript received November 23, 2009. This work was supported in
A lattice is a discrete subgroup (of maximal rank) in a
part by a scholarship from the Foundation for Science and Technology,
Euclidean space and can be defined in a number of ways, as
listed in Section I. We summarise here the most common
Francisco A. Monteiro is with the Department of Engineering and with
The Computer Laboratory, University of Cambridge, UK, and also with the
Telecommunications Institute at the Lisbon University Institute, Portugal
Ian J. Wassell is with The Computer Laboratory, University of
Cambridge, UK (e-mail:

[email protected]).

*Revised version of the paper in the ISCCSP 2010 proceedings: a QR is *
*moved into the receiver (Fig.3) and the implications from that. *
Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)
where

**y** are the points of the lattice,

**h***i * are the

*generating *
*vectors* where each corresponds to the

*i*th column of the

*n*×

*n*
*generator matrix* **H** (considering full-rank lattices only).

Consequently, one can state that

**G** induces a quadratic

Each integer

*xi* is an element of the column vectors

**x**. Thus, form and is definite positive because

a lattice defined as in (1) is the span of the column space of This permits us to say that
always has a

** LDL***T*
the generator matrix

**H**, when we restrict the input to integers. decomposition [21][20][22].

Note that the prevalent notation in MIMO literature
Obtaining a valid from is not simple. defines an
considers column vectors while in channel coding or in other abstract lattice, however, two versions of a lattice will have
fields in mathematics lattices are traditionally represented by the same Gram matrix and in general, for a given ,
the span of the row space of a generator matrix. Notice that
rows and columns of a given

**H** span different lattices.

can be transformed into one

*factorization problem*. When
representing an

*equivalent lattice* defined by
decomposition offers a good solution as it applies to
symmetric definite positive matrices [21]. For the general

*m*×

*n* case, obtaining a basis from a specified Gram matrix
had no available method in the literature until recently [11].
is an

*unitary matrix* (with real elements and
is a

*unimodular matrix* (with

*C. The volume of lattices *
Full rank lattices are specified by a full-rank generator
matrix and the volume of a lattice (e.g., [8]) is given by
With this in mind, the unitary transformation performs
a rigid rotation of the lattice structure (i.e., of its generating
vectors), while the unimodular matrix replaces a set of
is rank-deficient (which is the case when
generating vectors by a different set that still generates the non-square), the volume is
One of the hardest lattice problems is the

*lattice *
*distinguishing problem*, i.e., to discover if two lattices are
For a traditional complex representation with

*N*
and

*NR* antennas as outputs, the received signal is
question becomes trivial. But when both transformations are
unknown the question is difficult to solve in some particular
problems [18],[19]. This decision problem has a simple
solution when the lattices are

*rational lattices* (when all entries are in
). In that case two lattices are equivalent if
and only if their generating matrixes have the same Hermite
Normal Form [13],[20]. However, the real lattices that arise mean circularly symmetric Gaussian random variable with
in communication problems lead to numerical problems
given the large numerators and denominators in the fractions unitary variance. The noise vector is
representing fading coefficients. In that case the best
approach is to use the fact that the QR decomposition is
with independent circularly symmetric Gaussian
unique up to signs in the main diagonal.
random variables, each one with a certain variance
In the remainder of the paper we will resort to the
equivalent real model of complex lattices:
where

*H* is the Hermitian operator (conjugate and transpose)

*.*
between all generating vectors and thus is
unique to a lattice subject to unitary transformations
(albeit not unique for unimodular transformations
should be noticed that, by construction,
symmetric (because the inner product is commutative) and a where

**L** is a rational

*n*×

*n* lower triangular matrix with ones

*definite positive matrix*. This second property can be verified in the diagonal (i.e., is a unit matrix) and

**D** is a

*n*×

*n* diagonal

from the squared Euclidean norm of

**y** (using the Hermitian matrix with rational diagonal entries

Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)
number of squares of

*R* rational numbers, that is,
Using traditional singular value decomposition (SVD)
An exact expansion of these

*djj* can be accomplished by
applying a naive greedy algorithm as proposed for the first
time by Lenstra in [11]. Imposing an exact expansion for each
often leads to a large number of terms in the sum where is a diagonal matrix and
(10) and for that reason Lenstra also proposed the use of a matrices (i.e., norm-preserving rotations, and therefore
randomized algorithm given in [23], which assures the preserves the statistics of the noise term). A SVD-based bound
scheme implies one SVD decomposition (requiring at least
This paper proposes to replace an exact conversion from
flops [22]) at the Rx in addition to matrix
by an approximate conversion (leading to a
using a fixed complexity algorithm that can be applied in a requiring
flops). This technique achieves capacity
real time communication system. Based on the results in [23], through

*water filling* power allocation (according to the
we use a simple greedy algorithm for the expansion and singular values) [26]. truncate the number of terms to
In [27] it was shown that the

**LDL***T* decomposition

*truncated Lenstra algorithm*. One way of achieving this is by achieves better performance than standard SVD, while being
using

*R* equal terms in (10). One may notice that when

*R*=1, slightly less complex. Most importantly, that new approach
the algorithm resorts to an approximated Cholesky takes advantage of having a precoding matrix with
zero elements. In fact it requires a in the
One starts by constructing a

*tall matrix*
rows and

*n* columns. For the case with

*R*=4 terms for each of triangular matrix is fedback from the R
This paper proposes a technique that achieves the same
performance as [27] while removing most of the complexity
from the receiver side to the transmitter side, which is an
important feature in scenarios where the transmitter is a base
station and the receiver terminal should be made as simple as
possible, i.e., by avoiding expensive processing.
where each row has one and only one non-zero entry. Now,

*Remark*: as in [27], this paper is not assessing the
one can re-construct the diagonal matrix from using
capacity-achieving regime and thus, for simplicity, uniform
power is allocated to the transmit antennas.
Both this proposal and [27] have a pre-processing stage at
We have made this matrix multiplication explicit to Rx consisting of the generation of a definite positive matrix
emphasise how this ensures that each

*djj* is a sum of squares

**G,** the Gram matrix of the lattice defined by the columns of

as defined in (10). Finally, the approximated generator
. This Gram matrix is formed by the left multiplication (3)
In this proposal it is imperative CSIT so that Tx can
construct the precoding matrix . This paper shows that his
can be achieved with the feedback of a lower triangular
matrix only. Given the symmetry of , the Tx only needs to
reconstruct the entire matrix, achieving the same bandwidth
savings seen in [27] for the feedback channel. After
The complexity of this technique is cubic in the reconstructing the entire (by symmetry), the Tx can use
dimension

*n*, due to the

**LDL***T* steps, to which we should add the truncated Lenstra algorithm described in Section IV to

for the rational approximation steps. The overall obtain an equivalent generator matrix (Section II) for the
, however, as it will be shown lattice. This matrix,
later in Table 1, as the

**LDL***T* decomposition will be reused, equivalent generator matrix for the lattice, holding the same

only the

*n* term will add to the complexity of the technique Gram matrix. However, it is possible to obtain from them
the same and unique generator matrix resulting from QR
It should be noticed that, unlike Cholesky decomposition, decompositions remembering that a QR decomposition is
this technique is applicable to both square and non-square unique when imposing the positiveness of elements in the
matrixes, allowing us to retrieve a rectangular from .
Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)

independent transmission channels and

*ii)* geometrically it
corresponds to a communication problem over a rectangular
lead to and , which would be the same matrix (up signs lattice. Thus, it is convenient to think of a lattice as the result
in main diagonal) if there was no distortion associated with
of a linear transformation of the cubic lattice
in the truncated Lenstra algorithm. A central aspect is that
the same Gram matrix is also obtainable from alone,
alone or even a mixture of both, given their closeness:
orthogonal generating vectors which span a rectangular
lattice (this lattice would even have the simplest trellis
representation one may have for lattices, e.g., [16]). The
As indicated in Section II.B and in Section IV, would performance increase can be geometrically interpreted from
have a

**LDL***T* decomposition, which can be calculated not this insight. A rectangular lattice is obtained from a

only given

(in both cases applying (3)) but also deformation of a cubic lattice
, or both. However, this will not be the dimension according to each
matrix to be

**LDL***T* decomposed.

rectangles and thus even a zero-forcing detector experiences
Taking advantage of these facts the mode in (7) and be no performance penalty.
The right multiplication of the channel matrix by
(17) changes the power at the transmitter. The geometric
interpretation is also useful on this matter. The “row lattice”
The matrixes used in (18) will be presented and justified
on the fallowing. First, notice that this made a matrix
to appear (a permutation matrix may be needed
to have a unique QR), which, despite not
being the Gram matrix of the underlying lattice, it is the
(approximate) Gram matrix of the lattice spanned by the row
Subsequently, one also needs the insertion of a diagonal
lattice (as indicated in Section II). This matrix also has an

**LDL***T* decomposition and thus (18) can equivalently be scaling

at the precoding stage so that the volume of
both lattices underlying the transmission scheme becomes
Figure 1 depicts the overall transmission scheme that is
proposed while in Figure 2 and in Figure 3 one can observe
in detail the processing required respectively at the T
Finally, after the detection filter at the receiver, the entire
x as well as the fluxes of CSI between both of them.
At the Rx it is important to highlight that there are two
parallel processing occurring at different stages and each one
associated with a different fading block:

*i)* obtain

**G** that will

be sent back to Tx in the form of a triangular matrix and

*ii)*
construct the receive filter from a received strictly upper
triangular matrix and

**Q**. In fact,

triangular but also a unit lower triangular (ones in the
It should be noticed that, as

**Q** is orthogonal (unitary if diagonal). This saves the transmission of the diagonal and

further simplifies the computations at the receiver. The R
be forwarded to the Rx. These coefficients are denoted by
. The process is summarised in Algorithm 1. (The
will then just have to apply the filtering
to the channel is assumed to remain unchanged between adjacent
incoming precoded signal, i.e., the unavoidable filtering symbols as it is common in the slow fading assumption.)
multiplications that are present in all detectors. Besides that,
the Rx only needs to compute

**G** and (given its symmetry)

send back to the Tx only the lower or the upper parts of

**G**,

which will be denoted by

are computed and sent from the Tx to the Rx.
The resulting transmission chain (20) can be interpreted in
two ways:

*i)* algebraically it corresponds to a set of
Figure 1: Proposed closed loop transmission scheme.
Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)

coefficients flowing in both the uplink and downlink. The
number of operations in Table 1 is presented in a way that
shows the contribution of each individual processing stage to
the total number of operations of the Rx or Tx (matrix
complexity at the receiver comes from a QR decomposition
and two matrix multiplications: one to initially obtain
(similar to [27]) and then the unavoidable filtering
Figure 2: Processing at the transmitter.
multiplication by . One should remember that this last
multiplication is common to all types of receivers in both
To access the approximation one first computes the error
matrix of the Gram matrix involved (i.e., the Gram matrix
associated with the “row lattice”, as indicated in Section V)
5: Obtain approximate from using the algorithm
and one applies to it the squared

*Frobenius matrix norm* [22]
7: Compute the Gram matrix of the “row lattice” at Tx:
Figure 4 shows the distribution of this error for three
example cases having the number of real dimensions most
common in MIMO wireless communications (and with
(note: for a non-squared the non-zero rows must be variance 0.5 per real component).
Notice that despite the Gram matrix of the “row lattice”
10: Strictly lower triangular matrix is sent:
and the one of “column lattice” being different, they hold the
statistics and consequently they are interchangeable in (3).
(note: can be computed at the same time as steps 4-10)
For a

*NT *=4,

*NR *=4 configuration (i.e.,

*n*=8 dimensions)
under a Rayleigh fading channel and using 16-QAM
modulation, it was verified that with the

**LDL***T*
multiplies the received decomposition proposed in this paper the error shown in
Figure 4 leads to a negligible performance penalty in terms
of symbol error rate (SER) in respect to the results presented
The number of flops required by the

**LDL***T* decomposition in [27] for the same configuration when using the same

, which is half of the number of flops needed in minimum mean squared error (MMSE) receiver.
Gaussian elimination, the number of flops of QR
This paper shows how to reconstruct (with very low
more efficient algorithms for matrix multiplication). Table 1 distortion and fixed complexity) a generator matrix of a
contains a comparison of the proposed technique with SVD lattice from one given Gram matrix of the same lattice for
and with [27] in terms of the number of flops and number of non-square matrixes. This opens new possibilities in several
problems of engineering and computer science that rely on
Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)
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Presented at the Int. Symposium on Communications, Control and Signal Processing, Limassol, Cyprus, March 2010. (IEEE Signal Processing Society.)

Source: http://iul.pt/~frmo/ISCCSP_2010_revised.pdf

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