22-11-2012, 02:20 PM
Link Adaptation for Spatial Modulation with Limited Feedback
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Abstract
In this paper, link adaptation schemes,
where the transmit parameters are dynamically adapted
to the changing channel conditions, are developed
for spatial modulation (SM) transmission systems. The
proposed schemes are based on the adaptive modulation
(AM) and transmit mode switching (TMS)
techniques, so as to further exploit the spatial freedom
of the MIMO channel. More specifically, an optimal
hybrid-SM (OH-SM) scheme that jointly uses both AM
and TMS techniques is first developed to efficiently
utilize the channel resources. In OH-SM, the transmit
mode is adapted jointly with modulation orders
according to the channel condition. Moreover, a suboptimal
hybrid SM scheme, termed as concatenated-
SM (C-SM) scheme, is also proposed. The C-SM
scheme exploits the multi-stage adaptation strategy to
balance the tradeoff between computational complexity
and performance. The performance of the proposed
schemes is evaluated by using the nearest neighbor
approximations, in order to derive the parameter selection
criterions. Simulation results show that under
the same spectral efficiency, the proposed schemes
provide considerable system performance improvement
compared to the conventional SM systems, especially
in correlated channel conditions.
Introduction
Spatial modulation (SM), which exploits the antennaactivation
process as a means of conveying information
bits, is a promising transmission technique for multipleinput
multiple-output (MIMO) channels [1]-[3]. In SM
schemes, only one transmit antenna is active at one time
instant so that the inter-channel interference (ICI) and
inter-antenna synchronization (IAS) in conventional MIMO
systems are efficiently avoided [1]. As the transmit
antenna index carries additional information besides the
conventional signal modulation, the spectral efficiency is
also enhanced in the SM systems. Moreover, another
advantage of SM is that it efficiently works in any configuration
of transmit and receive antennas [2].
System Overview
In this section we briefly review the system model of
the SM scheme [1]. At the SM transmitter, information
bits are mapped with the aid of two different operations:
the transmit antenna activation and the conventional
modulation scheme. Let C denote the field of complex
numbers. Then, the transmitted SM symbol x 2 CNt×1
is given as x = sleq [1], where sl is the complex-valued
symbol of the conventional modulation scheme employed,
such as M-QAM or M-PSK, which is associated with
log2(M) input bits, while eq 2 CNt×1(1 q Nt) is
selected from the Nt-dimensions standard basis vectors
(i.e., e1 = [1, 0, · · · , 0] ), according to log2(Nt) input bits.
Thus a total of log2(NtM) source bits are mapped to each
SM symbol.
Simulation results
In this section, we evaluate the performance of the
proposed link-adaptation schemes over the conventional
SM and ASM schemes. The simulation setup is based
on 3 and 4 bits/s/Hz transmissions over MIMO channels
with frequency-flat block Rayleigh fading. In all cases
we assume that four transmit antennas (Nall = 4) and
two receive antennas (Nr = 2) are available. In addition,
the results are compared under both uncorrelated and
correlated channel conditions. To be specific, we consider a
spatial correlated channel model [14] in which the MIMO
channel matrix is modeled by Hcorr = R1/2
r HR1/2
t , where
Rt = [rij ]Nall×Nall and Rr = [rij ]Nr×Nr are the positive
definite Hermitian matrices that specify the transmit and
receive correlations, respectively. In the simulations, we
assume that the elements of these matrices are obtained
from the exponential correlation model [14], i.e., their
components are calculated as rij = r
ji = rj−i for i j
where r is the correlation coefficient (0 r 1).
Moreover, the receiver applies the optimal ML detector
[2] to estimate the active transmitter indexes and the
transmitted symbols. In the proposed schemes, the number
of the estimated bits may not equal to the number of the
transmitted bits. In this case, the received bit vector may
need to be padded with zeros or some of the bits need to
be discarded.
Conclusion
In this paper, we have proposed link adaptation schemes
to obtain better system performance for limited feedback
SM transmission system under a target spectral efficiency.
The proposed schemes explore different degrees of MIMO
channels and achieve different tradeoffs among system
performance, complexity and feedback load. It has been
shown that the achieved performance is quite attractive,
especially in correlated channel conditions. In addition,
although we have adopted different system modes in hybrid
adaptive SM schemes, the number of required RF
chains switching between different transmit antennas is
only one for an arbitrary number of transmit antennas.
Our future work will be focused on the integration of trellis
coding [16], space-time block coding [17] and space-time
shift keying [7] into the proposed schemes.