496 research outputs found
On BICM receivers for TCM transmission
Recent results have shown that the performance of bit-interleaved coded
modulation (BICM) using convolutional codes in nonfading channels can be
significantly improved when the interleaver takes a trivial form (BICM-T),
i.e., when it does not interleave the bits at all. In this paper, we give a
formal explanation for these results and show that BICM-T is in fact the
combination of a TCM transmitter and a BICM receiver. To predict the
performance of BICM-T, a new type of distance spectrum for convolutional codes
is introduced, analytical bounds based on this spectrum are developed, and
asymptotic approximations are also presented. It is shown that the minimum
distance of the code is not the relevant optimization criterion for BICM-T.
Optimal convolutional codes for different constrain lengths are tabulated and
asymptotic gains of about 2 dB are obtained. These gains are found to be the
same as those obtained by Ungerboeck's one-dimensional trellis coded modulation
(1D-TCM), and therefore, in nonfading channels, BICM-T is shown to be
asymptotically as good as 1D-TCM.Comment: Submitted to the IEEE Transactions on Communication
Bit-Interleaved Coded Modulation Revisited: A Mismatched Decoding Perspective
We revisit the information-theoretic analysis of bit-interleaved coded
modulation (BICM) by modeling the BICM decoder as a mismatched decoder. The
mismatched decoding model is well-defined for finite, yet arbitrary, block
lengths, and naturally captures the channel memory among the bits belonging to
the same symbol. We give two independent proofs of the achievability of the
BICM capacity calculated by Caire et al. where BICM was modeled as a set of
independent parallel binary-input channels whose output is the bitwise
log-likelihood ratio. Our first achievability proof uses typical sequences, and
shows that due to the random coding construction, the interleaver is not
required. The second proof is based on the random coding error exponents with
mismatched decoding, where the largest achievable rate is the generalized
mutual information. We show that the generalized mutual information of the
mismatched decoder coincides with the infinite-interleaver BICM capacity. We
also show that the error exponent -and hence the cutoff rate- of the BICM
mismatched decoder is upper bounded by that of coded modulation and may thus be
lower than in the infinite-interleaved model. We also consider the mutual
information appearing in the analysis of iterative decoding of BICM with EXIT
charts. We show that the corresponding symbol metric has knowledge of the
transmitted symbol and the EXIT mutual information admits a representation as a
pseudo-generalized mutual information, which is in general not achievable. A
different symbol decoding metric, for which the extrinsic side information
refers to the hypothesized symbol, induces a generalized mutual information
lower than the coded modulation capacity.Comment: submitted to the IEEE Transactions on Information Theory. Conference
version in 2008 IEEE International Symposium on Information Theory, Toronto,
Canada, July 200
Towards Fully Optimized BICM Transceivers
Bit-interleaved coded modulation (BICM) transceivers often use equally spaced
constellations and a random interleaver. In this paper, we propose a new BICM
design, which considers hierarchical (nonequally spaced) constellations, a
bit-level multiplexer, and multiple interleavers. It is shown that this new
scheme increases the degrees of freedom that can be exploited in order to
improve its performance. Analytical bounds on the bit error rate (BER) of the
system in terms of the constellation parameters and the multiplexing rules are
developed for the additive white Gaussian Noise (AWGN) and Nakagami- fading
channels. These bounds are then used to design the BICM transceiver. Numerical
results show that, compared to conventional BICM designs, and for a target BER
of , gains up to 3 dB in the AWGN channel are obtained. For fading
channels, the gains depend on the fading parameter, and reach 2 dB for a target
BER of and .Comment: Submitted to the IEEE Transactions on Communication
On the Information Loss of the Max-Log Approximation in BICM Systems
We present a comprehensive study of the information rate loss of the max-log
approximation for -ary pulse-amplitude modulation (PAM) in a bit-interleaved
coded modulation (BICM) system. It is widely assumed that the calculation of
L-values using the max-log approximation leads to an information loss. We prove
that this assumption is correct for all -PAM constellations and labelings
with the exception of a symmetric 4-PAM constellation labeled with a Gray code.
We also show that for max-log L-values, the BICM generalized mutual information
(GMI), which is an achievable rate for a standard BICM decoder, is too
pessimistic. In particular, it is proved that the so-called "harmonized" GMI,
which can be seen as the sum of bit-level GMIs, is achievable without any
modifications to the decoder. We then study how bit-level channel
symmetrization and mixing affect the mutual information (MI) and the GMI for
max-log L-values. Our results show that these operations, which are often used
when analyzing BICM systems, preserve the GMI. However, this is not necessarily
the case when the MI is considered. Necessary and sufficient conditions under
which these operations preserve the MI are provided
Signal Shaping for BICM at Low SNR
The mutual information of bit-interleaved coded modulation (BICM) systems,
sometimes called the BICM capacity, is investigated at low signal-to-noise
ratio (SNR), i.e., in the wideband regime. A new linear transform that depends
on bits' probabilities is introduced. This transform is used to prove the
asymptotical equivalence between certain BICM systems with uniform and
nonuniform input distributions. Using known results for BICM systems with a
uniform input distribution, we completely characterize the combinations of
input alphabet, input distribution, and binary labeling that achieve the
Shannon limit -1.59 dB. The main conclusion is that a BICM system achieves the
Shannon limit at low SNR if and only if it can be represented as a zero-mean
linear projection of a hypercube, which is the same condition as for uniform
input distributions. Hence, probabilistic shaping offers no extra degrees of
freedom to optimize the low-SNR mutual information of BICM systems, in addition
to what is provided by geometrical shaping. These analytical conclusions are
confirmed by numerical results, which also show that for a fixed input
alphabet, probabilistic shaping of BICM can improve the mutual information in
the low and medium SNR range over any coded modulation system with a uniform
input distribution
On the Convergence Speed of Turbo Demodulation with Turbo Decoding
Iterative processing is widely adopted nowadays in modern wireless receivers
for advanced channel codes like turbo and LDPC codes. Extension of this
principle with an additional iterative feedback loop to the demapping function
has proven to provide substantial error performance gain. However, the adoption
of iterative demodulation with turbo decoding is constrained by the additional
implied implementation complexity, heavily impacting latency and power
consumption. In this paper, we analyze the convergence speed of these combined
two iterative processes in order to determine the exact required number of
iterations at each level. Extrinsic information transfer (EXIT) charts are used
for a thorough analysis at different modulation orders and code rates. An
original iteration scheduling is proposed reducing two demapping iterations
with reasonable performance loss of less than 0.15 dB. Analyzing and
normalizing the computational and memory access complexity, which directly
impact latency and power consumption, demonstrates the considerable gains of
the proposed scheduling and the promising contributions of the proposed
analysis.Comment: Submitted to IEEE Transactions on Signal Processing on April 27, 201
Iterative Detection of Diagonal Block Space Time Trellis Codes, TCM and Reversible Variable Length Codes for Transmission over Rayleigh Fading Channels
Iterative detection of Diagonal Block Space Time Trellis Codes (DBSTTCs), Trellis Coded Modulation (TCM) and Reversible Variable Length Codes (RVLCs) is proposed. With the aid of efficient iterative decoding, the proposed scheme is capable of providing full transmit diversity and a near channel capacity performance. The performance of the proposed scheme was evaluated when communicating over uncorrelated Rayleigh fading channels. Explicitly, significant iteration gains were achieved by the proposed scheme, which was capable of performing within 2~dB from the channel capacity
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