22-09-2012, 12:41 PM
Some Aspects of Interleave Division Multiple Access in Ad Hoc
Networks
1Some Aspects.pdf (Size: 213.54 KB / Downloads: 17)
Abstract
We consider some aspects of interleave division multiple access (IDMA) in ad hoc networks. IDMA is a multiple
access technique which relies on an iterative multiuser detection and it has a close relation to CDMA. Traditionally,
the medium access control protocol sees the physical layer as a collision model, while in literature the potential of
multiuser detection techniques in ad hoc networks has been discussed. Higher efficiency can be achieved if a certain
amount of interference is allowed due to the multiuser detection. In this paper we propose a simple complexity
reduction technique for IDMA and a design of multiple user-distinct interleavers. The complexity reduction technique
is aimed to support a variety of receivers with different capabilities of handling interference. It allows to reduce the
complexity significantly while graceful performance degradation can be realized. The proposed multiple interleavers
are designed so as to minimize memory requirements and signaling overhead by deriving multiple interleavers from
one common interleaver.
Introduction
CDMA is a strong candidate to achieve high spectral
efficiency along with high power efficiency. In
order to obtain the potential benefits from CDMA,
iterative multiuser detection techniques based on the
turbo principle have been intensively studied in the
last years, e.g. [1]. Iterative multiuser detection techniques
comprise a multiuser detector (MUD) and a
bank of user-independent a posteriori probability (APP)
decoders. Both MUD and APP decoder are soft-in softout
(SISO) blocks which exchange soft information.
Multiple access interference (MAI) as well as inter
symbol interference (ISI) is iteratively mitigated.
CDMA
In [1], iterative multiuser detection technique for
CDMA based on the instantaneous minimum mean
square error (MMSE) filter has been proposed. This
technique is developed for BPSK modulation. We extend
it for higher order modulation like QAM/PSK
constellations. There are two steps to compute the
conditional probability P(y|s(k)
n = si).
Complexity Reduction Strategy
The complexity of the MUD for IDMA is relatively
low as we saw in the previous section. The complexity
of the decoder, on the other hand, can be quite high for
a large number of users because of the K independent
decoders. The decoding task for IDMA is two fold as
shown in Fig. 2: decoding the repetition code and the
convolutional code, where the former is far simpler than
the latter. We propose to limit the exploitation of the
decoding of convolutional code aiming at complexity
reduction. If we do not decode the convolutional code,
there will be still some improvements due to the decoding
of the repetition code. In case of CDMA, however,
no performance improvement can be obtained without
decoding the convolutional code. In the following, we
will show that our simple method is beneficial for
IDMA in the two different switching strategies.
User-Wise Switching Strategy
In ad hoc networks, not all network nodes are likely
to afford the full complexity of the multiuser detection.
Therefore, some form of complexity reduction
should be considered. Here, we propose to decode
the convolutional code for a limited number of users.
Computer simulations are performed to evaluate our
scheme. Nb = 128 information bits are encoded by the
rate Rc = 1/2 memory 4 standard [23, 35]8 convolutional
code. The trellis is terminated with 4 additional
termination bits. For CDMA the code bits are interleaved
by the random interleaver, mapped on the BPSK
symbols, and then spread by the Nu = 4 spreading
code that is constructed from the OVSF codes and the
UMTS uplink long scrambling sequence as specified
in [12]. For IDMA the convolutionally coded bits are
further encoded by the rate Rr = 1/4 repetition code,
interleaved, and then mapped on the BPSK symbols.
Both schemes have the same bandwidth-efficiency.
Iteration-Wise Switching Strategy
It is well known that the interleaver size must be large
enough for the iterative processing to be effective while
it is always limited in practice. Fig. 4 shows the BER
performance of IDMA on an AWGN channel for different
sizes of interleaver. The coding parameters are the
same as in Fig. 3. The code bits are QPSK modulated.
K = 8 users are synchronous. On the left figure the
performance is plotted when the convolutional code is
always decoded for all users. It can be seen that the
performance improves as the frame size gets larger. It
converges to the single user bound at Eb/N0 = 6 dB
for Nb = 4096. Plotted on the right is the performance
when only the repetition code is decoded in the first 4
iterations, then the convolutional code is also decoded
in the rest of iterations in addition to the repetition code.
Surprisingly, the performance already converges to the
single user bound at Eb/N0 = 6 dB for Nb = 1024.
Multiple Interleavers
IDMA requires user-specific interleaver. Each node,
which applies multiuser detection, needs to know all
interleavers that results in a significant signaling overhead
in ad hoc networks. There have been very few
studies on multiple interleavers. In [7], [8], the authors
proposed empirical criteria of goodness of a set of interleavers
for convolutionally coded CDMA systems. One
of the criteria is to choose congruential interleavers
(see [7]) for a given code having certain free distance
such that the interleaved codes have good asymptotic
distances. This approach requires an intensive search
for a large number of users. Another criterion in [7]
is ‘an iterative decoding suitability measure’ that takes
into account correlations of L-values, but it is defined
only for the case of two users. Moreover, no deterministic
method to find good interleavers is provided.
Concluding Remarks
In this paper we proposed a simple complexity reduction
technique for IDMA and a design of multiple userdistinct
interleavers. We believe that these aspects are
important to bring the potential of IDMA into ad hoc
networks while keeping in mind that not all receivers
are likely to afford the full complexity. There are still
many aspects which need studies that include: synchronization
of nodes to some extent, channel estimations,
and a design of MAC protocol for network nodes with
different capabilities of interference handling.