23 research outputs found
Nested Lattice Codes for Gaussian Relay Networks with Interference
In this paper, a class of relay networks is considered. We assume that, at a
node, outgoing channels to its neighbors are orthogonal, while incoming signals
from neighbors can interfere with each other. We are interested in the
multicast capacity of these networks. As a subclass, we first focus on Gaussian
relay networks with interference and find an achievable rate using a lattice
coding scheme. It is shown that there is a constant gap between our achievable
rate and the information theoretic cut-set bound. This is similar to the recent
result by Avestimehr, Diggavi, and Tse, who showed such an approximate
characterization of the capacity of general Gaussian relay networks. However,
our achievability uses a structured code instead of a random one. Using the
same idea used in the Gaussian case, we also consider linear finite-field
symmetric networks with interference and characterize the capacity using a
linear coding scheme.Comment: 23 pages, 5 figures, submitted to IEEE Transactions on Information
Theor
Lattice Coding for the Two-way Two-relay Channel
Lattice coding techniques may be used to derive achievable rate regions which
outperform known independent, identically distributed (i.i.d.) random codes in
multi-source relay networks and in particular the two-way relay channel. Gains
stem from the ability to decode the sum of codewords (or messages) using
lattice codes at higher rates than possible with i.i.d. random codes. Here we
develop a novel lattice coding scheme for the Two-way Two-relay Channel: 1
2 3 4, where Node 1 and 4 simultaneously communicate with each other
through two relay nodes 2 and 3. Each node only communicates with its
neighboring nodes. The key technical contribution is the lattice-based
achievability strategy, where each relay is able to remove the noise while
decoding the sum of several signals in a Block Markov strategy and then
re-encode the signal into another lattice codeword using the so-called
"Re-distribution Transform". This allows nodes further down the line to again
decode sums of lattice codewords. This transform is central to improving the
achievable rates, and ensures that the messages traveling in each of the two
directions fully utilize the relay's power, even under asymmetric channel
conditions. All decoders are lattice decoders and only a single nested lattice
codebook pair is needed. The symmetric rate achieved by the proposed lattice
coding scheme is within 0.5 log 3 bit/Hz/s of the symmetric rate capacity.Comment: submitted to IEEE Transactions on Information Theory on December 3,
201
A Novel User Pairing Scheme for Functional Decode-and-Forward Multi-way Relay Network
In this paper, we consider a functional decode and forward (FDF) multi-way
relay network (MWRN) where a common user facilitates each user in the network
to obtain messages from all other users. We propose a novel user pairing
scheme, which is based on the principle of selecting a common user with the
best average channel gain. This allows the user with the best channel
conditions to contribute to the overall system performance. Assuming lattice
code based transmissions, we derive upper bounds on the average common rate and
the average sum rate with the proposed pairing scheme. Considering M-ary
quadrature amplitude modulation with square constellation as a special case of
lattice code transmission, we derive asymptotic average symbol error rate (SER)
of the MWRN. We show that in terms of the achievable rates, the proposed
pairing scheme outperforms the existing pairing schemes under a wide range of
channel scenarios. The proposed pairing scheme also has lower average SER
compared to existing schemes. We show that overall, the MWRN performance with
the proposed pairing scheme is more robust, compared to existing pairing
schemes, especially under worst case channel conditions when majority of users
have poor average channel gains.Comment: 30 pages, 6 figures, submitted for journal publicatio
Lattice Codes for Many-to-One Interference Channels With and Without Cognitive Messages
A new achievable rate region is given for the Gaussian cognitive many-to-one
interference channel. The proposed novel coding scheme is based on the
compute-and-forward approach with lattice codes. Using the idea of decoding
sums of codewords, our scheme improves considerably upon the conventional
coding schemes which treat interference as noise or decode messages
simultaneously. Our strategy also extends directly to the usual many-to-one
interference channels without cognitive messages. Comparing to the usual
compute-and-forward scheme where a fixed lattice is used for the code
construction, the novel scheme employs scaled lattices and also encompasses key
ingredients of the existing schemes for the cognitive interference channel.
With this new component, our scheme achieves a larger rate region in general.
For some symmetric channel settings, new constant gap or capacity results are
established, which are independent of the number of users in the system.Comment: To appear in IEEE Transactions on Information Theor