417 research outputs found
The Capacity of the Gaussian Cooperative Two-user Multiple Access Channel to within a Constant Gap
The capacity region of the cooperative two-user Multiple Access Channel (MAC)
in Gaussian noise is determined to within a constant gap for both the
Full-Duplex (FD) and Half-Duplex (HD) case. The main contributions are: (a) for
both FD and HD: unilateral cooperation suffices to achieve capacity to within a
constant gap where only the user with the strongest link to the destination
needs to engage in cooperation, (b) for both FD and HD: backward joint decoding
is not necessary to achieve capacity to within a constant gap, and (c) for HD:
time sharing between the case where the two users do not cooperate and the case
where the user with the strongest link to the destination acts as pure relay
for the other user suffices to achieve capacity to within a constant gap. These
findings show that simple achievable strategies are approximately optimal for
all channel parameters with interesting implications for practical cooperative
schemes.Comment: Submitted to the 2013 IEEE International Conference on Communications
(ICC 2013
On the Benefits of Partial Channel State Information for Repetition Protocols in Block Fading Channels
This paper studies the throughput performance of HARQ (hybrid automatic
repeat request) protocols over block fading Gaussian channels. It proposes new
protocols that use the available feedback bit(s) not only to request a
retransmission, but also to inform the transmitter about the instantaneous
channel quality. An explicit protocol construction is given for any number of
retransmissions and any number of feedback bits. The novel protocol is shown to
simultaneously realize the gains of HARQ and of power control with partial CSI
(channel state information). Remarkable throughput improvements are shown,
especially at low and moderate SNR (signal to noise ratio), with respect to
protocols that use the feedback bits for retransmission request only. In
particular, for the case of a single retransmission and a single feedback bit,
it is shown that the repetition is not needed at low \snr where the
throughput improvement is due to power control only. On the other hand, at high
SNR, the repetition is useful and the performance gain comes form a combination
of power control and ability of make up for deep fades.Comment: Accepted for publication on IEEE Transactions on Information Theory;
Presented in parts at ITW 2007 and ICC 200
The Strongly Asynchronous Massive Access Channel
This paper considers a Strongly Asynchronous and Slotted Massive Access
Channel (SAS-MAC) where different users transmit a randomly
selected message among ones within a strong asynchronous window
of length blocks, where each block lasts channel uses. A
global probability of error is enforced, ensuring that all the users'
identities and messages are correctly identified and decoded. Achievability
bounds are derived for the case that different users have similar channels, the
case that users' channels can be chosen from a set which has polynomially many
elements in the blocklength , and the case with no restriction on the users'
channels. A general converse bound on the capacity region and a converse bound
on the maximum growth rate of the number of users are derived.Comment: under submissio
On Discrete Alphabets for the Two-user Gaussian Interference Channel with One Receiver Lacking Knowledge of the Interfering Codebook
In multi-user information theory it is often assumed that every node in the
network possesses all codebooks used in the network. This assumption is however
impractical in distributed ad-hoc and cognitive networks. This work considers
the two- user Gaussian Interference Channel with one Oblivious Receiver
(G-IC-OR), i.e., one receiver lacks knowledge of the interfering cookbook while
the other receiver knows both codebooks. We ask whether, and if so how much,
the channel capacity of the G-IC- OR is reduced compared to that of the
classical G-IC where both receivers know all codebooks. Intuitively, the
oblivious receiver should not be able to jointly decode its intended message
along with the unintended interfering message whose codebook is unavailable. We
demonstrate that in strong and very strong interference, where joint decoding
is capacity achieving for the classical G-IC, lack of codebook knowledge does
not reduce performance in terms of generalized degrees of freedom (gDoF).
Moreover, we show that the sum-capacity of the symmetric G-IC- OR is to within
O(log(log(SNR))) of that of the classical G-IC. The key novelty of the proposed
achievable scheme is the use of a discrete input alphabet for the non-oblivious
transmitter, whose cardinality is appropriately chosen as a function of SNR
- …