30 research outputs found
Block-Fading Channels with Delayed CSIT at Finite Blocklength
In many wireless systems, the channel state information at the transmitter
(CSIT) can not be learned until after a transmission has taken place and is
thereby outdated. In this paper, we study the benefits of delayed CSIT on a
block-fading channel at finite blocklength. First, the achievable rates of a
family of codes that allows the number of codewords to expand during
transmission, based on delayed CSIT, are characterized. A fixed-length and a
variable-length characterization of the rates are provided using the dependency
testing bound and the variable-length setting introduced by Polyanskiy et al.
Next, a communication protocol based on codes with expandable message space is
put forth, and numerically, it is shown that higher rates are achievable
compared to coding strategies that do not benefit from delayed CSIT.Comment: Extended version of a paper submitted to ISIT'1
Robust Beamforming and Rate-Splitting Design for Next Generation Ultra-Reliable and Low-Latency Communications
The next generation ultra-reliable and low-latency communications (xURLLC)
need novel design to provide satisfactory services to the emerging
mission-critical applications. To improve the spectrum efficiency and enhance
the robustness of xURLLC, this paper proposes a robust beamforming and
rate-splitting design in the finite blocklength (FBL) regime for downlink
multi-user multi-antenna xURLLC systems. In the design, adaptive rate-splitting
is introduced to flexibly handle the complex inter-user interference and thus
improve the spectrum efficiency. Taking the imperfection of the channel state
information at the transmitter (CSIT) into consideration, a max-min user rate
problem is formulated to optimize the common and private beamforming vectors
and the rate-splitting vector under the premise of ensuring the requirements of
transmission latency and reliability of all the users. The optimization problem
is intractable due to the non-convexity of the constraint set and the infinite
constraints caused by CSIT uncertainties. To solve it, we convert the infinite
constraints into finite ones by the S-Procedure method and transform the
original problem into a difference of convex (DC) programming. A constrained
concave convex procedure (CCCP) and the Gaussian randomization based iterative
algorithm is proposed to obtain a local minimum. Simulation results confirm the
convergence, robustness and effectiveness of the proposed robust beamforming
and rate-splitting design in the FBL regime. It is also shown that the proposed
robust design achieves considerable performance gain in the worst user rate
compared with existing transmission schemes under various blocklength and block
error rate requirements.Comment: 12 pages, 9 figure
Generalized HARQ Protocols with Delayed Channel State Information and Average Latency Constraints
In many wireless systems, the signal-to-interference-and-noise ratio that is
applicable to a certain transmission, referred to as channel state information
(CSI), can only be learned after the transmission has taken place and is
thereby delayed (outdated). In such systems, hybrid automatic repeat request
(HARQ) protocols are often used to achieve high throughput with low latency.
This paper put forth the family of expandable message space (EMS) protocols
that generalize the HARQ protocol and allow for rate adaptation based on
delayed CSI at the transmitter (CSIT). Assuming a block-fading channel, the
proposed EMS protocols are analyzed using dynamic programming. When full
delayed CSIT is available and there is a constraint on the average decoding
time, it is shown that the optimal zero outage EMS protocol has a particularly
simple operational interpretation and that the throughput is identical to that
of the backtrack retransmission request (BRQ) protocol. We also devise EMS
protocols for the case in which CSIT is only available through a finite number
of feedback messages. The numerical results demonstrate that the throughput of
BRQ approaches the ergodic capacity quickly compared to HARQ, while EMS
protocols with only three and four feedback messages achieve throughputs that
are only slightly worse than that of BRQ.Comment: 19 pages, 5 figure
Ergodic Interference Alignment
This paper develops a new communication strategy, ergodic interference
alignment, for the K-user interference channel with time-varying fading. At any
particular time, each receiver will see a superposition of the transmitted
signals plus noise. The standard approach to such a scenario results in each
transmitter-receiver pair achieving a rate proportional to 1/K its
interference-free ergodic capacity. However, given two well-chosen time
indices, the channel coefficients from interfering users can be made to exactly
cancel. By adding up these two observations, each receiver can obtain its
desired signal without any interference. If the channel gains have independent,
uniform phases, this technique allows each user to achieve at least 1/2 its
interference-free ergodic capacity at any signal-to-noise ratio. Prior
interference alignment techniques were only able to attain this performance as
the signal-to-noise ratio tended to infinity. Extensions are given for the case
where each receiver wants a message from more than one transmitter as well as
the "X channel" case (with two receivers) where each transmitter has an
independent message for each receiver. Finally, it is shown how to generalize
this strategy beyond Gaussian channel models. For a class of finite field
interference channels, this approach yields the ergodic capacity region.Comment: 16 pages, 6 figure, To appear in IEEE Transactions on Information
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