280 research outputs found
Extended Generalized DoF Optimality Regime of Treating Interference as Noise in the X Channel
The simple scheme of treating interference as noise (TIN) is studied in this
paper for the 3 x 2 X channel. A new sum-capacity upper bound is derived. This
upper bound is transformed into a generalized degrees-of-freedom (GDoF) upper
bound, and is shown to coincide with the achievable GDoF of scheme that
combines TDMA and TIN for some conditions on the channel parameters. These
conditions specify a noisy interference regime which extends noisy interference
regimes available in literature. As a by-product, the sum-capacity of the 3 x 2
X channel is characterized within a constant gap in the given noisy
interference regime.Comment: 5 pages, 2 figure
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GDoF of the MISO BC: Bridging the gap between finite precision CSIT and perfect CSIT
This work bridges the gap between sharply contrasting results on the degrees of freedom of the K user broadcast channel where the transmitter is equipped with K transmit antennas and each of the K receivers is equipped with a single antenna. This channel has K DoF when channel state information at the transmitter (CSIT) is perfect, but as shown recently, it has only 1 DoF when the CSIT is limited to finite precision. By considering the full range of partial CSIT assumptions parameterized by β ⋯ [0,1], such that the strength of the channel estimation error terms scales as ∼ SNR-β relative to the channel strengths which scale as ∼ SNR, it is shown that this channel has 1 - β + Kβ DoF. For K = 2 users with arbitrary βij parameters, the DoF are shown to be 1 + mini,j βij. To explore diversity of channel strengths, the results are further extended to the symmetric Generalized Degrees of Freedom setting where the direct channel strengths scale as ∼ SNR and the cross channel strengths scale as ∼ SNRα, α ⋯ [0,1], β ⋯ [0,α]. Here, the roles of α and β are shown to counter each other on equal terms, so that the sum GDoF value in the K user setting is (α - β) + K(1 - (α-β )) and for the 2 user setting with arbitrary βij, is 2 - α + mini,j βij
A Rate-Splitting Strategy for Max-Min Fair Multigroup Multicasting
We consider the problem of transmit beamforming to multiple cochannel
multicast groups. The conventional approach is to beamform a designated data
stream to each group, while treating potential inter-group interference as
noise at the receivers. In overloaded systems where the number of transmit
antennas is insufficient to perform interference nulling, we show that
inter-group interference dominates at high SNRs, leading to a saturating
max-min fair performance. We propose a rather unconventional approach to cope
with this issue based on the concept of Rate-Splitting (RS). In particular,
part of the interference is broadcasted to all groups such that it is decoded
and canceled before the designated beams are decoded. We show that the RS
strategy achieves significant performance gains over the conventional
multigroup multicast beamforming strategy.Comment: accepted to the 17th IEEE International workshop on Signal Processing
advances in Wireless Communications (SPAWC 2016
Rate-Splitting for Max-Min Fair Multigroup Multicast Beamforming in Overloaded Systems
In this paper, we consider the problem of achieving max-min fairness amongst
multiple co-channel multicast groups through transmit beamforming. We
explicitly focus on overloaded scenarios in which the number of transmitting
antennas is insufficient to neutralize all inter-group interference. Such
scenarios are becoming increasingly relevant in the light of growing
low-latency content delivery demands, and also commonly appear in multibeam
satellite systems. We derive performance limits of classical beamforming
strategies using DoF analysis unveiling their limitations; for example, rates
saturate in overloaded scenarios due to inter-group interference. To tackle
interference, we propose a strategy based on degraded beamforming and
successive interference cancellation. While the degraded strategy resolves the
rate-saturation issue, this comes at a price of sacrificing all spatial
multiplexing gains. This motivates the development of a unifying strategy that
combines the benefits of the two previous strategies. We propose a beamforming
strategy based on rate-splitting (RS) which divides the messages intended to
each group into a degraded part and a designated part, and transmits a
superposition of both degraded and designated beamformed streams. The
superiority of the proposed strategy is demonstrated through DoF analysis.
Finally, we solve the RS beamforming design problem and demonstrate significant
performance gains through simulations
Generalized Degrees of Freedom of the Symmetric Cache-Aided MISO Broadcast Channel with Partial CSIT
We consider the cache-aided MISO broadcast channel (BC) in which a
multi-antenna transmitter serves single-antenna receivers, each equipped
with a cache memory. The transmitter has access to partial knowledge of the
channel state information. For a symmetric setting, in terms of channel
strength levels, partial channel knowledge levels and cache sizes, we
characterize the generalized degrees of freedom (GDoF) up to a constant
multiplicative factor. The achievability scheme exploits the interplay between
spatial multiplexing gains and coded-multicasting gain. On the other hand, a
cut-set-based argument in conjunction with a GDoF outer bound for a parallel
MISO BC under channel uncertainty are used for the converse. We further show
that the characterized order-optimal GDoF is also attained in a decentralized
setting, where no coordination is required for content placement in the caches.Comment: first revisio
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