10 research outputs found
Degrees of Freedom and Achievable Rate of Wide-Band Multi-cell Multiple Access Channels With No CSIT
This paper considers a -cell multiple access channel with inter-symbol
interference. The primary finding of this paper is that, without instantaneous
channel state information at the transmitters (CSIT), the sum
degrees-of-freedom (DoF) of the considered channel is
with when the number of users per cell is sufficiently large,
where is the ratio of the maximum channel-impulse-response (CIR) length
of desired links to that of interfering links in each cell. Our finding implies
that even without instantaneous CSIT, \textit{interference-free DoF per cell}
is achievable as approaches infinity with a sufficiently large number
of users per cell. This achievability is shown by a blind interference
management method that exploits the relativity in delay spreads between desired
and interfering links. In this method, all inter-cell-interference signals are
aligned to the same direction by using a discrete-Fourier-transform-based
precoding with cyclic prefix that only depends on the number of CIR taps. Using
this method, we also characterize the achievable sum rate of the considered
channel, in a closed-form expression.Comment: Submitted to IEEE Transactions on Communication
Degrees of Freedom Region of the MIMO Interference Channel with Output Feedback and Delayed CSIT
The two-user multiple-input multiple-output (MIMO) interference channel (IC)
with arbitrary number of antennas at each terminal is considered and the
degrees of freedom (DoF) region is characterized in the presence of noiseless
channel output feedback from each receiver to its respective transmitter and
availability of delayed channel state information at the transmitters (CSIT).
It is shown that having output feedback and delayed CSIT can strictly enlarge
the DoF region of the MIMO IC when compared to the case in which only delayed
CSIT is present. The proposed coding schemes that achieve the corresponding DoF
region with feedback and delayed CSIT utilize both resources, i.e., feedback
and delayed CSIT in a non-trivial manner. It is also shown that the DoF region
with local feedback and delayed CSIT is equal to the DoF region with global
feedback and delayed CSIT, i.e., local feedback and delayed CSIT is equivalent
to global feedback and delayed CSIT from the perspective of the degrees of
freedom region. The converse is proved for a stronger setting in which the
channels to the two receivers need not be statistically equivalent.Comment: Accepted for publication in IEEE Transactions on Information Theor
On the Degrees of Freedom of -User SISO Interference and X Channels with Delayed CSIT
The -user single-input single-output (SISO) AWGN interference channel and
SISO AWGN X channel are considered where the transmitters have the
delayed channel state information (CSI) through noiseless feedback links.
Multi-phase transmission schemes are proposed for both channels which possess
novel ingredients, namely, multi-phase partial interference nulling,
distributed interference management via user scheduling, and distributed
higher-order symbol generation. The achieved degrees of freedom (DoF) values
are greater than the best previously known DoFs for both channels with delayed
CSI at transmitters.Comment: 30 pages, 6 figures, 2 tables; Published in IEEE Transactions on
Information Theory, Oct. 201
Degrees of freedom of wireless interference network
Wireless communication systems are different from the wired systems mainly in three aspects: fading, broadcast, and superposition. Wireless communication networks, and multi-user communication networks in general, have not been well understood from the information-theoretic perspective: the capacity limits of many multi-user networks are not known. For example, the capacity region of a two-user single-antenna interference channel is still not known, though recent result can bound the region up to a constant value. Characterizing the capacity limits of multi-user multiple-input multiple-output (MIMO) interference network is usually even more difficult than the single antenna setup.
To alleviate the difficulty in studying such networks, the concept of degrees of freedom (DoF) has been adopted, which captures the first order behavior of the capacities or capacity regions. One important technique developed recently for quantifying the DoF of multi-user networks is the so-called interference alignment. The purpose of interference alignment is to design the transmit signals structurally so that the interference signals from multiple interferers are aligned to reduce the signal dimensions occupied by interference.
In this thesis, we mainly study two problems related to DoF and interference alignment: 1) DoF region of MIMO full interference channel (FIC) and Z interference channel (ZIC) with reconfigurable antennas, and 2) the DoF region of an interference network with general message demands.
For the first problem, we derive the outer bound on the DoF region and show that it is achievable via time-sharing or beamforming except for one special case. As to this particular special case, we develop a systematic way of constructing the DoF-achieving nulling and beamforming matrices. Our results reveal the potential benefit of using the reconfigurable antenna in MIMO FIC and ZIC. In addition, the achievability scheme has an interesting space-frequency interpretation.
For the second problem, we derive the DoF region of a single antenna interference network with general message demands, which includes the multiple unicasts and multiple multicasts as special cases. We perform interference alignment using multiple base vectors and align the interference at each receiver to its largest interferer. Furthermore, we show that the DoF region is determined by a subset of receivers, and the DoF region can be achieved by considering a smaller number of interference alignment constraints so as to reduce the number of time expansion.
Finally, as a related research topic, we also include a result on the average throughput of a MIMO interference channel with single-user detector at receivers and without channel state information at transmitters. We present a piecewise linear approximation of the channel throughput under weak, moderate and strong interference regimes. Based on that we determine the optimal number of streams that a transmitter should use for different interference levels