Akbar Ghasemi

Interference alignment for the K user MIMO interference channel

We consider the K user Multiple Input Multiple Output (MIMO) Gaussian interference channel with M antennas at each transmitter and N antennas at each receiver. It is assumed that channel coefficients are fixed and are available at all transmitters and at all receivers. The main objective of this paper is to characterize the total Degrees Of Freedom (DOF) for this channel. Using a new interference alignment technique which has been recently introduced in [1], we show that MN over M+N K degrees of freedom can be achieved for almost all channel realizations. Also, a new upper-bound on the total DOF for this channel is derived. This upper-bound coincides with our achievable DOF for K ≥ Ku ≜ M+N over gcd(M,N) where gcd(M,N) denotes the greatest common divisor of M and N. This gives an exact characterization of DOF for MIMO Gaussian interference channel in the case of K ≥ Ku.

On the degrees of freedom of MIMO X channel with delayed CSIT

The multiple-input multiple-output (MIMO) Gaussian X channel in i.i.d. fading environment and with delayed channel state information at transmitters (delayed CSIT) is considered. It is assumed that each transmitter has M antennas and each receiver has N antennas. New achievable results on the sum degrees of freedom (DoF) of this channel are provided and shown to be tight for all possible values of M and N except for 1/2 <; N/M <; 4/3. It is noteworthy that for certain values of M and N, the channel DoF coincides with the DoF of the broadcast channel obtained by assuming perfect transmitter cooperation.

On the Degrees of Freedom of

The K-user single-input single-output (SISO) additive white Gaussian noise (AWGN) interference channel and 2×K SISO AWGN X channel are considered, where the transmitters have delayed channel state information (CSI) through noiseless feedback links. Multiphase transmission schemes are proposed for both channels which possess novel ingredients, namely, multiphase partial interference nulling, distributed interference management via user scheduling, and distributed higher order symbol generation. The achieved degree-of-freedom (DoF) values are greater than the best previously known DoFs for both channels with delayed CSI at the transmitters.

K

We investigate the three-user MIMO Gaussian broadcast channel with i.i.d. fading and the same number of antennas at each receiver, and with the delayed channel state information at the transmitter (CSIT). We obtain achievability results on the degrees of freedom (DoF) of this channel and also show that our achievable DoF is tight for some ranges of transmit-receive antenna ratio. It is observed that when the number of antennas at the transmitter is strictly greater than that at each receiver, the DoF with delayed CSIT lies strictly between the DoF with perfect CSIT and DoF with no CSIT.

-User SISO Interference and X Channels With Delayed CSIT

The SISO (single-input single-output) AWGN interference and X channels in i.i.d. fading environment are considered where the transmitters have the past channel state information (CSI) through noiseless feedback links. New transmission schemes are proposed for these channels that achieve degrees of freedom (DoF) values greater than one (except for two-user interference channel). The achieved DoFs are strictly increasing with the number of users and asymptotically approach limiting values of ≈ 1.2663 and ≈ 1.4427 for interference and X channels, respectively. The achieved DoFs are greater than the best previously reported DoFs for these channels with delayed CSI at transmitters.

On the degrees of freedom of three-user MIMO broadcast channel with delayed CSIT

In this letter, we study a two-user Gaussian Interference Channel (GIC) with constellation-based transmitters where both transmitters utilize PSK signaling and both receivers treat interference as noise. It is verified that if one user rotates its constellation appropriately compared to the constellation of the other user, the achievable sum-rate in the network considerably increases. Due to the fact that the noise plus interference at each receiver is mixed-Gaussian, there is no closed formula for the sum-rate. Using Fanos inequality, a lower bound is developed on the sum-rate in the network. The lower bound is tight in the sense that it identifies the optimal value for the angle of rotation. Moreover, it is demonstrated that the proposed lower bound can be larger than the exact value of sum-rate achieved by random Gaussian codes or Time-Division-Multiplexing (TDM).

On the degrees of freedom of SISO interference and X channels with delayed CSIT

The Gaussian K-user interference and M × K X channels are investigated with no instantaneous channel state information at transmitters (CSIT). First, it is assumed that the CSI is fed back to all nodes after a finite delay (delayed CSIT), and furthermore, the transmitters operate in full-duplex mode, i.e, they can transmit and receive simultaneously. Achievable results on the degrees of freedom (DoFs) of these channels under the above assumption are obtained. It is observed that, in contrast with no CSIT and full CSIT models, when CSIT is delayed, the achievable DoFs for both channels with the full-duplex transmitter cooperation are greater than the available achievable results on their DoF without transmitter cooperation. Then, K-user interference and K × K X channels are considered with output feedback, wherein the channel output of each receiver is causally fed back to its corresponding transmitter. Our achievable results with output feedback demonstrate strict DoF improvements over those with the full-duplex delayed CSIT when K 5 in the K-user interference channel and K > 2 in the K × K X channel. Next, the combination of delayed CSIT and output feedback, known as Shannon feedback, is studied and strictly higher DoFs compared with the output feedback model are achieved in the K-user interference channel when K = 5 or K 6, and in the K × K X channel when K > 2.

On Gaussian Interference Channels with Constellation-Based Transmitters

The Gaussian single-input single-output (SISO) K-user interference and M × K X channels are investigated in i.i.d. fading environment with no instantaneous channel state information (CSI) at transmitters. First, it is assumed that the CSI is fed back to all nodes after some delay (delayed CSIT), and furthermore, the transmitters operate in full-duplex mode. Achievable results on the degrees of freedom (DoF) of these channels under the above assumption are obtained. Then, achievable DoFs are obtained for the K-user interference and K × K X channels with output feedback and also Shannon feedback, which is a combination of output feedback and delayed CSIT, and compared with the achievable DoFs under the full-duplex delayed CSIT assumption.

Interference and X Networks With Noisy Cooperation and Feedback

This paper addresses the following question regarding Gaussian networks: Is there an alternative to decoding interference or treating interference as Gaussian noise? By answering this question we aim to establish a benchmark for practical systems where multiuser decoding is not a common practice. To state our result, we study a decentralized network of one Primary User (PU) and one Secondary User (SU) modeled by a two-user Gaussian interference channel. The primary transmitter is constellation-based, i.e., PU is equipped with a modulator and its code-book is constructed over a modulation signal set. SU utilizes random Gaussian codewords with controlled transmission power that guarantees a certain level of Interference-to-Noise Ratio (INR) at the primary receiver. Both users are unaware of each others code-book, however, SU is smart in the sense that it is aware of the constellation set of PU. While interference at the primary receiver is modeled as additive Gaussian noise, the secondary receiver can utilize the structure of PUs modulator as side information to decode its message without decoding the message of PU. The instantaneous realizations of symbols in a codeword transmitted by PU are unknown to both ends of SUs direct link, however, the sample space of such symbols is available to SU. This makes the interference plus noise at the secondary receiver be a mixed Gaussian process. Invoking entropy power inequality and an upper bound on the differential entropy of a mixed Gaussian vector, we develop an achievable rate for SU that is robust to the structure of PUs modulation signal set and only depends on its constellation size and the dimension of the euclidean space that the constellation points lie in. Moreover, we obtain an achievable rate for PU using Fanos inequality in conjunction with a Gallager-type upper bound on the probability of error in decoding constellation points at the primary receiver. The developed achievable rates for PU and SU enable us to show that the sum rate can be improved compared to a scenario where both users employ Gaussian codewords and treat each other as Gaussian noise.

Full-duplex transmitter cooperation, feedback, and the degrees of freedom of SISO Gaussian interference and X channels

We consider a K user Gaussian multiple access system with impulse radio ultra-wideband signaling. The receiver is assumed to treat the Multi-Access Interference (MAI) as additive noise and does not enjoy multiuser detection. It is well known that for such a system the MAI is neither Gaussian nor white. Although, the non-Gaussianity of the MAI has been extensively studied, little attention has been directed to the non-whiteness of it. In this paper, we show that by employing a new front-end filter at the receiver, the non-whiteness of the MAI can be exploited to improve the performance of the single-user detector. We use the signal-to-interference-plus-noise ratio (SINR) as a merit to evaluate the performance improvement brought to the system. The SINR improvement depends on the interference level as well as pulse waveform. The bit error rate of the system is also simulated and compared with that of conventional matched filter receiver.