K Giridhar

An iterative MIMO-DFE receiver with MLD for uplink SC-FDMA

Single carrier frequency division multiple access (SC-FDMA) based systems such as long term evolution (LTE) uplink experience severe inter symbol interference (ISI) in highly frequency selective channels. The conventional linear receivers such as the linear minimum mean square error(LMMSE) based receiver fail to address this issue, thereby degrading the performance of the system. This paper proposes a new iterative multiple input multiple output decision feedback equalizer (MIMO-DFE) which does not require the estimation of any parameters for canceling ISI. It also derives a closed form expression for the residual noise covariance after equalization which is used for maximum likelihood detection (MLD) across streams. Simulations show that the proposed receiver yields significant performance gains especially in the high signal-to-noise-ratio (SNR) regime with only a modest increase in computational complexity.

Precoder design for Fractional Interference Alignment

In this work, we consider a K user Interference Channel (IC), where all the transmitters employ Finite Alphabet (FA) signals. With M antennas at both transmitter and receiver nodes, the Interference Alignment (IA) scheme [1] was shown to provide 1/2 SpAC (Symbols transmitted per transmit Antenna per Channel use) per user, whereas, the Fractional Interference Alignment (FIA) scheme [2] provides a value of SpAC in the range [0,1]. When FA signals are used, the IA technique which uses a fixed value of SpAC = 1/2 is sub-optimal, and therefore will not provide the best possible rate and/or bit error rate (BER). Instead, the FIA scheme explicitly computes the maximum SpAC that can be supported for the interference channel model (assuming that accurate knowledge of the channel state information is available for the desired and the interfering signals). Hence, based on two different optimization criteria, namely (a) Coding Gain (CG) maximization, and (b) Goodput (GP) maximization, we use a simple conjugate gradient search approach to compute the sub-optimal SpAC value and the corresponding precoders for the FIA scheme. Numerical results show that for a 3 user IC, even at finite signal-to-interference ratio (SIR), (M1)/M SpAC (upper bound in FIA scheme) is achieved, as signal-to-noise ratio (SNR) tends to infinity. The BER performance is also compared between CG and GP maximization problems. Finally, the BER performance of these two optimization schemes are compared with the IA scheme, where it is shown that the proposed FIA with CG criterion outperforms the IA scheme, and will provide 0.7 dB SNR gain for the same rate.

Precoder Design for K-User Interference Channels with Finite Alphabet Signals

We consider a MIMO K-user interference channel with finite alphabet signaling, and design an optimal precoder for maximizing the sum mutual information between all transmitter-receiver pairs. Perfect channel knowledge is assumed to be available at all the transmitters, but, decoding of the interfering signals is not done at any receiver. A gradient descent algorithm is employed for maximizing the sum mutual information. Simulation results for the three user interference channel show that when compared to conventional schemes which maximize the signal-to-interference plus noise ratio or minimize the mean squared error, the proposed design achieves a significantly better bit error rate performance. Along with the proposed precoder, the minimum distance detector which decodes the desired signal (while treating interference as colored Gaussian noise with a known covariance) was utilized at all the receivers, in order to get this enhanced performance.

Capacity improvement for finite-input constellation using unitary precoding for two user channels

In [1], it was shown that introduction of a relative phase shift between the transmitted constellations of both the users in a two user Gaussian Multiple Access Channel (GMAC) improves the capacity region when finite input constellation is used for both the users. This idea was also shown to improve the capacity region of the two user Gaussian Interference Channel (GIC) under the strong interference assumption, with finite constellation [2]. In the current work, we extend the idea of introducing the phase shift to the two symbol case. We show that for both, GMAC and GIC with strong interference, if one user precodes the signal with a 2×2 unitary matrix, then the capacity improves when compared to [1] and [2]. In particular, it is shown that the entries of the unitary precoder have to be complex in order to improve the capacity region. Using simulations, the entries of the unitary precoder for certain ranges of SNR are computed.

On Improving the BLER for ML Receivers in Block Faded Channels through Random Phase Rotation

In this work, we apply the idea of random phase rotation of the modulated symbols to a system with interfering transmitters and a Maximum-likelihood (ML) receiver. Expressions for Bit Error Rate (BER) for a given channel realization and the average Block Error Rate (BLER) for an error correcting code are derived for the Hard-Decision based ML receiver (HDML). The derived expressions are verified through simulation results and it is observed that at high signal to noise ratio (SNR), when the channel is quasi-static (block fading), random phase rotation provides a gain in BLER when compared to the case without random phase rotation. Also it is observed that the BLER for a system with random phase rotation approaches the BLER for the case of no co-channel interference. The gain due to random phase rotation is also verified for an LTE framework with Soft-Decision based ML receiver (SD-ML).

Null-space of block convolution matrix

A novel, efficient, non-iterative algorithm to find the null-space of the block convolution matrix which gives rise to a particular block banded block Toeplitz matrix (BBTM) is developed. This BBTM structure arises in the context of a multiple input multiple output (MIMO) (Nr receivers and Nt transmitters) orthogonal frequency division multiplexing (OFDM) model where the effective L taps channel impulse response length is to be shortened using a N tap channel shortening prefilter. This computationally efficient algorithm to find the null-space is derived from standard Gaussian elimination and exploits the structure of the MIMO-OFDM channel matrix. When compared to standard Gaussian elimination with partial pivoting which has a complexity of O(N3) for a L-tap channel the proposed algorithm has a run-time complexity of only O(N).

Robust LLR aided low feedback precoding for interference alignment

Interference alignment, as proposed in [1], is a joint multi-base-station precoding technique where each interfering base-station determines its precoder so as to obtain aligned interfering signals at each user. Determination of such precoders requires complete channel state information of all the interfering links to be present at all the base-stations. In this paper, we present a modified form of interference alignment based on locally-available channel state information at the user equipment. We consider the downlink of a wireless system where basestations and user equipments are equiped with two antennas each and scheduling of users is done based on Fractional Frequency Reuse. Assuming an MMSE receiver at the user equipment, an expression for the optimal precoder to be used by the interfering base-stations is derived so that there is zero interference after MMSE equalization. We also give an example for the case where there is uncancelled interference despite the use of these precoders. The effect of this uncancelled interference is overcome by the use of a robust LLR developed in [2]. Simulation results that demonstrate the performance of the proposed technique are presented.