Rohit Budhiraja

Precoder design for asymmetric multi-user two-way AF relaying in cellular systems

Two-way relaying reduces the loss in spectral efficiency caused in a conventional half-duplex relay due to two channel uses per data unit transmitted to the destination. Two-way relaying is possible when two nodes exchange data simultaneously through a relay. In the case of cellular systems, data exchange between base station (BS) and users (UE) is usually not symmetric, e.g., a user (UE1) might have uplink data to transmit during multiple access (MAC) phase, but might not have downlink data to receive during broadcast (BC) phase. This asymmetry in data exchange will reduce the gains of two-way relaying. In the case of infrastructure relays, where there are multiple users communicating through a relay, we propose that the BC phase following the MAC phase of UE1 be used by the relay to transmit downlink data to a second user (UE2). Conventional two-way relaying with symmetric MAC and BC phases must now be modified to asymmetric MAC (BS → RS ← UE1) and BC phases (BS ← RS → UE2), respectively. This will result in UE2 not being able to cancel the back-propagating interference in the usual way. We design precoders using conventional zero-forcing and linear minimum-mean-square-error criteria to mitigate the back-propagating interference at UE2 for an amplify-and-forward (AF) relay. We also propose a novel precoder appropriate for the asymmetric two-way relaying. The sum-rate performance of the proposed precoder is shown to be better than the conventional precoders.

Linear Precoders for Nonregenerative Asymmetric Two-Way Relaying in Cellular Systems

In conventional two-way relaying (TWR), it is assumed that a user has data to send and receive simultaneously from the base station (BS) via a relay. In cellular systems, data flow between the BS and a user is usually not simultaneous, e.g., a transmit-only user (say, TUE) may have uplink data to send in the multiple access (MAC) phase, but may not have downlink data to receive in the broadcast (BC) phase. Such one-way data flow will reduce TWR to spectrally inefficient one-way relaying. The multiple-input-multiple-output (MIMO) asymmetric TWR (ATWR) protocol considered here restores the two-way data flow via a relay. In ATWR, the BC phase following the MAC phase of a TUE is used to send downlink data to a receive-only user (say, RUE). However, the RUE will not be able to cancel the back-propagating interference. We design a structured precoder at the relay to cancel this interference. The proposed precoder also triangularizes the end-to-end MIMO channels. The channel triangularization reduces the weighted sum-rate maximization and relay power minimization problems to power allocation problems, which are then cast as geometric programs. Simulation results illustrate the effectiveness of the proposed precoder when compared with conventional solutions.

Joint Precoder and Receiver Design for AF Non-Simultaneous Two-Way MIMO Relaying

We investigate a joint design of linear precoders and receivers for multiple-input multiple-output non-simultaneous two-way relaying (NS-TWR). Unlike conventional two-way relaying, the base station in NS-TWR performs two-way relaying with two different users-a transmit-only user and a receive-only user (RUE). The RUE experiences back-propagating interference (BI). The proposed design cancels this BI and provides beamforming gain over existing designs. For NS-TWR, we maximize the weighted sum-rate (WSR) through joint power allocation, by solving a sequence of geometric programs. The precoder and the receiver designs as well as the power allocation program are then extended for a multi-user system with multiple transmit-only and receive-only users. With exhaustive simulations, we show that the proposed design provides significantly better WSR than the existing ones. The proposed design is also evaluated in a cellular framework using realistic path loss models, to assess the system-level performance gain achievable.

Transceiver Design for Nonconcurrent Two-Way MIMO AF Relaying with QoS Guarantees

We consider a cellular system with amplify-and-forward (AF) nonconcurrent two-way relaying (ncTWR), where a base station serves a transmit-only user and a receive-only user. Most of the state-of-the-art transceiver designs for AF multiple-input multiple-output (MIMO) ncTWR optimize a system-wide objective function subject to the transmit power constraints. Transceiver designs that incorporate quality-of-service (QoS) constraints are not well investigated in ncTWR literature. In this paper, we design a MIMO AF transceiver that maximizes weighted sum-rate (WSR) while guaranteeing the QoS constraints that are cast as per-stream rate required by two ncTWR users. The WSR maximization is a nonconvex problem due to its nonconvex objective. We solve this problem by separately approximating the objective at low and high signal-to-noise ratios (SNRs), with each approximation cast as a geometric program. With extensive numerical evaluations, we first demonstrate the improved performance of the proposed transceiver over existing designs without QoS constraints. We later investigate the effect of QoS constraints on the system WSR.

Multiuser Two-Way Nonregenerative MIMO Relaying With Nonconcurrent Traffic

Multiuser two-way relaying (MWR) enables multiple users to exchange data with a base station (BS) via a half-duplex relay in two channel uses. MWR assumes a specific traffic pattern, where users concurrently transmit/receive data to/from the BS. In this paper, we design a multiple-input-multiple-output (MIMO) two-way relaying protocol for a nonconcurrent traffic scenario, where one set of users transmits data to the BS in the first channel use, whereas another set of users receives data from the BS in the second channel use. By designing a novel linear precoder at the nonregenerative relay and by applying dirty-paper coding (DPC) at the BS, we maximize the: 1) minimum rate among all users; and 2) weighted sum rate (WSR) of the system, via the formulation of two geometric programs. Simulation results demonstrate the improved performance of the proposed precoder over conventional ones.

Precoder design for asymmetric two-way AF shared relay

Two-way relaying (TWR) reduces the loss in spectral efficiency caused in a conventional half-duplex relay. TWR is possible when two nodes exchange data simultaneously through a relay. In the case of cellular systems, data exchange between base station (BS) and users is usually not symmetric, e.g., a user might have uplink data to transmit during multiple access (MAC) phase, but might not have downlink data to receive during broadcast (BC) phase. This asymmetry in data exchange will reduce the gains of TWR. With infrastructure relays, where multiple users communicate through a relay, the BC phase following the MAC phase of a transmitting user (UE1) can be used by the relay to transmit downlink data to a second user (UE2). This will result in the receiving user UE2 not being able to cancel the back-propagating interference in the usual way. Precoders are designed in [1] to mitigate the back-propagating interference at UE2 for an amplify-and-forward (AF) relay. The present work studies the asymmetric data-flow problem for a shared AF relay, wherein multiple BS and users communicate using a common relay with multiple antennas. In this case, UE2 will observe inter-user interference (IUI) in addition to the back-propagating interference. Also, BS will now observe the IUI. We propose a precoder to jointly mitigate the back-propagating interference for UE2 and IUI for BS and UE2. It is shown that the sum-rate performance is better for the proposed precoder than the conventional zero-forcing precoder.

Joint Transceiver Design for QoS-Constrained MIMO Two-Way Non-Regenerative Relaying Using Geometric Programming

Transceiver designs for multiple-input multiple-output (MIMO) two-way relaying are being actively explored. Most of the state-of-the-art studies optimize a system-wide objective function subject to the transmit power constraints on the two source nodes and the relay. Transceiver designs with quality-of-service (QoS) constraints have lacked attention in two-way relaying literature. In this paper, we study a MIMO transceiver design, based on the generalized singular value decomposition, that allocates power at the source and relay nodes to optimize the following per-stream rate-constrained objectives: 1) network transmit power, and 2) sum-rate. In addition, we also maximize the rate of the transmit stream with the worst signal-to-noise ratio. Through extensive numerical evaluations, we demonstrate the superior performance of proposed design over the existing ones, not only with QoS constraints but also without them.

PAPR analysis of superimposed training based SISO/MIMO-OFDM systems with orthogonal affine precoder

Abstract Superimposed training (ST) is used to estimate channel in orthogonal frequency division multiplexing (OFDM) systems which results in higher throughput than the time/frequency multiplexed training schemes. Orthogonal affine precoders (OAP) are used in ST-based SISO/MIMO-OFDM system to cancel the interference caused due to simultaneous data transmission. We analyze the peak to average power ratio (PAPR) of SISO/MIMO-OFDM systems which use OAP-based ST scheme. We show that the OAP-based ST scheme has higher PAPR than the conventional OFDM (without OAP-based ST) systems. We design an OAP using Zadoff–Chu (ZC) sequence, which has lower PAPR than the conventional OAPs. We further reduce the PAPR of the ST systems using selective mapping (SLM), but by increasing the system complexity. We show that the PAPR of the ST-based SISO/MIMO-OFDM systems with ZC-based OAP and SLM is identical to that of the conventional OFDM using SLM.

Improved Rate-Energy Tradeoff for Energy Harvesting Interference Alignment Networks

Energy harvesting (EH) from RF signals is being investigated to ensure the perpetual operation of energy-constrained wireless devices. Interference alignment (IA) achieves maximum degrees of freedom (DoF) in a

Two-way MIMO DF relaying for non-simultaneous traffic in cellular systems

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