Balachander Narasimhan

Digital Compensation of Frequency-Dependent Joint Tx/Rx I/Q Imbalance in OFDM Systems Under High Mobility

Direct conversion orthogonal frequency division multiplexing (OFDM) systems suffer from transmit and receive analog processing impairments such as in-phase/quadrature (I/Q) imbalance causing inter-carrier interference (ICI) among sub-carriers. Another source of performance-limiting ICI in OFDM systems is Doppler spread due to mobility. However, the nature of ICI due to each of them is quite different. Unlike previous work which considered these two impairments separately, we develop a unified mathematical framework to characterize, estimate, and jointly mitigate ICI due to I/Q imbalance and high mobility. Based on our general model, we derive a closed-form expression for the degradation in signal-to-interference-plus-noise ratio (SINR) due to the impairments. Moreover, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for joint transmit/receive frequency-independent (FI) and frequency-dependent (FD) I/Q imbalances under high-mobility conditions.

Reduced-complexity baseband compensation of joint Tx/Rx I/Q imbalance in mobile MIMO-OFDM

Direct-conversion multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) transceivers enjoy high data rates and reliability at practical implementation complexity. However, analog front-end impairments such as I/Q imbalance and high mobility requirements of next-generation broadband wireless standards result in performance-limiting inter-carrier interference (ICI). In this paper, we study the effects of ICI due to these impairments for OFDM with space frequency block codes and spatial multiplexing, derive a generalized linear model and propose a non-iterative reduced-complexity digital baseband joint compensation scheme. Furthermore, we present a pilot scheme for joint estimation of the channel and the I/Q imbalance parameters and evaluate its performance through simulations. Our proposed scheme is effective in estimating and compensating for frequency-independent and frequency-dependent transmit and receive I/Q imbalances even in the presence of a residual frequency offset.

Baseband estimation and compensation of joint TX/RX I/Q imbalance in SC-FDE transceivers

Previous work [1] on I/Q imbalance in SC-FDE transceivers investigates the effect of receiver I/Q imbalance but does not consider Doppler effects due to mobility. SCFDE in doubly-selective channels is studied in [2] and [3] but without considering I/Q imbalance. In this paper, we derive an equivalent channel model for SC-FDE in the presence of both joint Tx/Rx I/Q imbalance and mobility and present an effective low-complexity digital baseband compensation scheme. Furthermore, we propose a novel joint channel and I/Q imbalance parameter estimation scheme for SC-FDE transceivers.

Digital Baseband Compensation of I/Q Imbalance in Mobile OFDM

I/Q imbalance and high mobility in OFDM systems result in performance-limiting intercarrier interference (ICI). However, the nature of ICI due to each of them is quite different. Unlike previous works which considered these two impairments separately, we develop a unified mathematical framework to characterize and mitigate ICI when both impairments are present. In addition, we exploit the special ICI structure to design efficient OFDM channel estimation and digital baseband compensation schemes for I/Q imbalance under high-mobility conditions.

SFBC design tradeoffs for mobile SC-FDMA with application to LTE-advanced

Single-carrier frequency-division multiple-access (SC-FDMA) has been adopted in the uplink of the LTE standard due to its lower peak-to-average-power ratio (PAPR) compared to orthogonal frequency-division multiple access (OFDMA). Recent activities in the LTE-advanced (LTE-A) standardization have focused on effective uplink transmit diversity schemes with low PAPR [1] but without considering the performance degradation due to Doppler despite the fact that LTE-A is required to support high mobility. In this paper, we present an open-loop space-frequency block coding (SFBC) scheme for the SC-FDMA uplink and demonstrate its robustness to high Doppler and large multipath delay spread while enjoying full spatial diversity, low PAPR and practical decoding complexity by a suitable design of the frequency span of each SFBC codeword. In this paper, we study the design tradeoffs involved in SFBC design for mobile SC-FDMA.

Training sequence design for joint channel and I/Q imbalance parameter estimation in mobile SC-FDE transceivers

In our previous work [1], digital baseband compensation of joint Tx/Rx I/Q imbalance in mobile SC-FDE transceivers was investigated. In this sequel, the training sequence design for joint channel and I/Q imbalance parameter estimation in mobile SC-FDE transceivers is discussed in greater detail.

Reduced-Complexity ICI Mitigation for Mobile SFBC-OFDM with Application to DVB-H

Effective inter-carrier interference (ICI) mitigation for MIMO-OFDM requires accurate channel estimation which is very challenging due to the large number and fast time-varying nature of the channel parameters to be estimated using scattered pilots. We present a novel SFBC-OFDM scheme for doubly-selective channels and a reduced complexity channel estimation algorithm which exploits the banded and sparse structure of the channel in the frequency and time-domains, respectively. Furthermore, we design an FIR-MMSE ICI cancellation algorithm for mobile SFBC-OFDM and demonstrate its effectiveness for digital video broadcasting-handheld (DVB-H) systems.

Digital Baseband Compensation for Mobile SFBC-OFDM Systems with Receiver I/Q Imbalance

In-phase and quadrature-phase (I/Q) imbalance is a major performance-limiting impairment for direct- conversion orthogonal frequency division multiplexing (OFDM) transceivers. I/Q imbalance degrades the signal-to-noise-ratio in an OFDM system by causing inter-channel-interference (ICI) between image subcarriers. Doppler spread due to mobility also causes ICI but mainly between adjacent subcarriers. In this work, we investigate the combined effects of mobility and I/Q imbalance in a multiple-input-multiple-output-OFDM system with the alamouti space-frequency block-coding scheme and design an effective compensation technique. Furthermore, we propose a novel pilot allocation and channel estimation scheme, and exploit the problem structure to reduce complexity.

Digital baseband compensation of joint TX/RX frequency-dependent I/Q imbalance in mobile MIMO-OFDM transceivers

Direct-conversion Orthogonal Frequency Division Multiplexing (OFDM) systems suffer from transmit and receive analog processing impairments such as In-phase/ Quadrature (I/Q) imbalance causing Inter-Carrier Interference (ICI) between the sub-carriers. Another source of performance-limiting ICI, but with a different nature, in OFDM systems is Doppler spread due to mobility. Unlike previous work which considered these two problems separately, we develop a generalized analytical framework to characterize, estimate and jointly mitigate ICI due to both I/Q imbalance and high mobility. Based on our general model, we exploit the special ICI structure to design efficient channel and I/Q imbalance parameter estimation and digital baseband compensation schemes for joint transmit/receive frequency-independent and frequency-dependent I/Q imbalance under high-mobility conditions. Moreover, we extend the model, compensation and channel estimation methods to the Multiple Input Multiple Output (MIMO) case, Spatial Multiplexing (SM) in particular.

Digital Compensation of Tx/Rx I/Q Imbalance in TD-SCDMA Systems

TD-SCDMA (Time Division- Synchronous CDMA) systems show performance degradation due to I/Q imbalance resulting from direct-conversion transmitters and receivers. In overall system analysis, it is important to consider both transmitter and receiver I/Q imbalances to account for low cost base stations (BSs) such as femtocells. Two approaches are considered in this paper. The first one is a purely time-domain approach based on widely linear minimum mean square error estimation (WL-MMSE). The second one is a frequency-domain approach with reduced computational complexity. Simulation studies are performed and comparison results presented.