Adnan Qamar Kiayani

Digital Suppression of Power Amplifier Spurious Emissions at Receiver Band in FDD Transceivers

As the duplexing distances in emerging wireless systems are getting more and more narrow, achieving sufficient isolation between transmit and receive chains using radio frequency (RF) filtering alone becomes increasingly complex. Particularly challenging problem in this context is the spectral regrowth of nonlinear power amplifiers (PAs) in the transmit chain, and other transmitter out-of-band (OOB) emissions, which can heavily desensitize the receiver chain. In this letter, we first carry out detailed modeling of transmitter OOB emissions due to practical wideband PAs with memory effects. Stemming from this modeling, and using the known digital transmit data inside the transceiver as reference, we then propose an efficient nonlinear digital cancellation technique to suppress the transmitter OOB emissions in the receiver path. The proposed technique is verified and analyzed using extensive computer simulations, rendering excellent suppression properties, hence enabling sufficient TX-RX isolation in frequency division duplexing (FDD) transceivers without any extra analog/RF filtering or PA linearization.

Modeling and dynamic cancellation of TX-RX leakage in FDD transceivers

Frequency division duplex (FDD) transceivers employing the direct-conversion radio architecture are known to suffer from transmitter-receiver signal leakage problems. The presence of such leakage signal can impose stringent linearity requirements in receiver components that are difficult to fulfill in practice. Good example is second-order intermodulation distortion (IM2) due to transmitter leakage signal, stemming from finite IIP2 of receiver mixers, falling directly on top of the weak received signal. This paper carries out detailed modeling and proposes a dynamic cancellation technique for such TX-RX leakage, enabling sufficient TX-RX isolation in FDD mode without any extra analog/RF filtering. The technique is based on creating a replica of the undesired transmitter leakage signal and subtracting it from the down-converted signal in the receiver path, taking also the essential transmitter nonidealities into account. Simulation results show that the proposed method is able to effectively push transmitter leakage induced IM2 below the receiver noise floor.

Channel Estimation and Equalization in Multiuser Uplink OFDMA and SC-FDMA Systems Under Transmitter RF Impairments

Single-carrier frequency-division multiple access (SC-FDMA), which is a modified form of orthogonal frequency-division multiple access (OFDMA), has been adopted as the uplink physical-layer radio access technique for the Third-Generation Partnership Project Long-Term Evolution (3GPP-LTE) and LTE-Advanced. Radio transceiver implementations for such OFDM-based systems with the direct-conversion architecture are desirable to enable small-size, low-cost, and low-power-consumption terminals. However, the associated circuit impairments stemming from the processing of analog radio frequency (RF) signals, such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset errors, can severely degrade the obtainable link performance. In this paper, we analyze the effects of these radio impairments in a multiuser SC-FDMA uplink system and present digital-signal-processing-based methods for the joint estimation and equalization of impairments and channel distortions on the receiver side with an arbitrary number of receiver antennas. For the equalization, linear equalizers such as the zero-forcing (ZF) and the minimum mean square error (MMSE) equalizers that utilize pairs of mirror subcarriers are formulated, and the MMSE equalizer is developed to effectively handle mirror subband users with different power levels. Furthermore, for reduced computational complexity, the joint channel and impairment filter responses are efficiently approximated with polynomial-based basis function models. The parameters of the basis functions are then estimated by exploiting the time-multiplexed reference symbols in the LTE uplink subframe structure. The performance of the proposed estimation and equalization methods is assessed with extensive multiuser link simulations, with both single-antenna and dual-antenna base-station receivers, and the results show that the proposed algorithms are able to significantly reduce the impact of channel distortions and radio impairments. The resulting receiver implementation with the proposed techniques enables improved uplink link performance, even when the mobile terminals fulfill their emission requirements, in terms of I/Q images, with no changes in the LTE standards frame and pilot structures.

Digital Mitigation of Transmitter-Induced Receiver Desensitization in Carrier Aggregation FDD Transceivers

Carrier aggregation transmissions in frequency division duplexing devices reduce the duplexing distance between the transmitter (TX) and receiver (RX) bands. As a consequence, the spurious intermodulation distortion products created by the nonlinear RF front-end of the TX may easily extend over to the RX band, potentially causing own RX desensitization. In this paper, we propose an efficient and computationally feasible adaptive digital identification and cancellation technique to mitigate the RX desensitization. We first show that the spurious leakage signal at own RX band depends on an equivalent leakage channel that models the overall signal leakage path including the TX nonlinearities, duplexer filter responses, and RX path. The parameters of the equivalent leakage channel can be efficiently estimated with the least squares or the recursive least squares algorithm, using the actual digital transmit data as a reference, and then used to regenerate and cancel the leakage interference from the received signal. The performance of the proposed technique is evaluated with extensive computer simulations, as well as with practical real-world RF measurements, demonstrating excellent calibration properties with up to 19-dB improvement in singal-to-interference plus noise ratio of the desired received signal.

Advanced receiver design for mitigating multiple RF impairments in OFDM systems: algorithms and RF measurements

Direct-conversion architecture-based orthogonal frequency division multiplexing (OFDM) systems are troubled by impairments such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset (CFO). These impairments are unavoidable in any practical implementation and severely degrade the obtainable link performance. In this contribution, we study the joint impact of frequency-selective I/Q imbalance at both transmitter and receiver together with channel distortions and CFO error. Two estimation and compensation structures based on different pilot patterns are proposed for coping with such impairments. The first structure is based on preamble pilot pattern while the second one assumes a sparse pilot pattern. The proposed estimation/compensation structures are able to separate the individual impairments, which are then compensated in the reverse order of their appearance at the receiver. We present time-domain estimation and compensation algorithms for receiver I/Q imbalance and CFO and propose low-complexity algorithms for the compensation of channel distortions and transmitter IQ imbalance. The performance of the compensation algorithms is investigated with computer simulations as well as with practical radio frequency (RF) measurements. The performance results indicate that the proposed techniques provide close to the ideal performance both in simulations and measurements.

Mobile transmitters I/Q imbalances in LTE uplink: Analysis and digital mitigation

The single-carrier frequency-division multiple access (SC-FDMA), also known as discrete Fourier transform (DFT)- spread OFDMA, has been adopted as uplink transmission scheme for the 3GPP Long Term Evolution (LTE). Such OFDM-based systems are known to be particularly sensitive to radio frequency (RF) impairments. The major sources of these RF impairments are in-phase and quadrature-phase imbalance (IQI) and carrier frequency offset (CFO). In this paper, we investigate the impact of radio impairments on the LTE uplink by mathematically formulating the received signal as a function of transmitted signals, IQI, CFO, and channel distortions. Then, two alternative joint equalization schemes are proposed for coping with such impairments, stemming from the minimum mean square error (MMSE) and the zero forcing (ZF) principles, respectively. LTE uplink computer simulations are then carried out to assess the validity and performance of the proposed equalizers. The results demonstrate that the proposed schemes significantly improve the uplink performance, the performance of the MMSE solution being very close to the zero RF impairments scenario.

Hybrid time/frequency domain compensator for RF impairments in OFDM systems

I/Q signal processing based communication systems suffer from analog front-end (FE) imperfections such as in-phase and quadrature-phase (I/Q) imbalance and carrier frequency offset (CFO). These impairments are commonly encountered in all practical implementations, and severely degrade the obtainable link performance. Moreover, orthogonal frequency division multiplexing (OFDM)-based systems are particularly sensitive to radio frequency (RF) impairments. In this paper, we analyze the impact of transmitter and receiver I/Q imbalance together with channel distortion and CFO error on an ideal transmit signal, and propose low-complexity DSP algorithms and compensation structure for coping with such imperfections. Based on our proposed estimation/compensation structure, we are able to decouple the impairments and process them individually with rather low-complexity. More specifically, we first apply a blind algorithm for receiver I/Q imbalance compensation, followed by an efficient time domain CFO estimator and compensator. The transmitter I/Q imbalance and channel are then equalized jointly, in the frequency domain, with maximum-likelihood (ML) or zero-forcing (ZF) schemes, respectively. The applied algorithms are either blind working without aid of any training symbol or use only one OFDM symbol for impairments estimation, providing an efficient alternative solution with reduced complexity. The computer simulation results indicate a close to ideal performance of ZF scheme, and suggest that additional performance improvement due to frequency diversity can be obtained when ML estimation technique is employed.

Active RF cancellation of nonlinear TX leakage in FDD transceivers

In frequency division duplex (FDD) transceivers, transmitter (TX) passband leakage induced self-interference can cause own receiver (RX) desensitization, due to limited isolation of the duplexer RX filter and nonlinear components in the RX front-end. In this paper, an active RF cancellation technique for TX passband leakage is proposed, to efficiently suppress the nonlinear leakage signal at the RX chain input. The technique is based on generating an RF replica of the leakage signal using an auxiliary transmitter branch, together with appropriate digital preprocessing of the known transmit data. This RF replica is then combined with the received signal at the LNA input in the RX chain, properly phased, such that the level of the TX leakage signal is reduced. The performance of the proposed solution is evaluated with simulations as well as practical RF measurements, demonstrating excellent suppression of the TX leakage signal for wideband signals at high transmission power levels. Such novel self-interference cancellation solutions can facilitate enhanced efficiency and flexibility of the RF spectrum use in the emerging 5G radio networks, especially at the lower carrier frequencies.

Linearity Challenges of LTE-Advanced Mobile Transmitters: Requirements and Potential Solutions

In order to provide higher data rates and to improve radio spectrum utilization, 3GPP has introduced the concept of CA in its Release 10 and onward commonly known as LTE-Advanced. The CA technology, particularly when applied in a noncontiguous manner, poses serious design and implementation challenges for radio transceivers, mainly due to the allowed flexibility in the transmitted signal characteristics and the nonlinear RF components in the TX and RX chains. As a consequence, substantial nonlinear distortion may occur that not only degrades the transmitted signal quality but can also affect the concurrent operation of the coexisting receiver when operating in the FDD mode. In this article, the key technical design challenges in terms of linearity requirements for LTE-Advanced mobile terminals are reviewed, and the corresponding self-interference problem related to the potential desensitization of the devices own receiver is highlighted. Then technical solutions to mitigate the self-interference at the RX band due to a nonlinear PA in the transmitter chain are reviewed, with specific emphasis on digital self-interference cancellation methods. As demonstrated through simulation and actual RF measurement examples, the cancellation solutions can substantially mitigate the RX desensitization problem, thus relaxing the RF isolation requirements between the TX and RX chains. Such cancellation methods are one potential enabling technique toward the full exploitation of the fragmented RF spectrum and the CA technology in future LTE-Advanced and beyond mobile networks.

Adaptive Nonlinear RF Cancellation for Improved Isolation in Simultaneous Transmit–Receive Systems

This paper proposes an active radio frequency (RF) cancellation solution to suppress the transmitter (TX) passband leakage signal in radio transceivers supporting simultaneous transmission and reception. The proposed technique is based on creating an opposite-phase baseband equivalent replica of the TX leakage signal in the transceiver digital front-end through adaptive nonlinear filtering of the known transmit data, to facilitate highly accurate cancellation under a nonlinear power amplifier (PA). The active RF cancellation is then accomplished by employing an auxiliary TX chain to generate the actual RF cancellation signal, and combining it with the received signal at the receiver (RX) low-noise amplifier (LNA) input. A closed-loop parameter learning approach, based on the decorrelation learning rule, is also developed to efficiently estimate the coefficients of the nonlinear cancellation filter in the presence of a nonlinear PA with memory, finite passive isolation, and a nonlinear LNA. The performance of the proposed cancellation technique is evaluated through comprehensive RF measurements adopting commercial LTE-Advanced transceiver hardware components. The results show that the proposed technique can provide an additional suppression of up to 54 dB for the TX passband leakage signal at the LNA input, even at very high transmit power levels and with wide transmission bandwidths. Such a novel cancellation solution can, therefore, substantially improve the TX–RX isolation, hence reducing the requirements on passive isolation and RF component linearity, as well as increasing the efficiency and flexibility of the RF spectrum use in the emerging 5G radio networks.