Robert L. Olesen

Peak Power Characteristics of Single Carrier FDMA MIMO Precoding System

A salient advantage of single carrier frequency division multiple access (SC-FDMA) is low peak-to-average power ratio (PAPR) which is lower than that of OFDMA. Precoding using transmit beamforming (TxBF) is a MIMO spatial multiplexing method that increases the data throughput significantly. However, when applied to SC-FDMA, it increases the PAPR. In this paper, we describe how we can apply TxBF to an SC-FDMA system and show its effect on PAPR We then consider the effects of precoder averaging across subcarriers and quantization, typically used to reduce feedback overhead, on the PAPR Comparisons are made with single antenna, non-precoded spatial multiplexing (SM) and space-frequency block coding (SFBC) systems. Lastly, we show that a modest amount of amplitude clipping can reduce PAPR to an acceptable level with virtually no harmful effects on performance or spectral growth.

Uplink Single-User MIMO for 3GPP LTE

In this paper a multiple-input multiple-output (MIMO) scheme proposed for a single carrier frequency division multiple access (SC-FDMA) system is described. Transmit and receive algorithms are developed for SC-FDMA MIMO systems. A spatial precoding closed-loop transmit beamforming (TxBF) algorithm for SC-FDMA MIMO system which uses singular value decomposition (SVD) at the transmitter and linear minimum mean square error (LMMSE) equalization at the receiver is described. Quantization and feedback schemes to support closed-loop operation are discussed. The PAPR effects and a mitigation approach are presented. Simulation results are shown for throughput vs. signal-to-noise ratio (SNR). The performance for the new TxBF algorithm is compared to Single-Input-Multiple Output (SIMO). Also, single and dual- codeword performance is compared.

A comparison of implicit and explicit channel feedback methods for MU-MIMO WLAN systems

In this paper we compare the implicit and explicit methods of providing channel state information (CSI) to the transmitter in a multi-user multiple-input-multiple-output (MU-MIMO) system as specified in the draft specification IEEE 802.11ac [1]. The comparison is made on the basis of both overhead requirements and packet-error-rate (PER) performance. We also propose a hybrid feedback scheme that combines the explicit and implicit feedback methods thus benefiting from more frequent calibration without incurring extra overhead.

Efficient Feedback Design for MIMO SC-FDMA Systems

An efficient feedback design for single carrier frequency division multiple access (SC-FDMA) multiple input multiple output (MIMO) communication systems is described. We consider a codebook-based approach for MIMO pre-coding and quantization of feedback information. Differential processing is used to generate the updates iteratively for the feedback information between feedback instances. An approach of combined differential and non-differential feedback is considered for reduced feedback overhead and improved performance. A differential feedback with periodic error reset for differential processing is described. The performance of MIMO communication systems using pre-coding/transmit beamforming and the described feedback design is evaluated for SC-FDMA systems. The effects of pre-coding and feedback quantization, sub-carrier grouped feedback and feedback delay are investigated.

Flexible DFT-S-OFDM: Solutions and Challenges

Discrete Fourier transform spread orthogonal DFT-S-OFDM, adopted in 3GPP LTE uplink, enables the synthesis of block-based single carrier waveforms with various bandwidths by changing the size of the DFT-spread block. Conceptually, it also allows a transition between block-based multicarrier and single-carrier schemes when multiple DFT-spread blocks are employed in the structure. Recently, it has been shown that DFT-S-OFDM can also accommodate an internal guard period that offers flexibility on the duration of the guard periods without affecting the symbol duration. In this article, we present further modifications of DFT-S-OFDM that offer improvements in flexibility and discuss the latest enabling techniques on this topic. Considering the flexibility introduced by DFTs-OFDM and its variations, the DFT-S-OFDM family also offers a set of promising waveforms for 5G networks, which will require a flexible physical layer.

Multi-user Parallel Channel Access for high efficiency carrier grade wireless LANs

The IEEE 802.11 specification for Wireless Local Area Networks (WLANs) supports 20 MHz channels utilizing bandwidths up to 160 MHz. However, the support of devices simultaneously transmitting over different, non-overlapping channels, is not specifically addressed. Recently, there has been interest in developing a 802.11 specification for Carrier Grade Wi-Fi which has led to the requirement for solutions with efficient frequency resource utilization that are not currently supported. In this paper we propose a Medium Access Control (MAC) method, called Multi-User Parallel Channel Access (MU/PCA) that enables simultaneous transmission to multiple devices of various bandwidths, while maintaining backward compatibility with 802.11. We present analytical and simulation results demonstrating the throughput gains that can be achieved using the proposed MU/PCA scheme as compared to the current Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) scheme that is used in 802.11.

An Improved Unique Word DFT-Spread OFDM Scheme for 5G Systems

In this paper, we propose a new waveform based on discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM), in which the tail of the DFT-S-OFDM symbol contains a fixed sequence, i.e., a unique word (UW). The proposed waveform, called UW DFT-S-OFDM, improves upon both existing zero-tail (ZT) DFT-S-OFDM and UW OFDM schemes by removing the impact of data symbols on the tail of the transmitted signal. This is done by creating a redundant symbol vector that has 1% of the total transmitted energy approximately. As a result, the proposed UW DFT-S-OFDM scheme keeps the advantages of UW OFDM and ZT DFT-S-OFDM such as the circular convolution of the channel without the use of a cyclic prefix (CP), low peak-to-average power ratio (PAPR), and low out-of-band (OOB) emission. In addition, by generating the UW sequence at the input of the DFT process, simpler receiver operation is obtained.

Zero-Forcing Beamforming Codebook Design for MU-MIMO OFDM Systems

Zero-forcing (ZF) beamforming, an efficient technique for multiuser MIMO systems, requires perfect channel state information to be available at the base station. This is achieved by quantizing the channel and feeding back the information with a limited number of bits. A challenge of this method is the large downlink signaling overhead, which is due to the large size of the codebook that can be used at the base station for ZF beamforming. In this paper, we first analyze this problem and then present a technique designing codebooks with reduced size and minimal performance degradation. We also define the associated procedures required for the system implementation. Simulation results show that the size of the eNodeB codebook can be substantially reduced with only a small (5%) penalty in performance.

Blind Estimation and Compensation of Frequency-Flat I/Q Imbalance Using Cyclostationarity

I/Q imbalance is one of the major concerns in the design of direct-conversion front-end receivers in high data rate wireless networks. To address the challenge, various I/Q imbalance estimation and compensation algorithms have been proposed in the literature. In this paper, we propose a blind cyclostationary based estimation and compensation of frequency-flat I/Q imbalance. The proposed blind estimation algorithm uses second-order statistics to compensate I/Q imbalance without estimating the mismatch parameters and is an unbiased estimator when a DC offset exists at the receiver. The performance of our approach is evaluated and compared to other existing blind I/Q imbalance estimation algorithms.

Carrier Grade Wi-Fi: Air interface requirements and technologies

The IEEE 802.11 standardization group has recently ratified 802.11ac as the newest major amendment of the 802.11 family of Wi-Fi standards. While 802.11ac has specified a number of improvements over 802.11n such as: (i) 8 spatial streams (ii) mandatory bandwidth of 80 MHz and (iii) multi-user MIMO on the downlink, these improvements mostly target to improve the per-link throughput, and in case of MU-MIMO, traffic performance on the downlink. In order to satisfy the air interface high efficiency requirements and technologies, herein referred to broadly as 5G-Carrier Grade WiFi (5G-CGW), it is important to consider other metrics for system performance, such as area-throughput and Quality of Experience (QoE), which are more relevant in use cases where there can be a dense deployment of access points (APs), and stations (STAs). Recently, 802.11 started a study group called High Efficiency Wi-Fi (HEW) to develop the next generation of Wi-Fi physical (PHY) and medium access control (MAC) protocols that would satisfy these requirements. In this paper we will first provide an overview of the state-of-the-art in 802.11 standards, followed by a discussion on some of the limitations of 802.11ac in use cases of interest such as dense deployments in apartment buildings, stadiums and airports. We will provide an overview and preliminary simulation results of three technologies that have shown promise for meeting the requirements of CGW: (i) Multi-User Parallel Channel Access (MU-PCA) which would allow APs to simultaneously transmit to and to receive from a number of STAs in the frequency domain: enabled through multiplexing. This would alleviate the problem of underutilization of frequency resources caused by the need to support STAs of different bandwidths. (ii) Uplink Multi-User MIMO (UL MU-MIMO): IEEE 802.11ac standardized multi-user simultaneous transmissions in the downlink via downlink MU-MIMO. Uplink MU-MIMO needs to be defined to enable multiple users to share the spatial domain and transmit at the same time in the uplink. (iii) Fractional CSMA and Transmit Power Control (TPC): In a dense deployment of APs, the performance of overlapping basic service sets (BSSs) can be improved by coordinating the transmitted power in the adjacent APs in such a manner that STAs on the edge of coverage face reduced interference.