Frederick W. Vook

Millimeter-Wave Enhanced Local Area Systems: A High-Data-Rate Approach for Future Wireless Networks

Wireless data traffic is projected to skyrocket 10 000 fold within the next 20 years. To tackle this incredible increase in wireless data traffic, a first approach is to further improve spectrally efficient systems such as 4G LTE in bands below 6 GHz by using more advanced spectral efficiency techniques. However, the required substantial increase in system complexity along with fundamental limits on hardware implementation and channel conditions may limit the viability of this approach. Furthermore, the end result would be an extremely spectrally efficient system with little room for future improvement to meet the ever-growing wireless data usage. The second approach is to move up in frequency, into an unused nontraditional spectrum where enormous bandwidths are available, such as at millimeter wave (mmWave). The mmWave option enables the use of simple air interfaces since large bandwidths can be exploited (e.g., 2 GHz) to achieve high data rates rather than relying on highly complex techniques originally aimed at achieving a high spectral efficiency with smaller bandwidths. In addition, mmWave systems will easily evolve to even higher system capacities, because there will be plenty of margin to improve the spectral efficiency as data demands further increase. In this paper, a case is made for using mmWave for a fifth generation (5G) wireless system for ultradense networks by presenting an overview of enhanced local area (eLA) technology at mmWave with emphasis on 5G requirements, spectrum considerations, propagation and channel modeling, air-interface and multiantenna design, and network architecture solutions.

MIMO strategies for equal-rate data streams

This paper explores strategies for multiple-input/multiple-output (MIMO) communications where the number of data streams is fixed and each stream has the same modulation type. For the fixed data stream case, transmit and receive weights based on the singular value decomposition (SVD) approach may not give the best BER performance. This paper explores two alternate strategies to the SVD approach for jointly calculating the transmit and receive weights based on minimizing the mean-squared error summed over all data streams. The first strategy finds linear transmit and receive weights, and the second strategy finds linear transmit weights and successive cancellation receive weights.

System level modeling and performance of an outdoor mmWave local area access system

Millimeter wave (mmWave) frequencies offer the potential for very large capacity increases from traditional cellular frequencies because of the incredible amount of available spectrum (e.g., 10 GHz in the E-band alone). Additionally the latest channel measurements and research have shown that mmWave frequencies are useable for a 5G-type local area access system. However, employing mmWave frequencies for outdoor local area access presents a challenge particularly from blockage of the signal from the mobile to the access point. Using the latest path loss equations from measured data, a newly developed mmWave channel environment, and a model for distance-dependence LOS blocking probability, system-level capacity results are presented for a dense urban environment. The results show that with adequate access point density that low outage probability can be obtained with cell-edge rates well in excess of 100 Mbps and cell-average user throughputs of up to 5.12 Gbps.

Method for obtaining full channel state information for RF beamforming

Millimeter wave (mmWave) and massive MIMO systems will be characterized by the use of a large number of antennas at the base stations. In order to reduce costs and power consumption, the number of radio frequency (RF) chains (transceivers/baseband units) should be minimized for these arrays. One option to achieve this goal is with RF beamforming where the control of the gain and phase of each element in the array is done at RF instead of at baseband in typical digital beamformers. However obtaining the full channel knowledge for RF beamformers is difficult because sounding each antenna separately incurs a power loss. In this paper we propose using basis functions to create orthogonal beams which the base station array can sound in order to obtain the full transmit CSI between the transmit array and the UE array. We also explore a method of feedback reduction where the amount of CSI is reduced by feeding back CSI separately for the azimuth and elevation dimensions of the array. Simulation results at mm Wave show that the use of the basis functions can obtain performance very close to ideal channel knowledge and that feeding back CSI separately for azimuth and elevation is effective as long as the array is manifold calibrated.

Coverage and rate trends in moderate and high bandwidth 5G networks

Higher frequency bands (>6 GHz) look promising to meet the proposed 5G data rates, given the large amount of available spectrum in these bands. However, a rigorous understanding of some fundamental tradeoffs like network densification, sectorization, and bandwidths has only begun to be investigated at millimeter wave (mmW) bands. In this work, we investigate the coverage and rate performance of cellular networks with sectorized access points (APs) operating at high frequency bands, using tools of stochastic geometry. We observe that sectorizing the APs can significantly improve the data rates and thus can be used in conjunction with network densification, in order to achieve the 5G data rate requirements. However, the increased data rates come at the expense of increased interference in the network. We investigate the interference effects on a typical moderate (200 MHz) bandwidth network at 28 GHz and a high (2 GHz) bandwidth network at 72 GHz carrier frequency, with 4 sector APs and validate the trends observed with the help of detailed system-level simulations using METIS-like scenarios.

Finite Impulse Response Cyclic Shift Transmit Diversity for Broadband Mobile OFDM

This paper introduces an open-loop transmit diversity method called finite impulse response cyclic shift diversity (FIR-CSD) for broadband mobile OFDM systems. Ordinary CSD operates in OFDM by cyclically shifting the same time- domain signal by different cyclic shift values on each transmit antenna branch. Unfortunately, in some channels (such as a line of sight channel or a heavily spatially correlated channel), ordinary CSD introduces deep spectral nulls in the composite frequency response seen by the receiver, which results in a performance loss relative to the single transmit antenna case. FIR-CSD is an extension of the CSD concept wherein multiple weighted and circular shifted copies of the time-domain signal are combined and transmitted on each antenna. Each transmit antenna essentially uses a different FIR filter that combines multiple weighted and circularly-shifted copies of the time-domain signal to be transmitted. Careful selection of the circular shift values and weighting factors on each transmit antenna can significantly reduce or eliminate the deep spectral nulls that ordinary CSD would produce in the above mentioned channels. This paper shows that FIR-CSD provides a significant performance improvement over the original cyclic shift diversity method, especially in correlated channels and with high-rate coding.

Performance of trellis-coded OFDM with antenna diversity

This paper studies the link performance of coded OFDM systems via both analysis and simulation. We first show that the level of frequency diversity that can be exploited by coding across subcarriers is not only decided by the code design, but also limited by the channel delay spread. The analysis also shows the performance improvement of multi-antenna OFDM systems over single-antenna systems. In order to maximally exploit both spatial and frequency diversity, we then propose to use the Alamouti transmit diversity scheme to first optimally exploit the spatial diversity, and then use bit interleaved coded modulation (BICM) to obtain the maximum level of frequency diversity. This design is compared with many existing space-time codes that follow a joint spatiotemporal design philosophy. Simulation results show that our design outperforms the latter under both frequency-selective and frequency-flat channels, even though the space-time codes we simulated are designed for frequency-flat channels.

Investigation into the effects of polarization in the indoor mmWave environment

Recently a measurement campaign was performed and a subsequent channel model proposal was created for an indoor office environment at 73 GHz. The channel model included a detailed modeling of polarization which was designed to capture the expected effects of polarization observed in the literature. Using this new channel model we perform a study of how polarization impacts the performance of an indoor millimeterwave (mmWave) communication system in a small-office environment. Particular emphasis is given to having only a single polarization at both ends of the link. The impact of polarization on large-scale fading, such as angle spread, channel rank, and system-level performance is considered. Results show that some polarization combinations at each end of the link can increase the delay and angle spreads while other combinations can decrease the spread. Results also show the importance of having mixed polarization types at both ends of the link (e.g., circular on one end and linear on the other) when each end has a single polarization type.

Product codebook feedback for massive MIMO with cross-polarized 2D antenna arrays

Massive MIMO involves the use of large scale antenna arrays for high-gain adaptive beamforming and high-order spatial multiplexing. An important design challenge in Massive MIMO systems is the acquisition of channel state information at the transmit array, where accurate channel knowledge is critical for obtaining the best performance with Multi-User MIMO transmission on the downlink. In this paper, we explore the use of a product codebook feedback methodology for two-dimensional antenna arrays where the codebook feedback strategy is decomposed into two separate feedback processes, one for azimuth and one for elevation. We specifically address the case where the transmit array consists of cross-polarized antennas and show how two separate codebook feedback processes can reduce reference signal overhead and simplify the mobile complexity while providing significant gains in performance over existing LTE configurations.

System-Level Performance of Different Array Types for an Indoor mmWave System

The use of millimeter wave (mmWave) frequencies for access links promises to provide an incredible user experience given the large bandwidths available. Using a recently-developed mmWave indoor channel model for the lower E-band (71-76 GHz), we investigate the system-level performance of a mmWave system in a small office-type environment. Particular attention is paid to the performance with different antenna array types. The results show that peak data rates in excess of 14 Gbps are possible and everywhere data rates of more than 100 Mbps are seen for all indoor locations. Also the results show that it is best to position the antennas in the azimuth dimension at the access point as opposed to elevation, and that there is little performance difference between circular and linear arrays with the same number of antenna elements. Finally the results demonstrate that the mobile can benefit from having arrays of directional patch antennas as long as there are sufficient patch arrays around the mobile to provide omni-like coverage.