Philip J. Pietraski

The bandwidth crunch: Can wireless technology meet the skyrocketing demand for mobile data?

Predictions abound on the growth of mobile data traffic driven by smart phones and tablet PCs that come with an expectation of broadband Internet connectivity everywhere and a demand for high quality video. The FCCs National Broadband Plan indicated that mobile data traffic in North America would increase by roughly 20 to 45 times between 2009 and 2014, and current data suggests these numbers may be exceeded. This paper examines how wireless technology can address the bandwidth challenge. We consider not only the track of cellular technology, but new approaches that promise improved spectral efficiency, use new spectrum, and rely on new network architectures. We recap cellular evolution from second generation through LTE-Advanced, address enhancements such as Coordinated Multipoint Transmission (CoMP) and examine other innovative multi-cell technology. More ambitious changes to cellular architecture such as relay nodes and terminal-to-terminal communications are discussed. Pico-cells, femto-cells, and forward-looking dense deployments are presented as promising approaches. New spectrum, including the use of TV white space is considered. We provide a quantitative assessment -perhaps a goal or perhaps another prediction - of what can be achieved.

New results on SNR estimation of MPSK modulated signals

Signal-to-noise ratio (SNR) is an important parameter in many receivers. In a previous letter B Li et al., (2002) we derived a maximum-likelihood amplitude estimator and obtained a low bias SNR estimation algorithm for a binary phase-shift keying (BPSK) modulated signal. We now extend the result and obtain a low bias SNR estimator for multiple phase-shift keying (MPSK) modulation. We compare our algorithm with others, such as a decision-directed approach. Simulations show that the new algorithm provides not only much lower bias and but also much smaller mean square error (MSE).

Relaying with distributed interference alignment in multi-node networks

A system with multiple transmitters (i.e. base stations) and multiple receivers (mobile nodes) is considered. The transmitter-receiver pairs are assumed to operate using the same resources without any coordination and hence the receivers unavoidably experience interference from undesired transmitters. A MIMO relaying scheme is proposed to manage the interference in the network by judiciously optimizing the relay precoding matrix and aligning the interference at each receiver simultaneously. It is shown that the proposed scheme provides a closed-form optimal precoding matrix. Moreover, the numerical results for various scenarios show that the scheme performs better than the considered baseline schemes for most of the channel conditions.

60 GHz patch antenna array on low cost Liquid-Crystal Polymer (LCP) substrate

Millimeter wave communications promises to enable the very high data rates demanded of 5G networks, but there are several hurdles that must be negotiated first. One of these hurdles is the small energy collecting capability of the small aperture antennas at these wavelengths. High gain antennas are required to overcome this limitation. The 60 GHz band is of particular interest due to the large 7-9 GHz of unlicensed spectrum available. The often cited atmospheric absorption in the band (~13dB/km) is easily tolerated in the 100-200m links envisioned for 5G networks. In order to test the feasibility of a 60GHz network, a platform is being constructed and a low cost, high gain antenna is required. For a low cost and small size solution, a low profile microstrip phased array antenna has been designed and fabricated on low-loss, two-layer Liquid Crystal Polymer (LCP) substrate. 4×4, 8×8 antenna array prototypes have been characterized and the results are described.

Interference mitigation via successive cancellation in heterogeneous networks

In this paper, we present practical interference management schemes in heterogeneous networks (HetNets) based on interference decoding and cancellation at the receivers. The underlying idea is based on Han-Kobayashi type message splitting (MS) technique [3]. We develop relatively low-complexity precoders that facilitate interference mitigation and maximize sum-throughput in the network. System-level simulation results for a general HetNet system are presented. It is shown that the proposed MS design along with interference coordination among the cells provides up to 57% cell average throughput gain compared with the rank-1 coordinated beamforming (CBF) scheme. The design also provides substantial throughput gain in particular for Macro cells compared with rank-adaptive CBF transmission.

Dynamic Performance of a Chip Level Adaptive Equalizer in a UMTS High Speed Downlink Packet Access (HSDPA) Terminal

The HSDPA mode of the 3GPP UMTS FDD standard extends the peak downlink rate of the W-CDMA waveform to 14 Mbps. To achieve high rates across a multipath channel the demodulator must include an advanced receiver rather than a conventional Rake. This paper presents the performance of an NLMS-based adaptive equalizer that supports mobile velocities as high as 250 km/h. The equalizer works with radio and implementation impairments typical of a non-HSDPA receiver, so redesign of the terminal front end is not required. A key aspect of the design is dynamically adjusting the equalizer step size and leakage parameter. There is often concern about the ability of an adaptive algorithm to operate in a highly mobile environment. The transient behavior is discussed in the context of HSDPA operation and examples are provided of initial convergence and operation after a sudden change in propagation conditions.

mm Wave UE Antenna Configuration Study

Millimeter wave (mm wave) communication has emerged as a promising component of 5G cellular systems. The large losses in the Friis equation (due to the implied small apertures of mm wave antennas) suggest that high gain antennas are required. The high gain also implies steerability is required. Arrays of patch antennas are one possible low cost solution, but each array has a limited angular field of view. Multiple arrays per node can be used to improve the field of view. Choosing the correct antenna configuration requires a complex cost-benefit analysis and performance-complexity tradeoff. In this paper, we investigate these tradeoffs through system simulations. Our study uses ray tracing in 3D deployment scenarios and realistic patch antenna array models. MATLAB-based system simulation results are provided, and conclusions are drawn to provide insight into the performance vs. antenna complexity of a mm wave based system. Some of the key conclusions are: diminishing returns on performance as the number of arrays are increased, and the fact that a smaller number of larger arrays can outperform a larger number of smaller arrays. Iso-element curves are also presented as another way to perform a cost-benefit analysis.

Signal and noise power maximum likelihood estimation for fast AGC in packet based systems

Automatic Gain Control (AGC) system performance generally depends on the ability to accurately estimate receive signal power. The initial gain setting may be set high to ensure that low power signals can be detected. As the Variable Gain Amplifier (VGA) resides before the Analog to Digital Converter (ADC), there is a potential for heavy saturation at the output of the ADC, where this estimation is normally performed. This heavy saturation will cause power to be underestimated when using estimators that do not account for clipping, causing slow AGC convergence time. Fast AGC convergence time is desirable in general, but especially for high rate packet based systems (e.g. 802.11ad) which allow for only a limited number of updates in the early stages of a training period. Furthermore, optimal setting of the VGA gain requires statistics sufficient to estimate the received signal distribution. In this work, we propose Maximum Likelihood Estimation (MLE) based estimators of the signal power and noise power that account for saturation at the ADC. The performance of MLE based estimators of total power is compared to a traditional power estimator and results are shown through simulations.

Interference Mitigation through Successive Cancellation in Heterogeneous Networks

We present a practical interference management scheme for heterogeneous networks (HetNets). The underlying ideas are based on (i) Han-Kobayashi-type message splitting (MS) where the receivers decode and cancel “part” of the interference which is accordingly optimized by the transmitters to ensure decoding and (ii) opportunistic interference cancellation (OIC) where the interfering transmitters act independently of the receivers that employ interference cancellation. We develop a novel transmission and reception scheme, called joint MS and OIC (MS-OIC), that engages both MS and OIC to account for a practical HetNet system with multiple macrocells and femtocells. The MS component includes a precoder design that judiciously maximizes the weighted sum throughput via the enabling of interference cancellation. A system design along with a novel scheduler that facilitates MS-OIC in a general HetNet system is also developed. System level simulations for a general HetNet system are presented, and the proposed MS-OIC scheme is compared with benchmark schemes such as Coordinated Beamforming (CBF) and joint CBF and Almost Blank Subframes (CBF-ABS). It is observed that the proposed MS-OIC scheme improves the macrocell throughput substantially, balances the achievable rates between the macrocell and femtocell users, and provides significant outage performance improvement in the system.

BER estimation for MIMO HSDPA : Chip level Wiener equalizer and successive interference cancellation

In this paper we describe and simulate an accurate Bit Error Rate (BER) estimator for a chip level Wiener equalizer and combined equalizer/interference canceller for employment with the CDMA MIMO High Speed Downlink Packet Access (HSDPA) feature of the Third Generation Partnership Project (3GPP). In the 2 × 2 MIMO system two data streams may be transmitted in the downlink to each user equipment (UE) using assigned multiple orthogonal CDMA codes of Spreading Factor 16. The two streams are mapped to the two transmit antennas via a 2 × 2 weight matrix. Since the two streams use the same set of CDMA codes, it results in some direct interference between the two data streams at the receiver. The signal constellations from the two data streams interfere with each other and this complicates the BER estimation process. Treating this interference as AWGN is not sufficient for accurate BER estimation. The modulation type and order needs to be taken into account. Furthermore, the modulation order of the two data streams may also be different, e.g., QPSK may be used for one stream while 16QAM is used for the other. In this paper we describe the HSDPA transmitter and antenna weight matrix parameter calculations, as well as a chip level equalizer structure for which the BER is to be estimated. The transmit antenna weight matrix calculations are based on Channel State Information (CSI) and can be used, along with adaptive modulation and coding to optimize transmission. In the receiver a chip level 2 × 2 matrix Wiener equalizer is first used to recover the two transmit antenna streams in a minimum mean square error sense. The equalizer is implemented using a generalized overlap-save filtering method using FFT processing. Following this, the transmitter processing is reversed to estimate the transmitted data. The BER estimator must take into account white noise at the receiver, the interference between codes from loss of orthogonality due to the multipath channel, self noise from different time shifts of the given users code (also due to the multipath channel) and the crosstalk between the users two data streams, which use the same CDMA codes. The BER estimator for both the basic Wiener filter receiver and the Wiener filter plus interference canceller will be analyzed and compared with simulated BER measurements.