The most critical challenge facing mobile millimeter wave communication is the precise projection of high-gain and narrowly focused radiation toward the multi-paths leading to a device as it moves and rotates in the field. In order to design and evaluate such beam tracking mechanism, an accurate model of the propagation environment is required. Unfortunately, conventional channel models for system operating in carrier frequency lower than 6 GHz are typically statistically based with time-invariant parameters and thus inadequate to simulate the dynamics in spatial characteristics the millimeter wave channel may exhibit. In this paper, such shortcoming is overcome by the introduction of certain time-varying parameters to a widely adopted geometry based stochastic model. With the revised model, we proceed to tune its parameters to fit the statistics extracted from the latest measurement data and subsequently investigate the model's capability in evaluating beam tracking performance. The beam tracking algorithms are generic in nature and only assume certain imperfect knowledge of the channel's spatial profile. It is found that at 28 GHz carrier frequency, beam tracking can be maintained at a moving speed of 60 kmph and an angular speed of 720°/sec with around 20% loss comparing with the stationary reference if the spatial profile is updated at a rate of 50 Hz.

We address beam space reduction method inside which proper analog beamformers are selected for hybrid beamforming millimeter Wave (mmWave) system. We apply insights from spatial multiplexing theory on beam subspace selection and show that computational effort of searching for desired beamformers can be significantly reduced. A simplified channel throughput estimation measure is also devised to further lessen the complexity in the course of beamformer searching. Simulation results demonstrate that near-optimal performance can be achieved with significantly reduced complexity.

This paper investigates the design of physical-layer multicasting in the cellular downlink systems, where antennas at a base station are used to transmit a common signal to multiple single-antenna users. The central design problem of the physical multicasting is the search of the optimal beamforming vector that maximizes the multicast rate and has been known as an NP-hard problem in the literature. In this paper, a new method based on a two-user approximation is proposed for optimizing the beamforming vector. The method is shown to achieve a much higher multicast rate than the existing methods and provides an attractive tradeoff between performance and complexity, especially for the case of using a large number of antennas.

A frequently encountered problem in a cellular network is the selection for each cell in the network a set of parameter values that is uniquely distinguishable from or compatible with those selected by its neighboring cells. Examples include reuse frequency, reference signal offset, beamforming weights and transmit power, just to name a few. It may also be advantageous for the parameter selection to be carried out in a distributed manner so that no centralized controller is required. In this paper, we study a general framework for coordinating parameters in a distributed manner with the goal of applying such a framework to solving most of the above-mentioned problems that have well-defined cost functions. The main components of the framework are the exchange of the impact functions among neighboring nodes and the parameter update based on the weighted sum of these impact functions. The impact function is essentially an indication to the recipient of the information the impact of the recipient's parameter setting on the cost function of the node that transmits the information. Simulations using frequency reuse planning as an example show good convergence behavior and performance comparing with ideal assignment.

Cellular wireless systems have focused, to date, on system capacity, user data rates, latency and coverage as primary design criteria. Energy efficiency has recently received increased attention as another important criterion to be considered. The potential for energy savings in 3GPP LTE systems has been investigated in [1]-[3]. An important design element that determines the potential for energy efficient transmission is the structure of reference signals that need to be transmitted independent of the load in the network, such as the cell-specific reference signals (CRS) in 3GPP LTE systems. This paper discusses modifications to the reference signal structure in LTE that can enable greater energy efficiency. The time and frequency estimation performance implications of such modifications are evaluated and discussed.

Link level performance results for physical antenna port muting in LTE are presented. In this paper we examine the block error rate performance of the LTE downlink control channels (PBCH, PCFICH, PDCCH, and the PHICH) for different antenna muting configurations. Also the throughput performance of the LTE downlink data channel (PDSCH) is studied for 2Tx and 4Tx transmit diversity formats. The results show that the combination of logical antenna port merging with physical antenna port muting provides a very minor performance loss. Since antenna muting should only be used when the traffic in a cell is low, this performance loss is not significant in practice. With antenna muting we can save energy by activating radio units only when motivated by the traffic load in the cell.

In this paper, we present a pilot design for an OFDM cellular system based on Costas arrays. The design provides pilot symbols for both common and dedicated pilots for MIMO transmission. The pilot symbols are designed so that pilots from different cells intersect minimally. The design results in a large number of available distinct pilot symbol patterns with minimally overlapping resource elements that may be assigned to different cells which simplifies cell planning. The design is described in the context of IEEE 802.16m, but is applicable to any OFDM system. Performance of the pilot design in noise and interference limited environments is presented and is shown to be on par with currently used pilot designs while providing a larger number of patterns.

In this paper, we consider the problem of joint adaptive beamforming from multiple Access Points (APs), equipped with array antennas, to multiple single-antenna User Equipments (UEs). We derive an iterative algorithm for Multi-Cell Beam-Forming (MCBF) under per-cell power constraints and the constraint that a UE may receive data transmission from a single AP. The scheme is suitable for coordinated beamforming downlink transmission in a Coordinated Multi-Point (CoMP) cellular network. Simulation results indicate that as compared to uncoordinated transmission by means of a Grid of Beams (GoB) based Space Division Multiple Access (SDMA) scheme, MCBF offers much higher throughput for cell-edge UEs. For a single cluster of nine sectorized cells with four-element antenna arrays in an urban macro-cellular environment, MCBF obtains a 13 fold increase in 5 percentile user throughput at 2 bits/s/Hz/sector cell throughput as compared to GoB-SDMA.

The delay-Doppler correlator is most commonly used for target detection in a radar system. It is essentially a two-dimensional filter matched to the hypothesized delay and Doppler shifts of echoes reflected from the illuminating signal. More recently, it has been applied in communication to the detection of signature sequences generated from artificially introduced time-frequency shifts of a base sequence. In this paper, we derive a computationally efficient delay-Doppler correlator for a class of time-frequency hopping waveforms that can be represented by segments of equal-length sinusoids. These sequences can be found in applications such as continuous waveform radar and device identification in wireless communication. By applying sliding discrete Fourier transform and exploiting the structure of the waveform, the number of multiplications required for evaluating the entire delay-Doppler range can be reduced by a factor equaling the length of the sequence.

In a slot-synchronized wireless communication system, a set of signature sequences can be derived by circularly shifting a base sequence with good auto correlation properties. In a time-dispersive channel, the sequences with different circular time shifts can be uniquely identified as long as their minimum separation in time is greater than the channelpsilas maximum delay spread. In this paper, we extend this approach further to the Doppler domain by introducing an additional circular frequency offset to the delay shifted sequences. When the base sequence is carefully designed through applying some delay-Doppler radar signal design principles, the results of this two-dimensional generalization are a much larger set of sequences with better correlation properties than some existing designs. Moreover, the new design is also suitable in channels that are both time and frequency selective as long as the minimum Doppler separation of the sequences is greater than the channelpsilas maximum Doppler spread. The detection of the sequences essentially consists of correlating the received signal with all valid hypotheses, and can be carried out efficiently if the base sequence is properly designed. Simulation results show that given a sequence length that is large enough, exact number of multiple devices can be accurately detected in very challenging conditions with a single threshold test.

In an OFDM system, known symbols referred to as pilot are transmitted across the time-frequency plane for the receiving device to estimate the channelpsilas time-frequency response in order to perform coherent demodulation of data symbols. The placement of these pilot symbols largely falls into two categories, namely, the regular and irregular patterns. Inspired by synchronization techniques used in radars, derived a time-frequency hoping pattern, called Costas Array which provides a better frequency offset estimation and accommodates substantially higher number of unique patterns that can be used for Base Stations (BSs) and/or antenna identifications. In this paper, we investigate the link and system performance of both pilot designs. From a link perspective we examine if there is a need to boost the pilot signals in order to achieve an adequate Signal to Noise Ratio (SNR). From the radio network perspective we analyze the impact of power pilot boosting. Further we explore an alternative method to reduce the interference perceived by the pilot signals, by introducing a data frequency reuse at interfering BSs. Our results show that a pilot boosting of 9 dB is required and data frequency reuse yields minor SINR improvement for the pilot signals. Furthermore the Costas Array, in addition to having lower peak power, yields substantially better SINR statistics compared to the regular pilot patterns.

In the downlink of an Orthogonal Frequency Division Multiplexing (OFDM) system, pilot symbols are transmitted across the time-frequency plane for the receiving device to estimate the channel's response. The density of such pilot is often designed to accommodate the devices under most extreme channel conditions. This worst case scenario design results in unnecessary overhead that could have been used for data transmission otherwise. In this paper, we propose an adaptive pilot allocation scheme that, in addition to a low-rate periodically transmitted pilot symbols, sends device associated pilot adaptively only when called for by the channel condition, thereby reducing the overall pilot overhead. Furthermore, the placement of signaling channel and reduced complexity channel estimator required to facilitate this adaptive operation are also addressed.

In an OFDM system, channel estimation can be considered as sampling the time-frequency response of the channel through a number of known pilot symbols placed across the time-frequency plane. Sampling theory dictates that the pilot insertion frequency must be above the Nyquist rates in both time and frequency to avoid aliasing of the delay-Doppler response. Based on the regularly spaced pilot pattern, we can derive alternative patterns that preserve the non-aliasing property by hopping the scan lines in either time domain or frequency domain, but not both. From this extended set of patterns, we find ones with properties that, in addition to channel estimation, can achieve responsively other synchronization tasks such as initial time-frequency offset estimation and device identification. The ambiguity function analysis frequently used in radar signal design leads to a periodic time-hopping pattern based on the costas array that has minimal coincidences with its circular time-frequency shifts, which can be used for the identification of multiple devices. The hopping in time also greatly increases the pilot's time support, thus enabling the quick initial acquisition of timing and frequency offset with very short observation.

In an Orthogonal Frequency Division Multiplexing (OFDM) system, the channel is often modeled as a two-dimensional zero-mean Gaussian random process in time and frequency with known second order statistics. With pilot symbols placed across the time-frequency plane, the channel's response at a particular time-frequency point can be derived from its correlation with the nearby pilot locations. However, this a priori channel statistics may not always be available or the channel may not even be stationary to be adequately characterized by a correlation function. In this paper, we introduce a model of time-varying channel based on its delay-Doppler response. We show that a widely adopted time-frequency correlation model, upon which popular channel estimators for OFDM are often based, can be viewed as a special case of such a model with separable statistical profiles imposed on the delay-Doppler plane. Without using any statistical assumption on the channel, we derive a classical estimator based on the two-dimensional delay- Doppler correlator, which can be implemented using Discrete Fourier Transforms (DFT) of the pilot symbol measurements. Simulation results show promising performance even when the pilot symbols' insertion rate is at the channel's maximum delay-Doppler spread.

Channel estimation is critical to receiver performance in an OFDM system. Pilot symbols are typically interspersed with data symbols among the sub-carriers to aid channel estimation. Standard algorithms to estimate the channel based on these pilot symbols include minimum mean-square error (MMSE) and maximum-likelihood (ML) estimation. With a uniform density of pilots in frequency among the sub-carriers, the ML estimate exhibits a boundary effect whereby the quality of the estimates on sub-carriers towards the edges of the signal band degrades. While the use of MMSE mitigates this effect significantly it requires precise knowledge of channel statistics which are difficult to obtain. Also, it is often too complex to implement for common numbers of sub-carriers. This paper proposes mitigation of the boundary effect described above by transmitting extra pilot symbols in the sub-carriers towards the edges of the band in addition to the regularly spaced pilots, thereby increasing pilot density at the edges. Channel estimation is performed using a two-step algorithm. ML estimates are first formed using the uniform pilot set. The output of the ML estimator is then used to bias an MMSE estimator which is applied only to the edge pilot set. The two-step approach enhances performance as compared to ML estimation while keeping complexity much lower than that incurred when MMSE is used across the entire band.

This paper describes the physical layer for a high penetration paging (HPP) mode, that is a highly integrated adjoint to a cellular system. Through the use of very low rate coding and periodically inserted synchronization bursts, the HPP mode enables a cellular user to locate and maintain a minimal communication link with a cellular network at 15 to 20 dB below the normal operating signal to noise ratio (SNR). In this paper, the ANSI-136 air interface standard is used as the underlying cellular system. The same concept can be extended to other systems as well.

Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.

The properties of the ambiguity function and the uncertainty relation of Fourier transforms assert fundamental limitations on the ability of any single radar waveform of constrained time-bandwidth product to distinguish two or more targets closely spaced in both time-delay (range) and Doppler-shift (radial velocity). These same mechanisms place fundamental limits on the ability radar imaging systems to distinguish separate scatterers in delay and Doppler. In this paper, the problem of using multiple waveform sets to make enhanced discrimination delay-Doppler measurements is considered. While small coded waveform sets for enhanced discrimination delay-only measurement are known (e.g., the Golay sequences), these waveforms do not have good Doppler discrimination properties. The problem of designing multiple waveform sets for enhanced discrimination delay-Doppler measurement is investigated, and the composite ambiguity function (CAF) is introduced as a tool to measure the delay-Doppler discrimination characteristics of these waveform sets. The problem of designing optimal coded waveform sets under a time-bandwidth product constraint is considered, and explicit optimal phase, frequency, and joint phase-frequency coded waveform sets having constant amplitude are presented. Algorithms for the construction of such waveform sets of arbitrary size and practical implementation issues are also presented.