A key goal of next generation networks is to scale hardware design and signal processing algorithms to mmWave and THz arrays with a large number of elements. Imperfect manufacturing and limitations of circuit design introduce variations in the gain and relative phase offset of transmit and receive array elements that must be compensated prior to beam formation for either communication or sensing. We propose a novel method for calibrating large arrays in the field by exploiting the sparsity of the spatial channel. While conventional calibration methods are susceptible to multipath components in the wireless channel, our approach is shown to be robust to multipath interference if the measurements are gathered from a sufficiently diverse set of locations.

Millimeter wave MIMO combines the benefits of compact antenna arrays with a large number of elements and massive bandwidths, so that fully digital beamforming has the potential of supporting a large number of simultaneous users withper userdata rates of multiple gigabits/sec (Gbps). In this paper, we develop an analytical model for the impact of nonlinearities in such a system, and illustrate its utility in providing hardware design guidelines regarding two key challenges: the low available precision of analog-to-digital conversion at high sampling rates, and nonlinearities in ultra-high speed radio frequency (RF) and baseband circuits. We consider linear minimum mean square error (LMMSE) reception for a multiuser MIMO uplink, and provide performance guarantees based on two key concepts: (a) summarization of the impact of per-antenna nonlinearities via a quantity that we term the “intrinsic SNR”, (b) using linear MMSE performance in an ideal system without nonlinearities to bound that in our non-ideal system. For our numerical results, we employ nominal parameters corresponding to outdoor picocells operating at a carrier frequency of 140 GHz, with a data rate of 10 Gbps per user.

This paper investigates the effect of oscillator phase noise on a multiuser millimeter wave (mmWave) massive MIMO uplink as we scale up the number of base station antennas, fixing the load factor, defined as the ratio of the number of simultaneous users to the number of base station antennas. We consider a modular approach in which the base station employs an array of subarrays, or “tiles.” Each tile supports a fixed number of antennas, and can therefore be implemented using a separate radio frequency integrated circuit (RFIC), with synchronization across tiles accomplished by employing a phased locked loop in each tile to synthesize an on-chip oscillator at the carrier frequency by locking on to a common lower frequency reference clock. Assuming linear minimum mean squared error (LMMSE) multiuser detection, we provide an analytical framework that can be used to specify the required power spectral density (PSD) mask for phase noise for a target system performance. Our analysis for the phase noise at the output of the LMMSE receiver indicates two distinct effects: self-noise for each user which is inversely proportional to the number of tiles, and cross-talk between users which is insensitive to the number of tiles, and is proportional to the load factor. These analytical predictions, verified by simulations for a 140 GHz system targeting a per-user data rate of 10 Gbps, show that tiling is a robust approach for scaling. Numerical results for our proposed design approach yield relatively relaxed specifications for phase noise PSD masks.

Millimeter wave (mmWave) carrier frequencies offer the dual benefits of large bandwidth and physically compact antenna arrays with a large number of elements, so that a large number of simultaneous independently modulated users can be supported with digital beamforming. In this paper, we propose an efficient digital backend design for a wideband mmWave massive MIMO uplink taking full advantage of both features. The challenges in receiver design for a wideband system with a large antenna array include the following: (a) for each user, there is a multi-symbol delay spread across the array, hence the receiver must handle intersymbol as well as multiuser interference; (b) even though we consider channels with a single dominant path, the spatial frequency for a user becomes smeared across the band; (c) the natural strategy of MIMO-OFDM used at lower carrier frequencies is unattractive at mmWave frequencies because the linearity requirements due to large peak-to-average ratio are difficult to meet. In this paper, we introduce a novel technique that combines spatial domain FFT and time domain FFT, together with an interpolation technique for limiting the spread of spatial frequency across the band which significantly reduces the complexity of beamformer weight acquisition. We present simulation results for a system operating at 140 GHz with a bandwidth of 14 GHz and a 256-element horizontally-scanning linear base station array.

Highly scaled indium phosphide (InP) heterojunction bipolar transistor (HBT) technologies have been demonstrated with maximum frequencies of oscillation ( f max) of >1 THz and circuit operation has been extended into the lower end of the terahertz (THz) frequency band. InP HBTs offer high radio-frequency (RF) output power density, millivolt (mV) threshold uniformity, and high levels of integration. Integration with multilevel thin-film wiring permits the realization of compact and complex THz monolithic integrated circuits (TMICs). Circuit results reported from InP HBT technologies include: 200-mW power amplifiers at 210 GHz, 670-GHz amplifiers and fundamental oscillators, and fully integrated 600-GHz transmitter circuits. We review the state of the art in THz-capable InP HBT devices and integrated circuit (IC) technologies. Challenges in extending transistor bandwidth and in circuit design at THz frequencies will also be addressed.

We consider analog multiband as a means for scaling up the bandwidth for millimeter (mm) wave communication. Analog multiband sidesteps the difficulty of scaling analog-to-digital conversion (ADC) to higher sampling rates by channelizing the available bandwidth in the analog domain into subbands and using existing power-efficient ADCs to digitize each subband. In this paper, we address the issue of efficient channelization into subbands. A direct approach using a bank of mixers with independent frequency synthesizers has several disadvantages, including large power consumption and the potential for oscillator coupling. We explore an alternative approach based on polyphase sampling, using analog discrete Fourier transform (DFT) along with appropriately designed baseband filters. We quantify the inter-subband interference as a function of filter choice, and demonstrate that it can be handled using interference suppression strategies developed in our prior work. A natural comparison of such approaches, where analog channelization is followed by parallel ADCs running at slower rates for each subband, is with time-interleaved ADCs, which also use polyphase samplers, but use parallel sub-ADCs to digitize the entire band. We show that the proposed approach reduces the required dynamic range of the subband ADCs in comparison to the TI-ADC.

In this paper, we investigate spatial multiplexing at millimeter (mm) wave carrier frequencies for short-range indoor applications by quantifying fundamental limits in line-of-sight (LOS) environments and then investigating performance in the presence of multipath and LOS blockage. Our contributions are summarized as follows. For linear arrays with constrained form factor, an asymptotic analysis based on the properties of prolate spheroidal wave functions shows that a sparse array producing a spatially uncorrelated channel matrix effectively provides the maximum number of spatial degrees of freedom in a LOS environment, although substantial beamforming gains can be obtained by using denser arrays. This motivates our proposed mm-wave MIMO architecture, which utilizes arrays of subarrays to provide both directivity and spatial multiplexing gains. System performance is evaluated in a simulated indoor environment using a ray-tracing model that incorporates multipath effects and potential LOS blockage. Eigenmode transmission with waterfilling power allocation serves as a performance benchmark, and is compared to the simpler scheme of beamsteering transmission with MMSE reception and a fixed signal constellation. Our numerical results provide insight into the spatial variations of attainable capacity within a room, and the combinations of beamsteering and spatial multiplexing used in different scenarios.

Analog-to-digital converters (ADCs) with high sampling rates and output resolution are required for the design of mostly digital transceivers in emerging multi-Gigabit communication systems. A promising approach is to use a time-interleaved (TI) architecture with slower sub-ADCs in parallel, but mismatch among the sub-ADCs, if left uncompensated, can cause error floors in receiver performance. Conventional mismatch compensation schemes typically have complexity (in terms of number of multiplications) that increases with the desired resolution at the output of the TI-ADC. In this paper, we investigate an alternative approach, in which mismatch and channel dispersion are compensated jointly, with the performance metric being overall link reliability rather than ADC performance. For an OFDM system, we characterize the structure of mismatch-induced interference, and demonstrate the efficacy of a frequency-domain interference suppression scheme whose complexity is independent of constellation size (which determines the desired resolution). Numerical results from computer simulation and from experiments on a hardware prototype show that the performance with the proposed joint mismatch and channel compensation technique is close to that without mismatch. While the proposed technique works with offline estimates of mismatch parameters, we provide an iterative, online method for joint estimation of mismatch and channel parameters which leverages the training overhead already available in communication signals.

Spatial multiplexing for millimeter (mm) wave line of sight (LOS) links potentially enables data rates of the order of 10-100 Gbps. Most prior work in this area has focused on uniform transmit and receive arrays, for which it is known that the spatial responses seen by different transmitters can be made orthogonal by choosing the antenna spacing appropriately as a function of range and wavelength. In this paper, we show that variations in range can cause significant degradation in performance for such uniformly spaced arrays optimized for a given range, due to the appearance of high correlations between the spatial responses for different transmitters (and hence rank deficiency in the MIMO channel matrix) as a function of range. We then demonstrate that optimized nonuniform arrays alleviate this problem by keeping correlations between spatial responses small over a significantly larger set of ranges than is possible with uniform spacing.

We present a cross-layer modeling and design approach for multiGigabit indoor wireless personal area networks (WPANs) utilizing the unlicensed millimeter (mm) wave spectrum in the 60 GHz band. Our approach accounts for the following two characteristics that sharply distinguish mm wave networking from that at lower carrier frequencies. First, mm wave links are inherently directional: directivity is required to overcome the higher path loss at smaller wavelengths, and it is feasible with compact, low-cost circuit board antenna arrays. Second, indoor mm wave links are highly susceptible to blockage because of the limited ability to diffract around obstacles such as the human body and furniture. We develop a diffraction-based model to determine network link connectivity as a function of the locations of stationary and moving obstacles. For a centralized WPAN controlled by an access point, it is shown that multihop communication, with the introduction of a small number of relay nodes, is effective in maintaining network connectivity in scenarios where single-hop communication would suffer unacceptable outages. The proposed multihop MAC protocol accounts for the fact that every link in the WPAN is highly directional, and is shown, using packet level simulations, to maintain high network utilization with low overhead.

Time-interleaved (TI) analog-to-digital converters (ADCs) are a promising architecture for realizing the highspeed ADCs required to implement "mostly digital" receivers for emerging multiGigabit communication systems. Mismatch between the parallel ADCs comprising a TI-ADC is a fundamental performance bottleneck. While there exist mismatch correction techniques for generic applications of ADC, we illustrate in this paper that mismatch compensation can be subsumed within the overall receiver design for communication applications. We consider an orthogonal frequency division multiplexing (OFDM) link with TI-ADC used after downconversion. We show that mismatch results in frequency selective interference across subcarriers that can significantly degrade the performance of a standard OFDM receiver. However, this performance degradation can be alleviated significantly by joint channel and mismatch estimation and compensation, leveraging already available training or pilot sequences. This eliminates the necessity for dedicated hardware for mismatch correction. Specifically, we present an algorithm for estimating the channel gains and the mismatch, followed by low-complexity linear equalization to suppress the inter-subcarrier interference resulting from the mismatch. Our simulations show that the error floor due to mismatch-generated interference can be eliminated, permitting bandwidth-efficient operation with large constellations.

Indium phosphide heterojunction bipolar transistors (HBTs) find applications in very wide-band digital and mixed-signal integrated circuits (ICs). Devices fabricated in high-yield process flows at 500 nm feature size obtain 450 GHz cutoff frequencies and 5 V breakdown and enable high yield fabrication of integrated circuits having more than 3000 transistors. Laboratory devices at 250 nm feature size obtain 755 GHz . We describe device and circuit bandwidth limits associated with HBTs, develop scaling roadmaps for HBTs having lithographic minimum feature sizes between 512 and 64 nm, and identify key technological challenges in realizing 480-GHz digital ICs and 1000-GHz amplifiers. Key features of manufacturable self-aligned dielectric sidewall processes are described in detail.

In the above titled paper (ibid., vol. 96, no. 2, pp. 271-286, Feb 08), there are errors. Corrections are presented here.

Calibration of sub-converter mismatches is a challenging task for high-performance time-interleaved analog-to-digital converters (TIADC). Presently known blind correction methods can remove static gain and sampling time mismatches. However, actual subconverters significantly deviate from a simple gain-timing model, and the resulting modeling error strictly limits maximum output signal-to-noise ratio achievable. Generalized mismatch modeling is, therefore, necessary to break the limitation of gain-timing model. In this paper, we propose a blind method for correcting generalized mismatch errors for M=2 TIADC, which is the first in the literature to the authors' knowledge. Cyclostationary spectral analysis shows that unique identification is possible in most practical cases. Simulation results show significant performance improvement by the proposed generalized correction method.

The 7 GHz of unlicensed spectrum in the 60 GHz band offers the potential for multiGigabit indoor wireless personal area networking (WPAN). With recent advances in the speed of silicon (CMOS and SiGe) processes, low-cost transceiver realizations in this "millimeter (mm) wave" band are within reach. However, mm wave communication links are more fragile than those at lower frequencies (e.g., 2.4 or 5 GHz) because of larger propagation losses and reduced diffraction around obstacles. On the other hand, directional antennas that provide directivity gains and reduction in delay spread are far easier to implement at mm-scale wavelengths. In this paper, we present a cross-layer modeling methodology and a novel multihop medium access control (MAC) architecture for efficient utilization of 60 GHz spectrum, taking into account the preceding physical characteristics. We propose an in-room WPAN architecture in which every link is constrained to be directional, for improved power efficiency (due to directivity gains) and simplicity of implementation (due to reduced delay spread). We develop an elementary diffraction-based model to determine network link connectivity, and define a multihop MAC protocol that accounts for directional transmission/reception, procedures for topology discovery and recovery from link blockages.

Time-interleaved analog-to-digital converters (TIADC) require mismatch calibration to achieve high signal-to-noise ratios. In this paper, we present a new blind technique for M=2 TIADC's with following properties: (1) correction of general mismatches as well as static gain and timing error, (2) unique multi-parameter identification under realistic assumptions, and (3) analytic error measure and gradient. The proposed method overcomes the limitation of traditional gain-timing mismatch model. Time-frequency transformation is eliminated by analytic approach, enhancing efficiency.

We report experimental verification of the previously proposed blind method of correcting gain and timing mismatches for a two-channel time-interleaved analog-to-digital converter (TIADC). The experimental setup allows for two different M=2 TIADC configurations with 14-bit resolution and 200-MHz overall sampling rates. Mismatch parameters are estimated by the blind algorithm with representative narrowband and wideband signals. The spurious-free dynamic range (SFDR) performance is then evaluated by using sinusoids. We discuss performance gain, as well as the limitations of the proposed blind algorithm when applied to real world analog-to-digital (A/D) converters.

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTL's) generate /spl sim/4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTL's is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation.>