Remote Antenna Units Research Articles

Spectral and Energy Efficiency of Distributed Massive MIMO with Low-Resolution ADC

In this paper, considering a more realistic case where the low-resolution analog-to-digital convertors (ADCs) are employed at receiver antennas, we investigate the spectral and energy efficiency in multi-cell multi-user distributed massive multi-input multi-output (MIMO) systems with two linear receivers. An additive quantization noise model is provided first to study the effects of quantization noise. Using the model provided, the closed-form expressions for the uplink achievable rates with a zero-forcing (ZF) receiver and a maximum ratio combination (MRC) receiver under quantization noise and pilot contamination are derived. Furthermore, the asymptotic achievable rates are also given when the number of quantization bits, the per user transmit power, and the number of antennas per remote antenna unit (RAU) go to infinity, respectively. Numerical results prove that the theoretical analysis is accurate and show that quantization noise degrades the performance in spectral efficiency, but the growth in the number of antennas can compensate for the degradation. Furthermore, low-resolution ADCs with 3 or 4 bits outperform perfect ADCs in energy efficiency. Numerical results imply that it is preferable to use low-resolution ADCs in distributed massive MIMO systems.

CDMA RoFSO Links With Nonzero Boresight Pointing Errors Over M Turbulence Channels

Radio-on-Free-Space Optical systems constitute a very attractive option for interconnecting central base stations with remote antenna units. Nonetheless, the optical signal propagation through the atmosphere is tremendously affected by turbulence- and misalignment-induced fading. In the present study, we investigate the transmission of code division multiple access signals over a free-space optical (FSO) link. The atmospheric turbulence is modeled by the M (Malaga) distribution, which acts as a unifying statistical model encompassing all the well-known distributions that have been proposed in the FSO area. In addition, nonzero boresight pointing errors (PEs) are employed, which are described by an accurate approximation of the Beckmann distribution. Novel mathematical expressions are extracted regarding the average bit error rate and the outage probability of the forward and the reverse link. Moreover, appropriate numerical results are depicted for various turbulence and PE scenarios, along with their associated corroborative outcomes.

Single Sideband Signals for Phase Noise Mitigation in Wireless THz-Over-Fibre Systems

In future photonic-based THz backhaul links, integrating the optical local oscillator (LO) in a remote antenna unit can be beneficial in terms of optical bandwidth efficiency and higher compatibility with passive optical networks. In such a scenario, two approaches can be used to reduce the high phase noise associated with free-running lasers: 1) baseband (BB) signals & carrier recovery, and 2) single sideband (SSB) signals & envelope detection. In this paper, we compare the performance of the two approaches for various optical LO linewidths using 5 GBd 16-QAM signals. We find that, for a total linewidth wider than 0.55 MHz, the SSB approach yields better results. The superior performance, however, comes at the expense of reducing the net information spectral density (ISD) of the SSB signal by 39% compared to that of the BB signal. However, using signal–signal beat interference-mitigation algorithms, an ISD only 15% lower than the BB signal ISD was sufficient to meet the FEC requirement. Given these results, we believe that the envelope detection of SSB signals is a promising solution to mitigate the phase noise problem of THz links based on free-running lasers, without excessively compromising the spectral efficiency of the system.

Transmission of wireless signals using space division multiplexing in few mode fibers

Evolution of next generation wireless networks brings challenges to efficiently transmit a large amount of data from a base station to a remote antenna unit. We investigate a space division multiplexing technique that employs few mode fibers (FMFs) to transmit 3 × 3 MIMO wireless signals, aiming to employ a common digital signal processing (DSP) unit to equalize both the fiber and wireless channel. We optimize system parameters and obtain above 28 dB and 23 dB signal-to-interference and noise ratio (SINR) for 3 meters wireless systems with 500 m and 2 km FMF, which correspond to the transmission capacity of 578 Mb/s and 468 Mb/s using a 20 MHz bandwidth, respectively. Moreover, we analyze that the nonlinear spectrum distortion due to the combined effect of nonlinearity in the directly modulated laser and the differential mode delay in multimode fibers and validate it by simulations.

Optical Frequency Tuning for Coherent THz Wireless Signals

THz wireless signals have become of interest for future broadband wireless communication. In a scenario where the wireless signals are distributed to many small remote antenna units, this will require systems which allow flexible frequency tuning of the generated THz carrier. In this paper, we demonstrate experimentally the implementation of two tuning methods using an optical frequency comb generator for coherent optical frequency tuning in THz wireless-over-fiber systems. The first method is based on using a photonic integrated circuit optical phase lock loop (OPLL) subsystem implemented as a high quality optical filter for single comb line selection and optical amplification. The OPLL generates an optical carrier, which is frequency and phase stabilized in reference to one of the optical comb lines with a frequency offset precisely selectable between 4 and 12 GHz. The second method is based on optical single sideband suppressed carrier (SSB-SC) modulation from the filtered comb line using an optical IQ modulator. With this technique, it is possible to suppress the other unwanted optical tones by more than 40 dB. This generated optical carrier is then heterodyned with another filtered optical comb line to generate a tuneable and stable THz carrier. The full system implementations for both methods are demonstrated by transmitting THz wireless signal over fiber with 20 Gb/s data in QPSK modulation. The system performance and the quality of the generated THz carrier are evaluated for both methods at different tuned THz carrier frequencies. The demonstrated methods confirm that a high quality tuneable THz carrier can easily be implemented in systems where dynamic frequency allocation is required.

Composite vector quantization for optimizing antenna locations

In this paper, we study the location optimization problem of remote antenna units (RAUs) in generalized distributed antenna systems (GDASs). We propose a composite vector quantization (CVQ) algorithm that consists of unsupervised and supervised terms for RAU location optimization. We show that the CVQ can be used i) to minimize an \textit{upper bound} to the cell-averaged SNR error for a desired/demanded location-specific SNR function, and ii) to maximize the cell-averaged \textit{effective} \textit{SNR}. The CVQ-DAS includes the standard VQ, and thus the well-known squared distance criterion (SDC) as a special case. Computer simulations confirm the findings and suggest that the proposed CVQ-DAS outperforms the SDC in terms of cell-averaged ``effective SNR''.

Passive Opto-Antenna as Downlink Remote Antenna Unit for Radio Frequency Over Fiber

A novel remote antenna unit, intended for application in the downlink of an analog radio frequency over fiber system, is proposed, whose radiated power originates entirely from the optical signal supplied by a single multimode fiber. By operating the photodetector of the optical receiver at zero bias voltage, while omitting typical active components, such as transimpedance amplifiers, a fully passive unit, requiring no external power supply, is obtained. Instead, a careful co-design is performed to maximize the power transfer from photodetector to antenna within the frequency range of interest using an impedance matching network. A wideband cavity-backed slot antenna is implemented in an air-filled substrate-integrated-waveguide technology. The antenna feed plane serves as an integration platform for the optical receiver. The resulting downlink remote antenna unit is compact, cost-effective, energy-efficient, and extremely reliable. Therefore, it is an ideal building block for novel, highly specialized, ultrahigh density wireless communication systems, which require massive amounts of remote antenna units, deployed in attocells as small as ${\text{15 cm}} \times {\text{15 cm}}$ . A prototype operating in a wide frequency bandwidth ranging from 3.30 to 3.70 GHz is constructed and validated. In free-space conditions, a broadside gain of $-$ 0.2 dBi, a front-to-back ratio of 8.8 dB and linear polarization with a cross-polarization level of $-$ 28.8 dB are measured at 3.50 GHz. Furthermore, a $-$ 3 dB gain bandwidth of 500 MHz is observed. Finally, the prototype is deployed in a unidirectional data link, achieving a symbol rate of 80 MBd over a distance of 20 cm.

Analog Radio-Over-Fiber Supported Increased RAU Spacing for 60 GHz Distributed MIMO Employing Spatial Diversity and Multiplexing

The improvements in coverage through spatial diversity and increased data rates through spatial multiplexing using a distributed multiple-input multiple-output (MIMO) system are important targets for future wireless communications. Here, the appropriate separation of remote antenna units (RAUs) at several user locations in a millimeter-wave system is demonstrated. An analog radio-over-fiber (RoF) fronthaul is used to achieve flexible spacing of distributed RAUs and transports two Gb/s data streams over 2.2 km of fiber and up to 8 m of 60 GHz wireless transmission distance. A performance comparison is performed between single-input single-output and MIMO operation using different antenna spacing and transmission distance. Results show that the wider RAU spacing enabled by the RoF distribution provides improved results at longer distances, for both spatial diversity and for spatial multiplexing. Verification of a method for measuring each channel coefficient individually and using subsequent MIMO processing on these coefficients enables an extension to the results showing the feasibility of 30 m indoor transmission.

A novel bidirectional half-duplex fronthaul system using MMW/RoF and analog network coding

A novel bidirectional half-duplex fiber-wireless fronthaul system is proposed in this paper. Bidirectional relaying transmission is deployed in both optical fiber and wireless link thanks to millimeter-wave radio over fiber (MMW/RoF) and analog network coding (ANC) techniques. The architecture of the proposed fronthaul system is simplified significantly since it requires only an optical fiber with a single wavelength and a wireless link with a single MMW frequency. The detailed architectures of the subsystems such as the central station, the remote antenna unit, and the remote radio head are also designed. Mathematical expressions for the performance analysis of the proposed fronthaul system are derived considering the effects of various physical layer impairments. The numerical results demonstrate not only the feasibility of our proposed system but also the gain in terms of throughput of ANC-based relaying compared to conventional and digital network coding (DNC)-based relaying.

Energy-efficient power allocation scheme for distributed MISO system with transmit correlation

ABSTRACTThe energy efficiency (EE) of distributed multiple-input single-output (MISO) system is investigated in spatially-correlated Rayleigh channels, where large-scale fading including the path loss and the shadow fading is considered. The target is to maximize the EE which is defined as the ratio of the transmission rate to the total power consumed by the system subject to the maximum transmit power of each remote antenna unit (RAU) constraint. The EE optimization can be formulated as a nonlinear fractional programming problem. An optimal power allocation (PA) scheme for maximizing the EE is proposed. With this scheme, an iterative algorithm using Dinkelbach method is presented. To avoid iterative calculation, a low-complexity simplified PA scheme is developed based on the good channel information for antenna selection; it can provide a computational efficient method to acquire the optimal numbers of active RAUs. Therefore, the multidimensional optimization problem is converted to a single-dimensional optimization problem. As a result, the closed-form PA is attained. The simulation results validate the effectiveness of the proposed two schemes. The results indicate that these two schemes can achieve very similar performance, and the simplified scheme has lower complexity because no iteration is required.

Fully-differential transimpedance amplifier for reliable wireless communications

The demand of mobile and wireless devices has been continuously growing over the last years, driving the development of new wireless communication systems to provide high data capacity and good accessibility at high data transmission rates. Wireless communication systems must also show high immunity to interference, and in this context distributed systems formed by several remote antenna units (RAU) fed by multi-mode fibers are becoming the preferred solution. At high transmission data rate increases, issues brought by the variability in the fabrication of the circuits and ambient conditions (PVT variations) become more important, and it is necessary to design the systems to minimize their effect. In this work, a fully-differential transimpedance amplifier (TIA) for the receiver in a RAU is presented. The TIA has been designed in a 180-nm RF CMOS technology with a 1.8-V voltage supply and it achieves a bandwidth of 2.5 GHz with a power consumption of 26 mW. To overcome the effect of PVT variations, the TIA features a programmable transimpedance of 20-dB linear-in-dB programmable transimpedance, also increasing the input dynamic range to improve the overall linear range of operation.

Miniaturized 38GHz Circular Substrate Integrated Waveguide Band Pass Filter using Low Temperature Co-Fired Ceramic Technology

This study presents design approach for realizing miniaturized Substrate Integrated Waveguide (SIW) Band Pass Filter (BPF) using Low Temperature Co-fired Ceramic (LTCC) technology at TMRND's LTCC Lab. Design method for the SIW BPF is based on the circular cavity structure with four pole Chebyshev and operating at center frequency of 38 GHz. This SIW BPF is an important part of the Remote Antenna Unit (RAU) transceiver for the Radio over Fiber (RoF) system. Two types of circular SIW BPF have been designed and investigated in term of performance and structure size which are planar SIW BPF and compact SIW BPF. Both SIW BPF were developed using LTCC Ferro A6M materials with dielectric constant of 5.8, loss tangent of 0.002 and metallization of gold. The insertion loss for planar SIW BPF and compact SIW BPF were measured at 6.2 dB and 6.3 dB, respectively. The passband return losses for both types of the SIW BPF were measured at more than 10 dB. The size of the compact SIW BPF is 6.94×6.94 mm² meanwhile size for planar SIW BPF is 12.15×4.145 mm². The size of the compact SIW BPF is reduced to nearly 10% compared to a planar SIW BPF structure.

Downlink Spectral Efficiency of Distributed Massive MIMO Systems With Linear Beamforming Under Pilot Contamination

In this paper, the downlink spectral efficiency of multicell multiuser distributed massive MIMO systems with linear beamforming is studied in the presence of pilot contamination. According to the levels of effective channel gain information at user side, we provide the lower bound and upper bound on user ergodic achievable downlink rate. Due to the different access distance from each user to different remote antenna units, the entries of user channel vectors are no longer identically distributed in distributed massive MIMO systems, which makes the spectral efficiency analysis challenging. Using the properties of Gamma distributions together with the approximate methods for nonisotropic vectors, we derive tractable, but accurate closed-form expressions for the rate bounds with maximum ratio transmission (MRT) and zero-forcing (ZF) beamforming in distributed massive MIMO systems. Based on these expressions, user ultimate achievable rates are also given when the ratio of the total number of transmit antennas to the number of users goes to infinity. It is shown that MRT and ZF beamforming achieve the same ultimate rate no matter what levels of effective channel gain information at user side. Numerical results show that ZF achieves better performance gain and faster convergence speed than MRT. When the coherence interval is large, the downlink beamforming training scheme is more preferable for the distributed massive MIMO systems.

Energy Efficiency Optimization for MIMO Distributed Antenna Systems With Pilot Contamination

In this paper, we study the energy-efficiency (EE) maximization problem for a multiple-input multiple-output distributed antenna system (DAS) with pilot contamination. With per-user quality of service constraints and per remote antenna unit (RAU) power requirements, we formulate the EE maximization problem as a joint optimization of sparse transmit beamforming, RAU selection, and RAU clustering. The considered problem is a non-convex multivariate optimization problem. To solve the problem, we transform it to an equivalent parametric programming problem (PPP) with a given EE parameter and design a two-layer optimization scheme to solve the original problem. The outer layer involves two kinds of algorithms to iteratively update the EE parameter based on Dinkelbach’s algorithm and bi-section search, respectively. The more challenging issue lies in the inner loop, where a non-convex multivariate PPP needs to be tackled. A series of techniques, including the reweighted $\ell_\mathbf {1}$ -norm, D.C. function, and semidefinite relaxation (SDR), is adopted to approximate the non-convex multivariate PPP with a convex SDR problem. Furthermore, a heuristic algorithm is proposed to reduce the complexity of a two-layer scheme. Simulation results show that the proposed algorithms significantly improve the EE and demonstrate that RAU selection and RAU clustering contribute to a higher EE.

50 GHz optical true time delay beamforming in hybrid optical/mm-wave access networks with multi-core optical fiber distribution

We propose and experimentally validate an optical true time delay beamforming scheme with straightforward integration into hybrid optical/millimeter (mm)-wave access networks. In the proposed approach, the most complex functions, including the beamforming network, are implemented in a central office, reducing the complexity and cost of remote antenna units. Different cores in a multi-core fiber are used to distribute the modulated signals to high-speed photodetectors acting as heterodyne mixers. The mm-wave carrier frequency is fixed to 50 GHz (V-Band), thereby imposing a progressive delay between antenna elements of a few picoseconds. That true time delay is achieved with an accuracy lower than 1 ps and low phase noise.

Benefits of Beamforming Training Scheme in Distributed Large-Scale MIMO Systems

The downlink training yields rather modest performance gains due to the high degree of channel hardening effect in co-located large-scale multi-input multi-output (MIMO) systems. However, in distributed large-scale MIMO systems, the potential benefits of the downlink training become larger, since there is a high probability that each user is very close to only a part of remote antenna units and is effectively served by them which results in less channel hardening effect. In this paper, we study the benefits of the beamforming training (BT) scheme (users estimate the channel state information by using the received training symbols, which are processed by linear beamforming in advance) in distributed large-scale MIMO systems. We derive accurate and tractable closed-form analytical expressions for the spectral efficiency (SE) of the BT scheme in distributed large-scale MIMO systems with maximum ratio transmission and zero-forcing (ZF) beamforming. Based on these expressions, we analyze the benefits of the BT scheme and investigate how the number of transmit antennas and the length of coherence interval affect the SE of the BT scheme in the co-located and distributed large-scale MIMO systems. The analytical results show that the BT scheme is more suitable for distributed large-scale MIMO systems with ZF beamforming, and a less number of transmit antennas and longer coherence interval can improve the performance gain of BT scheme.

Image-Free Microwave Photonic Down-Conversion Approach for Fiber-Optic Antenna Remoting

For commercial and military applications, it is highly desired to acquire broadband radio frequency (RF) signals from remotely located receiving antennas to the central office (CO) for centralized signal processing such as down-conversion. In this paper, a photonic approach for image-free microwave frequency down-conversion is proposed for antenna remoting scenarios. The RF signal is captured by the phase modulator (PM) at remote antenna unit and transmitted over a long fiber link to the CO for performing frequency down-conversion. In analog, with the Hartley architecture in the electronic domain, two in-phase (I) and quadrature (Q) intermediate frequency (IF) components are generated by using an electro-optic polarization modulator (PolM) and two polarizers. To achieve large image rejection ratio (IRR), the digital post-processing technique is introduced to accurately compensate the amplitude and phase imbalances between the I and Q IF signals. In the experiments, a 2-km single-mode fiber link is inserted between the PM and PolM. As the local oscillator signals are set as 35 and 3 GHz, two target sinusoidal signals at 35.5 and 3.1 GHz are applied with two image signals, a sinusoidal signal at 34.5 GHz and a broadband RF signal centered at 2.9 GHz, respectively. Then, both real-time analog and off-line digital processing methods are used to process the generated I and Q IF signals for image rejection, yielding an IRR over 45 and 60 dB respectively, when the sinusoidal image signal is applied. The distortions from the broadband 2.9-GHz image RF signal are also effectively suppressed by using the two processing methods. The proposed approach is capable of covering a wide frequency range from 5 to 40 GHz.

A Detection‐Switching‐Assisted DCF Mechanism for Simulcast WLAN‐over‐Fiber Based Distributed Antenna Systems

Distributed antenna systems (DASs) using Radio-over-fiber (RoF) links is a promising candidate to provide flexible coverage and high data rates. For simulcast Wireless local-area network (WLAN) use in RoF systems, the stations covered by different Remote antenna units (RAUs) contend for the wireless medium using Media access control (MAC) mechanism. Severe inter-RAU hidden nodes problem will happen due to the stations in different RAUs cannot detect each other very well. In order to alleviate this inter-RAU collision problem, a promising detection-switching-assisted Distributed coordination function (DCF) mechanism is presented using in the uplink. The control delay issues of the proposed mechanism are also discussed in this paper.

Energy and electromagnetic pollution considerations in ARoF-based multi-operator multi-service systems

Energy-efficient system design with controlled levels of electromagnetic pollution, without compromising on the user quality of experience and operator revenue, are essential considerations in the design of next-generation wireless communication networks. Integration of multiple services and multiple operator transmissions onto a common shared infrastructure is the solution forward to satisfy the contradictory subscriber and operator requirements and also to derive a future-proof fronthaul to support new techniques like opportunistic, co-operative and cognitive communications. This paper reports the link budget analysis, energy efficiency estimation and electromagnetic radiation calculation for a Multi-Operator Multi-Service Analog Radio over Fiber fronthaul for a small cell configuration. The BER analysis and the link budget analysis are presented for the central base station to remote antenna unit (RAU) link as well as the RAU to user equipment link. The estimated minimum transmission power sufficient to establish and maintain a quality connection suggests the possibility of eliminating the energy consuming power amplifiers in the transceivers and also achieve a reduced electromagnetic pollution in the environment.

Numerical Analysis of the Performance of Millimeter-Wave RoF-Based Cellular Backhaul Links

AbstractIn this paper, we study the performance of a next-generation cellular backhaul network that is based on a hybrid architecture using radio-over-fiber (RoF) and millimeter-wave (MMW) techniques. We develop a mathematic model and comprehensively analyze the performance of a MMW/RoF-based backhaul downlink under the impacts of various physical layer impairments originated from both optical fiber and wireless links. More specifically, the effects of nonlinear distortion, chromatic dispersion, fading, and many types of noises including shot noise, thermal noise, amplifier noise, and relative intensity noise are investigated. The numerical results show that the nonlinear distortion, fiber dispersion, and wireless fading are key factors that limit the system performance. Setting the modulation index properly helps minimize the effect of nonlinear distortion while implementing dispersion shifted optical fibers could be used to reduce the impact of dispersion and as a result, they can improve the bit-error rate. Moreover, it is also verified that, to mitigate the effect of multipath fading, remote radio heads should be located as near the remote antenna units as possible.