A First Experimental Demonstration of Analog MIMO Radio-over-Copper

09/14/2018 ∙ by Andrea Matera, et al. ∙ TIM Politecnico di Milano Consiglio Nazionale delle Ricerche 0

Analog MIMO Radio-over-Copper is an effective fronthaul architecture that exploits pre-existing LAN cabling building infrastructures to provide enhanced indoor coverage. Although LAN cables contain 4 twisted-pairs with up to 500 MHz bandwidth/each over 200 m, today's commercial in-building solutions employ mainly the low frequency portion of LAN cable bandwidth, mostly due to the moderate cable attenuation and crosstalk among wire pairs. Several works proposed simulations that support the possibility to use LAN cables up to several hundreds of MHz. However, an experimental evaluation is still missing. In this letter, we demonstrate experimentally, and for the first time, the feasibility of the analog relaying of MIMO radio signals over LAN cables up to 400 MHz, thus efficiently exploiting the LAN cables transport capabilities for 5G indoor applications.

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I Introduction

[findent=2pt]C loud Radio Access Network (C-RAN) architecture is a very attractive solution to handle a large number of users and antennas in the same radio resources, as demanded by next generation (5G and beyond) networks [1, 2]. C-RAN architecture is enabled by: i) the colocation of BaseBand units (BBUs) in so-called BBU pools, where centralized processing and interference mitigation is performed; ii) the remotization of antennas and RF equipment (Remote Radio Unit, RRU) that are moved away from the BBU and geographically distributed both indoor and outdoor close to the end-users. C-RAN is already deployed in current (4G) mobile networks, in which BBU and RRUs exchange over the so-called FrontHaul (FH) link digital In-phase and Quadrature (I/Q) signals [3]. However, the expected increase in radio signal bandwidth due to digitization calls into question the effectiveness of digital I/Q streaming, especially for multiple-antenna RAUs, thus favoring a fully analog FH link where the RRUs directly relay the analog signals to/from the BBU. Analog relaying of RF signals over the FH is a promising solution for ultra-low latency applications where latency is ultimately limited to signal propagation, and also enables precise bit/carrier-frequency synchronization among multiple RAUs for MIMO joint processing. [4, 5, 6, 7].

In this direction, C-RAN with analog FH based on LAN cables, namely Analog Radio-over-Copper (A-RoC), has been shown to be a cost-effective and bandwidth-efficient solution for upcoming 5G indoor networks [8, 9].

LAN cables contain 4 twisted-pairs bonded together that have a transport capability that depends on cable lengths, such as 500 MHz bandwidth/pair for 200 m cable. These twisted-pairs provide 4 space-separated and mutually interfering channels leading the Analog MIMO RoC (A-MIMO-RoC) concept [8]. Existing in-building commercial solutions based on LAN cables for the FH transmission use mainly the first few tens of MHz of LAN cable frequency bandwidth [5], for which cable attenuation and intra-cable crosstalk are mild. On the contrary, A-MIMO-RoC considered here, by leveraging a smart mapping between radio and cable resources, pushes the frequency cable usage up to several hundreds of MHz, as shown in [8, 10, 11, 12]. Although these theoretical works succeeded in showing the effectiveness of A-MIMO RoC in terms of equivalent wireless capacity by numerical analysis, an experimental A-MIMO-RoC demonstration is still missing, and is the focus of this letter.

This letter demonstrates experimentally, and for the first time, the potential of A-MIMO-RoC architecture as an effective solution for perspective 5G indoor networks. A prototype has been developed in order to prove the capability of LAN cables to carry multiple LTE signals over the same LAN cable. In particular, the experiment demonstrates the feasibility of transporting, in an all-analog fashion, multiple MIMO LTE signals over multi-pair copper cables at high cable frequency (i.e., impaired by high intra-cable crosstalk and cable attenuation [8]) with negligible performance degradation, which enables a more efficient exploitation of LAN cables transport capabilities. For the purposes of this experiment, the TRIANGLE testbed [13, 14] played a fundamental role in measuring the end-to-end performance degradation introduced by the proposed A-MIMO-RoC architecture.

Ii The TRIANGLE Testbed

Figure 1: Experimental Setup for A-MIMO-RoC

The TRIANGLE testbed is the main core of the H2020 TRIANGLE project [14], whose objective is to promote the testing and benchmarking of 5G mobile applications and User Equipments (UEs). In particular, the TRIANGLE testbed allows to run multiple tests in a controlled environment that encompasses all the elements of the telecommunication chain, from radio signal generation to the end-to-end testing of mobile applications. An extensive description of the TRIANGLE testbed is outside the scope of this letter. Details can be found in [13, 14], while here we review only the main elements that have been employed for the A-MIMO-RoC system.

Ii-a Keysight UXM RAN Emulator

The UXM Wireless Test Platform device by Keysight Technologies allows to emulate multiple cellular networks in a controlled manner and supports multiple Radio Access Technologies (multi-RAT), including GSM/GPRS, UMTS and LTE-Advanced networks (i.e., 2G, 3G, 4G and 4.5G). The UXM features include intra-RAT and inter-RAT handovers, protocol debugging, IP end-to-end delay, BLock Error Rate (BLER) and throughput measurements, in addition to the possibility of performing RF conformance tests. The UXM offers great customization capabilities and a very intuitive user interface, which allow to easily evaluate the Key Performance Indicators (KPIs) of interest under any possible system setting defined by the 3GPP. Moreover, the UXM features an advanced fading engine with the main channels models defined by 3GPP [15]. In our specific case, the UXM has been employed to generate the MIMO LTE radio signal and to emulate specific propagating channel conditions, e.g., the ETSI Pedestrian at 5 Hz (EPA5) channel model, which fairly describes the indoor office scenario targeted by the A-MIMO-RoC system.

Ii-B Mobile Device Monitoring

In order to measure the KPIs perceived by the end-device, some additional tools, detailed below, are needed on the smartphones under test (i.e., the UE), according to the scenario in Fig. 1.

Ii-B1 Performance Tool

In order to guarantee high resolution reporting of the target QoS KPIs, especially in 5G scenarios, the DEKRA Performance Tool has been integrated into the TRIANGLE testbed. Beside providing accurate one-way measurements, the DEKRA Performance Tool tool includes a built-in traffic generator and enables the automation and testing of Android mobile Apps by measuring relevant Quality of Experience (QoE) KPI. In particular, the DEKRA Performance Tool was employed to perform end-to-end YouTube application testing, which allowed us to quantify the performance degradation introduced by the relaying over cable in terms of YouTube video streaming quality. These results have not been reported here for brevity.

Ii-B2 TestelDroid Mobile Monitoring App

TestelDroid is a software tool developed by the University of Malaga (UMA) [16] that enables passive monitoring of radio parameters and data traffic in Android-based devices. Logging is implemented as an Android service that can be running in the background logging all the information while the application under test, e.g., YouTube, is being executed. This functionality enables monitoring of the traffic information generated by any application, which extends the testing to a very wide range of use cases. The logged parameters (network, neighbor cells, GPS, traffic) can be flexibly configured.

Iii A-MIMO-RoC Experiment Setup

For testing A-MIMO-RoC, the TRIANGLE testbed has been used in the typical device-testing configuration [14], with the only difference that a 4-pairs RJ45 Cat-5e LAN cable has been inserted between the RF output ports of the UXM and the RF connections at the device (see Fig. 1) to evaluate experimentally the performance degradation introduced by the all-analog relaying over copper. As shown in Fig. 1, the A-MIMO-RoC system is composed by two identical LAN-to-coax converters that perform impedance adaptation, cable equalization, coax-to-pairs mapping/demapping, and RF/IF conversion. Both converters include all-passive and all-analog devices along the signal path in order to perform fully bi-directional operations. For all wireless communications performed by exploiting the over A-MIMO-RoC platform, we used a 50 m long Cat-5e LAN cable (which is the cable type commonly deployed in buildings) with a cable bandwidth experimented here up to 400 MHz per twisted-pair.

The experiment setup is represented in Fig. 1, together with a simplified block diagram detailing the role of each of the components used for the experiments (i.e., RAN Emulator, LAN-to-coax converters, LAN cables, and UE). In particular, the experiment setup is as follows (only the downlink is described, being the uplink symmetrical):

  • up to 4 RF LTE signals are generated by the UXM;

  • RF cables are connected at each RF output of the UXM;

  • the signal carried on each RF cable is down-converted to Intermediate Frequency (IF) to match the bandwidth over the LAN cable (e.g., in the 10 ÷ 400 MHz frequency range), possibly multiplexed in frequency over cable by the first LAN-to-coax converter;

  • each IF-converted signal is conveyed by one of the 4 twisted-pairs: cable adaptation/equalization, coax-to-pairs mapping and RF/IF down-conversion between coax and twisted-pair is performed by the LAN-to-coax converter;

  • at the other end of the LAN cable, RF cables are connected to the RF connectors of the second LAN-to-coax converter that performs cable adaptation/equalization, pairs-to-coax de-mapping and IF/RF up-conversion to interface with the UE under test;

  • DEKRA Performance Tool and TestelDroid, integrated into the TRIANGLE testbed, are used to test the UE.

Iv Experimental Results

The goal of this experiment is to demonstrate the viability of remotizing RF antennas by relaying multiple LTE signals over the same 50 m Cat-5e LAN at high frequency, and to evaluate the end-to-end performance degradation introduced by the copper cable. The LTE system settings parameters are reported in Table I.

Table I: LTE system settings

In particular, we tested the relaying over copper cable of a MIMO LTE signal. The performance have been evaluated in the DownLink (DL) direction in terms of throughput and BLER for different Modulation and Coding Schemes (MCS), ranging from 0-QPSK to 17-16QAM [17]. Two main experiments, detailed in the following sections, have been performed for testing the A-MIMO-RoC architecture with LTE signals

Iv-a A-MIMO-RoC and cable IF

The goal of this experiment was two-fold: i) to prove the feasibility of transporting the 2 RF bands, corresponding to the 2 MIMO TX ports, over 2 different twisted-pairs of the LAN cable, but at the same cable IF frequency (), thus interfering each other over cable (see Fig. 2); and ii) to evaluate the performance degradation by increasing , i.e., by employing for which interference among pairs and attenuation are more severe [8].

Figure 2: RF signal mapping over cable: 2 LTE bands with bandwidth mapped over two different cable-pairs at the same cable-IF.
Figure 3: A-MIMO-RoC performances (Throughput and BLER) vs. for MHz and different modulation schemes.
Figure 4: A-MIMO-RoC performances (Throughput and BLER)vs. for MHz and different modulation schemes.

In Fig. 3 and 4 throughput and BLER are shown considering a Static MIMO channel, -20 dBm/ input power, and three different cable values for , as shown in Fig. 2: MHz (blue curves), MHz (red curves) and MHz (yellow curves). The maximum theoretical throughput achievable by each MCS [17] (black dashed line) over the considered channel bandwidth () is shown as reference, thus quantifying the performance degradation introduced by the proposed system. In case of MHz, shown in Fig. 3, the loss in terms of throughput introduced by A-MIMO-RoC is almost negligible for all the considered MCS and cable . As expected, BLER increases for high MCS, but the degradation with respect to the maximum achievable throughput is still small. However, for MHz, shown in Fig. 4, the maximum throughput is achieved for all the MCS only for MHz, while for MHz and MHz the maximum MCS that can be employed with fairly low BLER are 15-16QAM and 11-16QAM, respectively. This performance degradation, which is more pronounced for higher , is totally expected, and it is explained by the fact that both cable crosstalk and attenuation increase with cable frequency [8]. Moreover, the input power is the same for both settings, i.e., MHz and MHz (see Table I). Hence, to smaller channel bandwidth it corresponds higher Signal-to-Noise Ratio (SNR) per subcarrier, which explains the better performance obtained in case of MHz. Notice that the LAN-to-coax converter is an all-passive device, and the relaying over cable introduces a significant attenuation (in the order of 60 dB) which forces the whole communication system to work close to the UE sensitivity [17], even by setting the input power to the maximum value allowed by the hardware devices. As a consequence, it is expected that the performance loss observed in case of MHz can be avoided by introducing some signal amplification in the system design, which is left as future works. In any case, Fig. 4 confirms the feasibility of relaying LTE signals over high frequency copper cable bands: even in the worst case of 2 LTE bands transported over 2 different cable twisted-pairs at the same MHz, it is still possible to achieve a throughput of approx. 10 Mbps over MHz.

Iv-B A-MIMO-RoC and Channel Models

The goal of this experiment was to evaluate how the A-MIMO-RoC platform performs in different MIMO channel model scenarios. In particular, we adopted the ETSI Pedestrian at 5 Hz (EPA5) channel, as it fairly describes the propagating conditions of an indoor office environment, which is the target application scenario for the A-MIMO-RoC platform. We considered a MIMO LTE signal with MHz, where the 2 RF bands were transported over 2 different twisted-pairs at the same frequency MHz. In this experiment the input power was set to -15 dBm/.

Figure 5: A-MIMO-RoC vs Channel Model and MCS, MHz

Fig. 5 shows that, in the case of Static MIMO channel, even for MHz, the performance degradation introduced by the cable is negligible (only a small loss can be appreciated for high MCS). However, in case of EPA5 channel model, performance get worse, especially for medium and high channel correlation. Once again, the reason for this is the all-passive circuitry implementation of our system, that simplifies the hardware design of the prototype but introduces a significant attenuation. In fact, the power fluctuations introduced by the EPA5 channel make the system oscillate around the UE sensitivity threshold for high MCS, which explains the curves behavior in the rightmost portion of the figure. Fig. 5 shows that, even in the worst case of high-correlated EPA5 channel, it is still possible to transmit 16QAM modulation achieving slightly less than 20 Mbps.

V Concluding Remarks & Future Works

This paper demonstrates experimentally, and for the first time, the feasibility of an Analog MIMO Radio-over-Copper prototype to transparently relay multiple LTE signals over a single LAN cable in an all-analog fashion (i.e., without latency), even at high cable frequency. In particular, the conclusions drawn by the experiments can be summarized as follows: i) LAN cables bandwidth capability can be exploited up to several hundreds of MHz for transparently transporting MIMO RF signals over the last 100-200m, and this happens even when such signals are transported over different cable pairs but at the same IF, thus being affected by cable-crosstalk interference; ii) the performance degradation experienced for high MCS and high IF is mainly due to the low signal power received at the user device due to the attenuation introduced by the analog relay over cable; iii) again, due to the whole system attenuation, the prototype developed appears to be quite sensitive to signal power variations, and this reflects in some performance degradation when employing non-static MIMO channel models, e.g., ETSI Pedestrian at 5 Hz. For future developments of our technology, although the fully passive implementation considered here has substantially simplified the hardware design, the experiment results suggest that most of the issues encountered might be easily solved by introducing some active circuitry such as active equalization, amplification and power adaptations in the LAN-to-coax converters.

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