Wireless Communications and Mobile Computing

Volume 2018, Article ID 2423837, 11 pages

https://doi.org/10.1155/2018/2423837

## Real-Time Emulation of Nonstationary Channels in Safety-Relevant Vehicular Scenarios

^{1}Department of Electronic Systems, Norwegian University of Science and Technology, Trondheim, Norway^{2}Institute of Telecommunications, TU Wien, Vienna, Austria^{3}Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal

Correspondence should be addressed to Thomas Blazek; ta.ca.neiwut@kezalb.samoht

Received 29 September 2017; Accepted 18 February 2018; Published 8 May 2018

Academic Editor: Marceau Coupechoux

Copyright © 2018 Golsa Ghiaasi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper proposes and discusses the architecture for a real-time vehicular channel emulator capable of reproducing the input/output behavior of nonstationary time-variant radio propagation channels in safety-relevant vehicular scenarios. The vehicular channel emulator architecture aims at a hardware implementation which requires minimal hardware complexity for emulating channels with the varying delay-Doppler characteristics of safety-relevant vehicular scenarios. The varying delay-Doppler characteristics require real-time updates to the multipath propagation model for each local stationarity region. The vehicular channel emulator is used for benchmarking the packet error performance of commercial off-the-shelf (COTS) vehicular IEEE 802.11p modems and a fully software-defined radio-based IEEE 802.11p modem stack. The packet error ratio (PER) estimated from* temporal averaging* over a* single* virtual drive and the packet error probability (PEP) estimated from* ensemble averaging* over* repeated* virtual drives are evaluated and compared for the same vehicular scenario. The proposed architecture is realized as a virtual instrument on National Instruments™ LabVIEW. The National Instrument universal software radio peripheral with reconfigurable input-output (USRP-Rio) 2953R is used as the software-defined radio platform for implementation; however, the results and considerations reported are of general purpose and can be applied to other platforms. Finally, we discuss the PER performance of the modem for two categories of vehicular channel models: a vehicular nonstationary channel model derived for urban single lane street crossing scenario of the DRIVEWAY’09 measurement campaign and the stationary ETSI models.

#### 1. Introduction

The key characteristics of vehicle-to-anything (V2X) propagation channels are their nonstationarity, shadowing by other vehicles, and high Doppler shifts [1].

On a small scale in time and space, the channel characteristics in safety-relevant vehicular scenarios exhibit significant delay-Doppler spreads and the channel’s spreading is varying over time and frequency. These are due to several factors: major Doppler shifts are caused by moving transmitters, receivers, and mobile interacting objects in the multipath propagation channel. Many other details shape the local delay spread and Doppler spectral characteristics of V2X channels, for example, antenna patterns and the angular statistics of individual multipath components (MPCs) [2].

On a larger spatial and temporal scale, the* statistics* of the channel characteristics due to the surrounding environment, link and shadowing geometry, and velocities of physical objects evolve. On this coarser scale, the V2X multipath propagation channel exhibits nonstationarity [3, 4] and such channels do not belong to the well-known class of wide-sense stationary uncorrelated scattering (WSSUS) models. Both the small- and the large-scale effects have significant influence on the reliability and latency of V2X packet transmission.

Aiming at standardized conformance tests of V2X radio modems, both IEEE and ETSI have proposed channel models [5, 6] which reproduce the short-term delay-Doppler characteristics in various vehicular scenarios [2, 7]. These channel models belong to the class of WSSUS models because neither the channel’s tap delays nor their Doppler spectrum evolves in time. As a consequence, the local scattering function [3, 4] of these channel models becomes independent of time and frequency. The channel models in [6] will be referred to as ETSI models in this work for simplicity.

Recent research on real-world channel sounder data acquired in vehicular scenarios [3] proved that the WSSUS channel assumptions are violated for V2X transmission durations longer than 40 ms and bandwidths larger than 40 MHz. Thus, the ETSI models are not necessarily relevant to vehicular scenarios of longer duration and/or larger bandwidth.

In order to reproduce the varying Doppler and delay statistics of V2X channels on a larger scale, it is essential to allow non-WSSUS channel models featuring nonstationary fading on individual MPCs. For emulating such V2X channels, finite regions in the time-frequency plane are defined in which the process is assumed to be locally stationary [4]. Within such local stationarity region, the V2X channel is modeled adequately by means of the well-known tapped-delay line architecture [8]. In this contribution, we report on a V2X channel emulator for transmissions occupying up to 20 MHz of bandwidth and scenario durations much longer than 40 ms. For the computation of the channel output signal in real time, this means that the gain coefficients of a tapped-delay line representation need to be updated whenever the signal leaves the local stationarity region in time.

To reproduce the V2X channel characteristics in a lab setting, a channel emulator is placed between the transmitter (TX) and the receiver (RX) and connected by RF coaxial lines. This facilitates reproducible tests and measurements and has proved to be a cost-effective means for benchmarking of the V2X radio modem performance before heading for costly real-word drive tests. For safety-relevant applications, the emulated channel models for modem benchmarking shall represent somewhat harsher than typical V2X channel conditions. This approach is more conservative than aiming at accurately representing the V2X channel.

There are several commercial solutions available for real-time channel emulation [9, 10]. They offer adequate solutions for the emulation of WSSUS channels and their specifications are targeted for cellular network applications. In [11], researchers have reported a field-programmable gate array- (FPGA-) based emulator for stationary vehicular environments based on the Ingram-Acosta models [12] which belong to the WSSUS class. To the best of our knowledge, all these lack the capability to emulate non-WSSUS channels as observed in safety-related vehicular scenarios [3]. In [13], a channel emulator for vehicular modem stress tests is reported. For the stress test, an idealized channel with just two taps of equal gain is emulated. The first tap models the LOS component with zero delay and zero Doppler and the second tap models a specular reflection with delay and Doppler . Reference [13] served as a starting point for our design.

In this work, we present the design and implementation of an emulator on a software-defined radio (SDR) platform. FPGAs are suitable options which offer flexibility in design process along with shorter time needed for prototyping; however, their finite resources, such as gate count and data and clock rates, impose limitation on the design of the emulator. Therefore the main challenge is to optimize the trade-off between reconfigurability of the system while keeping the computational complexity as low as possible. The chosen platform is National Instruments universal software radio peripheral with reconfigurable input-output (USRP-Rio) 2953R which operates at frequencies up to 6 GHz and processes signals with up to 40 MHz bandwidth. It features two multiple-input multiple-output (MIMO) RF chains along with a Xilinx Kintex-7 FPGA which is programmable with the National Instruments LabVIEW FPGA Module [14]. The emulator documented in this work can either playback* recorded* channel impulse responses or emulate stochastic channel models [2, 6]. In doing so, the real-time emulator supports taps with fixed and variable delays and Doppler shifts and facilitates the Monte Carlo realizations of path-loss and phase variables.

We use the emulator to evaluate the benchmarking experiments in terms of the packet error ratio (PER) and the packet error probability (PEP) for several vehicular commercial off-the-shelf (COTS) modems and two sets of fading channels: ETSI channel models and a low complexity nonstationary channel model derived from the V2V urban crossing scenario channel impulses measured in the DRIVEWAY’09 campaign [15].

This manuscript is organized as follows: firstly, we present the overview of a continuous time delay- and time-variant channel as an input-output system. In Section 3, we discuss the structure of the emulator with a focus on generating sampled traces for accurate representation of the fading channels while abiding to the physical restrictions of the hardware. The design of the real-time emulator is presented in Section 4. The testbed configuration for the benchmarking tests is explained in Section 5. The description of the tests along with the experimental results is presented in Section 6. The measurements are conducted by analyzing the emulation performance for both presented model types, and then benchmark measurements of two types of modems are conducted.

#### 2. Channel Emulation

The small-scale fading characteristics of the V2X channel within a stationarity region are modeled by complex-valued stochastic processes representing the gains and phase shifts on individual MPCs. The continuous time delay- and time-variant impulse response of the V2X channel is represented as a superposition of MPCs: where is the number of the relevant MPCs defined by significant received power level, and is in which is a positive constant attenuation and is a real number representing the Doppler shift of the th MPC, respectively. Furthermore, the small-scale fading of the th MPC is modeled by the pair of stochastic processes and . The delays of the MPCs themselves are also time-dependent. To carry out the calculation of the time-discrete output signal sample stream representing the continuous-time function sampled at a sampling period of , the V2X channel emulator implements a discrete-time convolution of the time-varying channel impulse response (1) and the input signal sample stream , that is, The output needs only be calculated for with :

#### 3. Emulator Structure

In our previous work in [17], we presented a general systematic approach to emulation of a geometry-based stochastic channel model. This structure or its subsets can serve as a platform for other classes of channel models, such as stochastic models and models formed by recorded impulse response. Equation (4) indicates that in order to perform a real-time emulation, it is required to generate a sampled trace which realizes the fading properties of the channel and then convolve the trace with samples of the transmitted signal at the emulator. Since the data exchange and sampling rates in the emulator are limited, we would need to choose the parameters of the trace in an effective manner, so as to maximize the accuracy of the model while optimizing the implementation complexity and usage of hardware resources. To accurately compute the vehicular channel models, it is required to evaluate a large number of MPC parameters (namely, attenuation, path delays, and angles of arrival and departure) and to sum them up in real-time. Therefore, we apply a complexity reduction method aiming at determining the minimum value for which still achieves the accuracy desired for the channel models, hence reducing the number of gates needed on the FPGA for real-time computation. For instance, in [16], LASSO regression has been applied to sets of recorded impulse responses in DRIVEWAY’09 vehicular measurement campaign [3, 7], resulting in a nonstationary parsimonious model which has been benchmarking using the emulator in Section 6. Similarly, the model reduction deployed in [18, 19] utilizes clustering technique in which the paths with similar properties in terms of delay and Doppler spread are grouped and represented as a cluster. For instance one cluster could represent different points of a single moving object which obviously exhibit similar delay and Doppler spreads. This model was used in [17] for benchmarking the set of commercial modems in V2V scenario on highway with obstructed line of sight.

After setting the parameters such as center frequency and the set of relevant MPCs and their associated parameters, we would need to determine the required update rates for each set of parameters in order to maximize the channel accuracy and to account for rapid variation in the vehicular channel. Depending on the vehicular scenario, it may be required to account for time-varying antenna patterns due to changes in their orientation and antenna adaptivity. These parameters have different variation rates: the variation in shadowing relates to large-scale fading and is characterized by the stationarity time of the channel. The required update rate is low compared to the channel’s complex-valued tap weights which are modeled as stochastic processes with the Doppler bandwidth of the channel.

For the purpose of in-lab tests, the traces are calculated offline and the resulting samples forming the s are stored in a fast memory, which streams the corresponding parameters to the FPGA (referred to as time-variant convolution module), which carries out the real-time calculation of output (RX) samples by convolving the discrete-time varying impulse response of the channel with the input samples (from TX) according to (4). We will look into architecture of the time-varying convolution module in Section 5.

#### 4. Real-Time Emulator Design

Since time-variant convolution is the module which performs the real-time calculation in the FPGA, its architecture is the key to achieving the trade-off between complexity and accuracy. The first option is to implement the most general from of convolution by calculation of the discrete version of (4) (shown in (5)). The impulse response coefficients are complex-valued and given by (2). This architecture is capable of emulating any linear time-varying channel response by simple operations: complex number multiplication and additions: Here, ; therefore is the number of samples that a tap is offset. The drawback is that the rate of update of s is determined by the necessary rate for accurate modeling of elements. This rate may become too high to stream to FPGAs in typical SDRs.

For this reason, we implement a tapped-delay line architecture where each tap is parameterized by its delay, weighted in magnitude, and phase-rotated. The Doppler shift associated with a tap is explicitly parameterized which significantly reduces the required update rate for the channel parameters. Due to the time-discrete nature of the FPGA implementation, the architecture realizes only discrete delays and assumes that delay lags are on the sampling grid; hence, the MPC delay lags which are off-grid are approximated by shifting them to one of the neighboring on-grid delay (in a future extension of this work, fractional delays are foreseen as well).

The architecture shown in Figure 1 implements this tapped-delay line architecture restricted to on-grid delays according to the time-discrete approximation of (4), that is,