Pseudo-Analog

A Design of SDR-based Pseudo-Analog Wireless Video Transmission System
Xiao-Wei Tang, Student Member, IEEE, Xin-Lin Huang*, Senior Member, IEEE,

arXiv:2005.04558v1 [eess.SP] 10 May 2020
Abstract—The pseudo-analog wireless video transmission tech- nology can improve the effectiveness, reliability, and robustness of the conventional digital system in video broadcast scenarios. Although some prototypes of IEEE 802.11 series have been developed for researchers to do simulations and experiments, they are usually expensive and provide very limited access to the physical layer. More importantly, these prototypes cannot be used to verify the correctness of the new proposed pseudo- analog wireless video transmission algorithms directly due to limited modulation modes they can support. In this paper, we present a novel design of software radio platform (SDR)-based pseudo-analog wireless video transceiver which is completely transparent and allows users to learn all the implementation details. Firstly, we prove that the analog method can also achieve the optimal performance as the digital method from the perspective of the rate-distortion theory. Then, we describe the two hardware implementation difficulties existed in the designing process including the data format modification and the non- linear distortion. Next, we introduce the implementation details of the designed transceiver. Finally, we analyze the performance of the designed transceiver. Specifically, the results show that the designed system can work effectively in both simulations and experiments.
Index Terms—SDR, Pseudo analog transmission, GNU Radio, USRP.

1. 
   INTRODUCTION
   

The software radio (SDR) platform is a programmable radio that allows users to access all data in the form of physical waveforms and to implement all signal processing steps on software [1]. This feature makes the SDR platform particularly suitable for developing early transceivers which can be used to verify the effectiveness of new proposed algorithms. More importantly, the open source SDR platform can switch between simulations and experiments seamlessly, which helps bridge the gap between theory and practice and reveal potential defects in the system design.
Although many companies, e.g., Cohda Wireless, NEC, and Denso, have developed prototypes of IEEE 802.11 series [2], these prototypes are usually expensive and provide lim- ited access to the physical layer. Differently, the completely transparent SDR-based transceiver can allow users to access all the implementation details and even to modify the de- tails when necessary, whose general structure is presented in Fig. 1. There are mainly eight modules in the SDR- based transceiver [3] including: 1) the communication module
Xiao-Wei Tang (email: [e-mail]) is with the De- partment of Control Science and Engineering, Tongji University, Shanghai 201804, China.
Xin-Lin Huang (email: [e-mail]) is with the Department of Information and Communication Engineering, Tongji University, Shanghai 201804, China (corresponding author).
(e.g., SDR platform), 2) the driver module (e.g., universal software radio peripheral (USRP) hardware driver (UHD)),
3) the operating system module (e.g., system library, system interface, and kernel), 4) the interface module (e.g., universal serial bus (USB)/peripheral component interconnect express (PCIe)), 5) the control module (e.g., receive control), 6) the conversion module (e.g., digital up converter (DUC), digital down converter (DDC), digital-to-analog converter (DAC), and analog-to-digital converter (ADC)), 7) the filter module (e.g., filter), and 8) the power amplifier module (e.g., power amplifier).

Fig. 1 The general structure of SDR-based wireless transceivers.

Existing SDR platforms can be divided into two different categories according to the implementation type of physi- cal layer including field programmable gate array (FPGA)- based SDR platforms and general purpose processor (GPP)- based SDR platforms [4]. The wireless open-access research platform (WARP) [5] and the LabVIEW-based USRP-RIO series [6] are two popular FPGA-based SDR platforms. FPGA- based SDR platforms can meet the strict delay constraints of current communication standards owing to their characteristics of deterministic timing and low latency. However, the ultra- high licensing fees and low flexibility of the physical layer prevent the FPGA-based SDR platforms being widely used by researchers. Instead, GPP-based SDR platforms are especially suitable for rapid prototype development and physical layer simulations [7]. Specifically, GPP-based SDR platforms are usually written in C++ or Python and can be implemented on personal computers (PCs), while the FPGA-based SDR platforms often use very high speed integrated circuit hardware description language (VHDL) or Verilog hardware description language which is hard to write, compile and test.

Sora [8] and GNU Radio [9] are two popular GPP-based SDR platforms, which can be connected to USRP devices provided by Ettus Research [10]. Sora is a fully programmable and high-performance SDR platform that can be used to im- plement most advanced wireless communication technologies [11]. GNU Radio provides a graphical interface to configure and debug transceivers, which can display physical waveforms of signals in real time. In addition, GNU Radio can be used on a variety of operating systems such as MacOS and Linux. Compared with Sora, GNU Radio’s simple and practical graphical programming interface makes it a preferred choice for researchers.
In GNU Radio, all modules are designed based on the concept of unlimited data flow. There are mainly two data streaming mechanisms in GUN Radio including 1) message mechanism and 2) tag mechanism. In the message mech- anism, protocol data units (PDUs) are transferred between modules in asynchronous mode. In addition, the message interfaces provide a convenient communication connection between external applications and GNU Radio modules. The main disadvantages of message mechanism are that it works asynchronously and the reliability of message transmission between modules can not be guaranteed. In tag mechanism, one tag stream is transmitted in parallel with the data stream. Unlike the data stream, the tag stream is used to transmit control information and data-related information. The tags can help modules identify the sampling points that need to be processed, thus enhancing the reliability of inter module transmission.
In this paper, we present a design of SDR-based pseudo- analog wireless video transmission system. The designed system can be used for both simulations and experiments, thus providing seamless switch between theory and practice. Specifically, we redefine the data format transmitted in the physical layer while retaining most of the original functions of the orthogonal frequency division multiplexing (OFDM) system (e.g., channel estimation and channel equalization). Compared with the conventional digital transmission frame- work (e.g., IEEE 802.11a), the designed system removes the quantization module and the channel coding module in the

introduces the related work including the rate-distortion theory and the pseudo-analog wireless transmission technology. In Section III, we analyze the difficulties of the design and implementation of the proposed system. Section IV gives the details of the designed system including the overview of the system and the detailed functions of each module in the order of transmitter, channel, and receiver. In Section V, we provide the implementation details and analyze the simulation and experiment results obtained by the designed system. Finally, we summarize this paper in Section VI.

2. 
   RELATED WORK
   

In this section, we will introduce the rate-distortion the- ory and the existing pseudo-analog transmission systems. Specifically, we will firstly prove that the analog method can also achieve the optimal performance as digital method from the perspective of the rate-distortion theory. Then, we will introduce the difference between the pseudo-analog transmis- sion system and the existing digital system, as well as the development of the pseudo-analog transmission systems.

1. 
   Rate-Distortion Theory
   

∼N
Shannon theory indicates that separate source and channel coding can achieve asymptotically optimal performance in the point-to-point communication [12]. Such conclusion promotes the design of layered communication system, where the phys- ical layer is responsible for processing channel characteristic (e.g., path loss and fading) so that the application layer does not need to deal with bit errors. On the contrary, the application layer handles domain-specific issues (e.g., quantization and coding), making the physical layer invisible to the transmitted data. Although such a separate structure greatly reduces the complexity of the overall system design, it also destroys the essential relationships between the transmitted bitstream and the original pixels. Denote a memoryless Gaussian source as X and its distribution as X (0, λ) where λ represents the variance of X. Consequently, the corresponding rate-distortion function can be denoted as follows.

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