Implementation of Low Cost SDR in HF Band for Emergency Application

When events are uncontrolled due to hurricanes, the communication will breakdown. During this strenuous situation there is a need for reliable communication system to provide alert and emergency services. There remains a particular attention towards portability, recongurability, and interoperability in a communication system to reduce response time during emergencies. The real-time implementation helps to come across alternatives to overcome practical diculties and challenges. This motivates us to implement a Relay Node in real-time using SDR , hence an experiment is carried out in order to gain rst hand experience between different types of architecture of SDR i.e.VUSDR, HackRF One, and USRP. The architecture of Low-cost VUSDR and comparison between different types of SDR has been presented in this paper. Future scope and applications are presented.


Introduction
SDR was introduced by Joseph Mitola [1] that include both Analog and Digital parts of the radio. Initially, the design of SDR is to meet the requirements of the military, later it was extended to other applications like GSM,NOOHA, Satellite Communication etc.
Software De ned Radio (SDR) replace a traditional hardware radio by a software one and acts as a bridge between incompatible radios [27]. It uses DSP/FPGA and computer systems with suitable software. The SDR design initiated to use programmable processing, to incorporate new coding, in order to provide secure communication with modularity and inter-operability.
Software De ned Radio comprise of generic hardware. i.e. RF FrontEnd and Open Source software which offer exibility that in turn enables the engineers to ful l cognitive functions. .VU-SDR is a Low-Cost Transceiver developed by using open-source rmware by Amateurs Hams. This will operate in HF band through HDSDR software.
During natural disaster [1]for re-establishing communication a relay node is used as temporary station to communicate to the nearby station,that help quick rescue operation. To address several issues such as interference , path loss, multi path fading, shadowing, etc, one better choice is to use SDR as a relay node.
The signi cance of Relay can be understood by studying some similar cases and the applications of relay concept in various communication system have been discussed as follows.

Relay for satellite ground station:
Connection failure occurs due to the interference from adjacent frequency band congestion. This will be predominant with the reception of satellite signals and also degrade the performance at the terrestrial terminal. Hence relay is deployed for the satellite to ground signal. For implementing this testbed three USRP and GNU Radio being used in [2]. The two approaches called Amplify and Forward relay, Decode and Forward relay has been tested using GNU Radio.

Cooperative Relay communication
Cooperative communication enhances link capacity and reliability and end-to-end throughput. It exploits spatial and user diversity and the signals through different paths and users are combined at the receiver node. Single relay and multi-relay cooperation test bed been evaluated using USRP and GNU Radio. This technique in [3] overcomes fading and interference and improves system performance.It requires stringent synchronization of devices at their physical layer.

Orbiter Spacecraft to Lander
The NASA Europa Clipper mission added the features of a relay communication system to clipper Frontier Radio, to deploy for relay communications to a lander and Earth communications in [25].

AF Relay for Multi user MIMO
A specially designed AF (Amplify and Forward) Transceiver been implemented as AF Relay to overcome practical issues, such as pre-synchronization for cluster transmission. Recon gurable relays is demonstrated for the rst time in a realistic indoor environment with a commercial software radio platform in [23] 1.5. Multimedia applications through AF and DF relay Technique To mitigate multi path fading affects and to improve SNR, relay is used especially for multimedia applications such as Audio and Video.Basically it deploys two different cooperative relaying techniques (Amplify and Forward (AF) and Selective Decode and Forward (SDF)).the Experiment setup includes USRP and GNU Radio, and it proves that the performance of cooperative communication is better than that of direct communication presented in [22]. The receiver chain consists of two blocks. First block is to decimate the input signal to Intermediate Frequency IF, second block is to convert from IF to baseband sample, so that it could be supported by an ADC. The DSP stage involved in the architecture to shift IF signals to baseband using a demodulator and decimator/ lter is called Direct Digital Down Converter.

Keywords Used In
There are two types of SDRs namely QSD/QSE based SDR and DDC/DUC based SDR.In the QSD based SDR, RF is sampled and converted to an analog baseband AF signal in a single step. The resulting "IQ" output can be demodulated and ltered entirely within the host computer -without further hardware. DDC/DUC Radios employ FPGAs and is superior to QSD/QSE radio.The function of FPGAs is to digitize the input RF signals and process in the range MHz to GHz, and it is expensive.
High-performance SDR (HPSDR) was developed in 2005 and Flex Radio in 2006 after the introduction of the Tayloe detector in 2001. To process the signal to digital form, DSP Chip has been included between the audio ampli er and nal mixer, after it is fed to the speaker.
The purpose of the paper is to Discuss various architectures of SDR and experimenting audio signal transmission using low-cost VUSDR to Relay unit. A comparison of the architecture of SDR helps for improvements and enables a new design to overcome the limitations of the previous version.

Related Work
In 1970s US and Europe, both used SDR in the defence sector, espe cially for military application.
Software De ned Radio (SDR) replace a traditional hardware radio by a software one and acts as a bridge between incompatible radios [27]. It uses DSP/FPGA and computer systems with suitable software. It was initiated to use programmable processing, to incorporate new coding, that also acts as nodes in a network in order to provide secure communication with modularity and also provides interoperability.
Several companies develop SDR with different RF front end and tuning facilities like Flex Radio Systems, SunSDR, BladeRF etc. An open-source hardware USRP develops software radio using Open-source software GNU Radio. Except for Standalone SDR, all Other SDR platforms are Programmable Hardware to meet high-speed wireless [30] communication.
SDR gives the solution through inter-operability [27] by implementing Multi-data and multiple protocols communication through software and generic hardware without any constraint. Overview of the various design approach such as USRP N-series that includes GPPs, GPUs, DSPs, FPGAs are discussed and co-design approaches mentioned in [28] help to implement different communicationstandard through programming capability.
The various challenges of the SDR such as a) Security Issues like Insertion of malicious software to SDR terminals b)alteration or destruction of the con guration data c)high speed of ADCs and DACsand their synchronization with DSP, d) Increased complexity and development cost e)Increased power consumption f) Designing of antennas over a wide range of frequencies [29] are also presented byauthors.
The current research is towards the design of Software De ned Radio with low cost, compact, and also should consume less power. It should be operable during disaster condition in order to address the challenges encounter in communication.VUSDR which is deployed in this experiment ful ll all the above requirements such as low cost and is operable during emergency.
Classi cation of SDR based on its front end : 1)Stand Alone: Front end provided with Knobs and does not require any PC 2)Front End requires a PC for operation and control.
TCXO stands for Temperature Controlled Oscillator, controls theTemp-erature of the device to drift slightly by 0.5 parts per million so that it stay on frequency.
Super Heterodyne Radio architecture include double and triple conversion, mixers, Oscillators and other units like ampli ers, usually followed by DSP chip. Where as the new approach include Direct Digital Sampling(DDS) or Quadrature Sampling Detectors/ Exciters. The initial design of SDR concentrates especially on the audio output. Therefore the rst generation SDR is also called Sound card SDR which is economical, thus most of the functions are carried on PC.
In second-generation radio, ADC Chips have been deployed for faster conversion and better performance thus it also allows wider bandwidth than rst-generation SDR. Third generation SDR still uses QSD, that include onboard ADC and DSP chips. Several companies develop SDR with different RF front end and tuning facilities like Flex Radio Systems, SunSDR, BladeRF etc.

System Model
Several issues such as multipath loss, Fading, Low signal strength etc, and distance are the factors that will degrade the communication system.
The following are the reasons for deploying relay unit in communication system.
All the SDRs require dual sound cards or an expensive Sound card with twin input -outputs(like M-Audio).The Peaberry open source rmware got a sound card functions built-in. All the control functions and audio functions are handled by PSoc3 -a system on chip and a CODEC.
VUSDR is a QSD based SDR i.e. operates on the principle given by Tayloe and thus it is called as Tayloe detector. It consist of switch rotates at the carrier frequency so that each capacitor samples the signal once each revolution. The 0o and 180° capacitors differentially sum to provide the in-phase (/) signal and the 90o and 270o capacitors sum to provide the quadrature (Q) signal.

Conclusion
SDR as Relay Node in the wireless communication can supports greater exibility and helpful during emergency situation for Multiband Operation rather than conventional radio. Conventional relay has limitations, which may not be suitable for wide tuning applications. Therefore for best performance and Recon gurability ,the real-time implementation of SDR as Relay Node has proved that, it is operable even they differ both in architecture and software platform.

Declarations
Author Contributions:Madhuri Gummineni performed the experiment and wrote the paper, and Trinatha Rao Polipall helped to revise and improve the whole paper. All authors read and approved the nal manuscript.
Funding: The authors declare that they have no competing nancial interests or relationships that could have appeared to in uence the work reported in this paper. Experiment set up photograph of signal transmission from VUSDR to HackRF One