- Table of contents
- CONCEPTS
CONCEPTS¶
This project focuses in the implementation of a satellite communication chain in NI LabVIEW Communications System Design Suite [1], and demonstrate on two USRPs [2]. Transmission and Reception were devised using DVB-S2 [3] standard as syllabus. QPSK modulation was implemented and prior channel coding in BCH and LDPC. The approach was to start with the evaluation of BCH and LDPC coding and calculate the performance of FEC coding in terms of BER, secondly implement Tx and Rx using typical satellite waveforms and observe the QPSK constellations in both transmission and reception, and final merging the two into a communication chain for demonstration and simulation in the two USRPs.
Software Defined Radio¶
The term “Software Radio” was coined by Joe Mitola in 1992 [4] in order to underline the transition from digital radio to the new generation innovative radio, a multi-mode and multi-standard radio definable via software. The Federal Communications Commission (FCC) https://www.fcc.gov defines the SDR as a generation of device radio that can be reprogrammed in order to transmit and receive on every frequency of a specific range. The International Telecommunications Union (ITU) http://www.itu.int has defined SDR as a radio system in which the operative parameters (e.g. frequency, modulation, power) can be set or altered by software. The research reference of this area is the SDR Forum, now called wireless innovation form http://www.wirelessinnovation.org, an international non-profit organization founded in 1996 in order to accelerate the development of radio communication systems based on the concept of SDR. The forum gives another definition: SDR is a collection of hardware and software technologies able to reconfigure the architecture of wireless networks and user terminals.
Figure 2.1 Software Defined Radio Schematic
As mentioned before, the SDR captures the RF signal and digitizes it near the antenna. An ideal receiver would have an antenna and an AD converter. A DSP processor would read the data and transform the stream. However there are still limitation to that scheme due to technology of the AD-DA converters relevant to rate and accuracy. So the common architecture consists of a bandpass filter to eliminate spurious signals, a LNA to amplify low power signals (mW), an oscillator mixer and filter to tune the received signal to tune the signal where it can be then sampled by the AD converter.
Figure 2.2 SDR Ideal vs Real
Universal Software Radio Peripheral¶
The SDR used for this simulation is the NI USRP 2920 (Figure 2.3). USRPs (Universal Software Radio Peripheral) are designed by Ettus Research http://www.ettus.com and its parent company National Instruments (NI).
Figure 2.3 NI USRP2920
In Figure 2.4 we present the NI USRP 2920 schematic and in Table 2.1 its specifications.
Figure 2.4 NI USRP2920 Schematic
Frequency accuracy $f_a$ is given in parts per million (ppm) of the sample rate and is a good example of the pricision dynamics of the USRP. Frequency accuracy $f_a$, frequency error $f_e$ and the signal frequency are related with the following formulas $f_m=f_s \pm f_e=f_s/(1 \pm fa) \Rightarrow f_e=f_s*f_a$. Considering this for a 100KHz sine wave with 2.5ppm we have fe=0.25Hz and therefore frequency accuracy of 100KHz+_0.25Hz which proves the high performance that can be attained with USRP2920. The USRP 2920 offers MIMO Expansion Connectivity for synchronized data transfer through the Ethernet Gb connection allowing TX - RX simulation with two USRPs. The RF signal output TX1 is connected to the RF signal input RX1 of each USRP through SMA (SubMiniature version A) cohexial RF connector with use of a 30dB SMA attenuator for power safety.
| Table 2.1 Specifications | |
| Frequency Range | 50MHz-2.2GHz |
| Frequency Accuracy | 2.5ppm |
| Frequncy Step | <1kHz |
| Noise Figure | 5-7dB |
| Maximum Bandwidth(16bit sampling) | 20MHz |
| Maximum IQ sampling rate(16bit) | 25MS/s |
| Max output Power | 15dBm-20dBm |
| Max input Power | 0dBm |
| Ethernet connection | 1 Gigabyte |
DVB-S2 standard¶
References¶
[1] NI LabVIEW Communications System Design Suite
[2] USRP 2920
[3] DVB-S2
[4] Mitola III, J. (1992). Software radios-survey, critical evaluation and future directions. National Telesystems Conference