Our department currently develops several physical layer prototyping platforms, namely:
- Filterbank Modulation based communication systems
- PHY for Femto-based systems based on LTE and beyond
- Deep space modems for telecommand and telemetry
More details are provided below:
Filterbank Modulation based communication systems
Many mobile communication systems today use the OFDM (Orthogonal Frequency Division Multiplexing) technology for broadband communications. As a multicarrier modulation OFDM has many advantages: it can be efficiently implemented using an IFFT/FFT algorithm and allows for simple equalization. However, it also presents some strong disadvantages: it is vulnerable to narrowband interferers, it has a decreased bandwidth efficiency due to the Cyclic Prefix (CP) and it suffers from a high out of band emission. These shortcomings can be solved by using a Filterbank Modulation scheme instead of OFDM. The Filterbank modulation scheme has the following advantages:
- Optimum bandwidth efficiency
- Narrowband interferers only affect a contiguous group of subcarriers
- Multiple user access is alleviated especially in the uplink with different timing offsets
- Flexibility in the choice of the number of subcarriers
- Per-subcarrier equalization yields appealing results
All these advantages come at the expense of increased latency and increased computational complexity (equalizer and filters). This complexity is usually implemented in an FPGA or a DSP.
The MINOTAUR test-bed is a real-time transmitter-receiver filterbank communications modem that enables the prototyping and evaluation of physical layer algorithms targeting Multi-Carrier-based wireline communications. The main application of the MINOTAUR is high voltage powerline communications, but it could be scaled up for research on medium frequency, broadband over power line or wireless transmission.
The purpose of the MINOTAUR demonstrator is to explore high efficiency signal processing algorithms that maximize the data rate using high efficiency modulation schemes on bandwidth constrained wireline channels. Since MINOTAUR uses filterbank communications, a type of modulation that will be used in future mobile communications, this test-bed could also be modified for research on 5G wireless communications. By using MINOTAUR we can evaluate the implementation costs on FPGA and DSP-digital signal processing boards and the system performance compared to the Matlab simulations. The MINOTAUR test-bed is implemented as two separate full-duplex modems. Each modem is implemented using one Virtex-5 FPGA and three floating-point TMS320C6727 DSPs. All the baseband signal processing chain including the I/Q (Phase/Quadrature) generation and I/Q demodulation, upsampling and downsampling is performed digitally (using the SDR concept). Therefore all the signal processing strategies are highly customizable.
The MINOTAUR modem includes all the acquisition, synchronization and bit loading algorithms necessary for the modem to synchronize automatically once the modem is started, resynchronize automatically whenever the communication is interrupted and perform a dynamic real-time adaptation of the communication speed depending on the channel impairments (noise and interfering signals).
On top of that the MINOTAUR testbed includes PC software to monitor the performance and the behaviour of the Filterbank algorithm on real-time, such as the data rate, the bit error rate, the constellation, the channel estimation and the CFO (Carrier Frequency Offset).
PHY for Femto-based systems based on LTE and beyond
Consistently with research on 4G and 5G beyond 2020 wireless systems, we carried out a collaborative company project where we implemented the LTE (Long Term Evolution) System Information Decoding functionality for a femtocell. This is the functionality by which a femtocell obtains information about all the LTE base stations that lie in the neighbourhood. The LTE System Information Decoding functionality is part of any LTE femtocell capabilities. This prototype implements the functions that decode the LTE system information (the MIB, SIB1 and SIBx blocks).
LTE is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed by using a different radio interface together with core network improvements. The LTE standard is developed by the 3GPP (3rd Generation Partnership Project) and is specified in its Release 9 document series.
A femtocell is a small, low-power cellular base station, typically designed for use in a home or small business. A femtocell allows service providers to extend service coverage indoors or at the cell edge, especially where access would otherwise be limited or unavailable. Designs typically support two to four active mobile phones in a residential setting, and eight to 16 active mobile phones in enterprise settings.
The System Information decoding module of a femtocell has two software components: a control software and the LTE system information decoding functions. The Control Software that runs in the femtocell is simulated. The LTE system information decoding functions are implemented in C++. All the software implementation, compilation and testing is done in software using a personal computer. However the LTE system information decoding functions are intended to run as embedded software in a LTE femtocell.
Deep space modems for telecommand and telemetry
One of our objectives for 2014 is to complete a full FPGA-based receiver for deep space communications, including all the necessary acquisition and synchronization procedures. The purpose of this receiver is:
- To investigate, implement and evaluate possible algorithms to allow ESA telemetry receivers using standard TT&C (Telemetry, Tracking and Command) to operate and acquire in real-time in the areas of extremely low Es/No.
- To improve the BER/FER performance of the Turbo codes decoding algorithms, especially in the error floor region.
The receiver will achieve its goals by using two modified turbo-decoder algorithms: the Coupled-Turbo Decoder and the CIM (Correction Impulse Method). The receiver will contain all the required analog/digital electronics, and firmware/software to decode the deep space communications modulated in IF (Intermediate Frequency). This receiver implements standard telemetry modulation schemes. This development will be carried out in the framework of the project “Coupled enhanced turbo codes demodulator and decoder study” with the European Space Agency (ESA).