Hybrid Modulation for Impulse Radio
Impulse radio systems communicate by encoding each symbol as a sequence of short pulses. Due to the absence of frequency upconversion, low-cost transceiver structures are possible. Currently our project aims to make use of the low-cost transceiver capabilities of impulse radio systems by investigating modulation and coding techniques that are appropriate for embedded networking and sensor network applications.
Background & Objective: With the start of the IEEE 802.15.4a standard task group in 2003 there has been increased interest in the application of UWB technologies to the field of low cost, low data rate sensing and control networks. Particularly those applications which require accurate indoor geolocation (asset tracking, security, fire and police first responder systems, ...). The use of UWB signaling has clear advantages in these scenarios due to it's robustness to multipath channels typically observed indoors and more importantly it's ability to resolve a great number of individual multipath components (MPC). The accuracy with which any radio signal can be used to identify individual MPCs is proportional to the signal bandwidth and this motivates the use of UWB signals for geolocation purposes.
Technical Discussion: Our previous work sought to solve the cost and complexity issues associated with coherent based impulse radio receivers by developing a modulation that combined features BPSK, Transmitted Reference (TR) and Pulse Position Modulation (PPM). The result was modulation technique we termed Hybrid-Impulse Radio (H-IR). The key feature of H-IR modulation is that is allows simultaneous reception by variety of receivers types ranging from simple energy detectors to complex coherent rake receivers. This in turn allows for heterogeneous networks of devices that can trade-off cost/complexity/performance to meet system requirements.
The ability of the H-IR modulation to reception at variety of receivers is achieved via a systematic coding of the information bits and subsequent modulation of the IR pulses so that the systematic bits can be recovered via non-coherent means or TR type auto-correlation receivers. This allows the redundant bits to be modulated onto the phase of the pulses and these can be used by coherent receivers to obtain a coding gain.
After our initial discovery of the H-IR modulation is was observed that the technique may be viewed as a form of Trellis coded modulation (TCM) which led us to explore methods that could further improve the performance of both coherent and non coherent receivers. A simple approach might be to include a forward error correcting code (FEC) prior to the H-IR modulation. However, the optimal use of FEC with H-IR is not the trivial application in which one simply codes the bits prior to H-IR modulation. This is due to the fact that the H-IR is already a coded system. In essence the addition of FEC led us to consider serially concatenated codes and a class of iterative decoders that can give performance results that approach those of turbo codes. The H-IR technique has been extended to consider the use of an outer FEC and interleaver along with the H-IR modulation that still enables the simultaneous reception by coherent, TR, and non-coherent receivers and also offers improved performance to all three types of receivers. The greatest improvement is seen in coherent receivers which can make use of advanced iterative decoding. In addition, the reduced complexity receivers (TR, and noncoherent) now get the error protection provided by the outer FEC encoder.
Publications:
Technology Area: Digital Communications
Modification Date: July 3, 2007