Development and evaluation of low complexity physical layer concepts for energy efficient UWB communications
Zusammenfassung der Projektergebnisse
A large bandwidth — in the case of ultra wideband (UWB) at least 500 MHz — may have many benefits for wireless communications, but it also imposes some serious problems. By increasing the bandwidth, more and more multipath arrivals with different path gains are resolvable at the receiver, which makes it difficult to collect the multipath energy coherently. Especially in non line-of-sight (NLOS) scenarios, a coherent RAKE receiver requires a very large number of RAKE fingers (and thus a large number of multiplications per symbol) and a precise channel knowledge (dozens or even hundreds of complex path weights) to efficiently capture the multipath energy. Within the project, we have focused on non-coherent UWB transmission, which is an attractive approach especially if simple and robust implementations with a small power consumption are required. Main application fields are low data rate sensor or personal area networks, which require low cost devices and a long battery life time. The main advantage of a non-coherent receiver is clearly the dramatically reduced effort which is required for channel estimation, synchronization and multipath diversity combining. These advantages are, however, bought by a serious drawback: non-coherent detectors are more susceptible to narrowband interference (NBI), multi-user interference (MUI) and inter-symbol interference (ISI). During the project, we have investigated non-coherent transmission schemes, which promise robustness even in the presence of NBI or MUI. In the first two years, we have derived important performance limits, where we focused on analog receiver implementations. “Analog” refers to the fact that a major part of the (non-coherent) multipath combing will be accomplished by means of an analog “integrate and dump” filter. In phase II of the project, we have concentrated on digital receiver implementations. Here, the multipath diversity combing is completely performed in the digital domain. Non-coherent detection can either rely on energy/envelope detection or on differential detection. Both schemes are commonly used in state of the art fiber optical transmission systems with optical amplifiers. It is therefore evident that basic knowledge from fiber optics can be used here - especially when it comes to estimate the bit error rate. We have considered pulse-position modulation (2-PPM) and differential phase-shift keying (DPSK) as straightforward modulation schemes to be combined with either envelope detection or with differential detection, respectively. In a multipath channel with additive white Gaussian noise (AWGN), both schemes perform very similar. For a given path diversity combining technique (such as single window combining or weighted subwindow combining), the Eb/N0 -performance differs by exactly 3 dB in favor of DPSK. The performance loss compared to ideal coherent (2-PPM or DPSK) detection increases with the signal bandwidth B and the integration interval Tint within which the multipath arrivals are collected. Since the fading resistance also increases with B, an appropriate bandwidth must be chosen very carefully. We have addressed this topic in. In contrast to fiber optics, where an optical delay interferometer can be used to compare a modulated symbol with a preceding modulated symbol, analog implementations of differential receivers are very difficult to build, since UWB-signals demand for electrical broadband delays. Although the magnitude of the delay can be reduced by transmitted reference (TR) signaling, where the modulated symbol to be detected is compared to an unmodulated reference pulse periodically inserted in the transmit signal, it is more than unlikely that analog implementations of differential receivers will ever have a chance to be applied in low cost products. The situation may change if completely digital non-coherent receivers are considered, where delays can be realized very easily. We have investigated digital receiver implementations in phase II of the project. These have several further advantages. First of all, they offer a superior interference rejection capability, since user specific filtering (“digital code matched filter” - DCMF) - that is a coherent operation - can take place prior to the non-coherent signal processing. This restricts the non-coherent combing loss to the multipath arrivals (which exhibit stochastic path weights), whereas that part of the signal energy, which is already spread by a user-specific code at the transmitter, is coherently summed up. Furthermore, digital receiver implementations enable advanced modulations such as Walsh-modulation or advanced NBI-suppression strategies based on soft-limiting.
Projektbezogene Publikationen (Auswahl)
- “Low-complexity and Energy Efficient Non-coherent Receivers for UWB Communications,” in Proc. 18-th Annual IEEE International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC07), Greece, Sept. 2007
N. Song, M. Wolf, and M. Haardt
- “A Digital Code Matched Filter-based Non-Coherent Receiver for Low Data Rate TH-PPM-UWB Systems in the Presence of MUI,” in IEEE International Conference on Ultra Wideband (ICUWB 2009), Vancouver, Canada, Sept. 2009
N. Song, M. Wolf, and M. Haardt
- “Chapter 5: Non-Coherent Detection,” in Short-Range Wireless Communications: Emerging Technologies and Applications, R. Kraemer and M. D. Katz, Eds. Wiley, 2009
M. Wolf and N. Song
- “Non-Coherent UWB Communications,” FREQUENZ Journal of RF-Engineering and Telecommunications, Oct. 2009, special issue on Ultra-Wideband Radio Technologies for Communications, Localization and Sensor applications
M. Wolf, N. Song, and M. Haardt
- “On the choice of the bandwidth for low-complexity UWB communications with non-coherent detection,” in Proc. of 23rd Wireless World Research Forum (WWRF), Beijing, China, October 2009
N. Song, M. Wolf, and M. Haardt
- “Performance of PPM-Based Non-Coherent Impulse Radio UWB Systems using Sparse Codes in the Presence of Multi-User Interference,” in Proc. of IEEE Wireless Communications and Networking Conference (WCNC 2009), Budapest, Hungary, April 2009
N. Song, M. Wolf, and M. Haardt
- “A b-bit Non-Coherent Receiver based on a Digital Code Matched Filter for Low Data Rate TH-PPM-UWB Systems in the Presence of MUI,” in Proc. of IEEE International Conference on Communications (ICC 2010), Cape Town, South Africa, May 2010
N. Song, M. Wolf, and M. Haardt
- “A Digital Non-Coherent Ultra-Wideband Receiver using a Soft- Limiter for Narrowband Interference Suppression,” in Proc. 7-th International Symposium on Wireless Communications Systems (ISWCS 2010), York, United Kingdom, Sept. 2010
N. Song, M. Wolf, and M. Haardt
- “Adaptive Reduced-Rank Interference Suppression for DS-UWB Systems based on the Widely Linear Multistage Wiener Filter,” in Proc. 7-th International Symposium on Wireless Communications Systems (ISWCS 2010), York, United Kingdom, Sept. 2010
N. Song, R. de Lamare, M. Wolf, and M. Haardt
- “An Iterative Widely Linear Interference Suppression Algorithm based on Auxiliary Vector Filtering,” in Proc. of the 44th Asilomar Conference on Signals, Systems and Computers, Pacific Grove, CA, Nov. 2010
L. Wang, N. Song, R. C. de Lamare, and M. Haardt
- “Time-Hopping M-Walsh UWB Transmission Scheme for a One-Bit Non-Coherent Receiver,” in Proc. 7-th International Symposium on Wireless Communications Systems (ISWCS 2010), York, United Kingdom, Sept. 2010
N. Song, M. Wolf, and M. Haardt
- “Non-data-aided adaptive beamforming algorithm based on the widely linear auxiliary vector filter ,” in Proc. International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2011), Prague, Czech Republic, May 2011
N. Song, J. Steinwandt, L. Wang, R. C. de Lamare, and M. Haardt
