Spectral efficient IR-UWB communication design for low complexity transceivers

Vijaya Yajnanarayana 0 Satyam Dwivedi 0 Alessio De Angelis 1 Peter Hndel 0 0 Department of Signal Processing, KTH Royal Institute of Technology , Fack, 100 44 Stockholm, Sweden 1 Engineering Department, University of Perugia , Perugia, Italy Ultra wideband (UWB) radio for communication has several challenges. From the physical layer perspective, a signaling technique should be optimally designed to work in synergy with the underneath hardware to achieve maximum performance. In this paper, we propose a variant of pulse position modulation (PPM) for physical layer signaling, which can achieve raw bitrate in excess of 150 Mbps on a low complexity in-house developed impulse radio UWB platform. The signaling system is optimized to maximize bitrate under practical constraints of low complexity hardware and regulatory bodies. We propose a detector and derive its theoretical performance bounds and compare the performance in simulation in terms of symbol error rates (SER). Modifications to the signaling, which can increase the range by 4 times with a slight increase in hardware complexity, is proposed. Detectors for this modification and a comparative study of the performance of the proposed UWB physical layer signaling schemes in terms of symbol error rates are discussed. 1 Introduction The radio technologies for communication systems generally employ a non-overlapping radio frequency (RF) spectrum. That is, every radio technology like GSM, 3G, Bluetooth, etc. uses a distinct RF spectrum. There are several radio technologies, and several new ones are emerging; as a result, RF spectrum is becoming more premium and more scarce. Communication systems using ultra wideband (UWB) offer a promising solution which can coexist with other radio technologies. This coexistence also saves expensive spectrum licensing fees [1,2]. The Federal Communications Commission (FCC) adopted licensefree UWB operation in the United States of America [3]. This has resulted in 7.5 GHz of spectrum available for UWB systems. One of the direct consequences of this large bandwidth is the ability to achieve very high data rates, as given by the Shannon-Hartley theorem. Wide bandwidth also enables innovative system design such as trading data rate to avoid costly channel estimation techniques in [4] or designing the analog transmit and receive structure with non-idealities in [5]. In general, there is a wide scope of data rate, range, and other parameters that can be traded off based on the application [6-8]. There are several ways in which a signal can be spread to large bandwidths. The most popular methods include frequency hopping (FH) [9], orthogonal frequency-division multiplexing (OFDM) [10], direct-sequence spread spectrum (DS-SS) [11], and time-hopping impulse radio (TH-IR) [12]. UWB based on OFDM and TH-IR have gone in to IEEE 802.15.3a and IEEE 802.15.4a standards. TH-IR schemes are most popular as they provide better performance and complexity trade-offs [7]. The use of impulse signaling (TH-IR) was proposed by Win and Scholtz in the 1990s. Their work published in [12-14] contributed significantly toward the adaptation of TH-IR for UWB. High bandwidth enables the UWB transceiver to generate narrow impulse signals; this fine time resolution can yield accurate position localization and ranging. This has enabled the application of UWB for high-precision ranging and localization. Our objective is to utilize the same platform for both communication and localization. Figure 1 shows a graphical depiction of an inhouse developed IR-UWB platform for ranging and communication. It uses a low-cost pulse generator to generate Figure 1 Iconic model of the in-house developed impulse radio UWB-platform of size 6 4 cm. Iconic model of the in-house developed impulse radio UWB-platform of size 6 4 cm for ranging and communication working in the 6-GHz regime with separate RX and TX antennas from Greenwave Scientific (details available at [19]). sub-nanosecond pulses using step recovery diode (SRD), as described in [15]. The characterization and modeling of the UWB platform for a distance measurement system can be found in [16,17]. A detailed architectural description and experimental ranging result from a prototype of the platform have been published in [18]. The power and range of the transceiver can be easily traded by controlling the amplitude, duty cycle, and number of pulses per bit of transmission. There are several commercial companies which develop IR-UWB products, including [20-24]. Companies like DecaWave and BeSpoon develop 802.15.4a standard specific IR-UWB products [23,25]. The physical layer signals of these UWB radios are defined by the standard. There are some companies like Time Domain and Ubisense which develop non-standard or custom-made communication and localization solutions [26,27]. In these UWB radios, the physical layer signaling does not adhere to any standards. The work proposed in this paper considers a methodology to maximize the communication rate through custom physical layer signaling, subject to hardware, and regulatory constraints. The main motivation for the work is from the requirement that many UWB applications need to perform localization and communication using the same radio module [6,28]. The UWB radios of Time Domain and Ubisense both have localization and communication capabilities; however, these radios have minimal communication capabilities of few Kbps and physical layer signaling in them is not made public. This paper is also motivated by the fact that extensive research can be found on the design of hardware platforms and algorithms for localization and communication strategies in [6,18,29], However, how to optimize the physical layer signaling for communication in view of constraints from cost-effective hardware and regulatory bodies is not a well studied problem. The achievable bitrate for the proposed methods in this paper depends on the hardware parameters of the UWB platform. The proposed methods suggest that the in-house developed UWB radio shown in Figure 1 can achieve bitrates up to 150 Mbps. The in-house UWB platform uses pulse round-trip time (RTT) for localization. It has a range of about 10 m with an accuracy of 30 cm in practical scenarios. It has a digital processing section based on a field-programmable gate array (FPGA), which interfaces with analog UWB sections to generate required analog pulsed waveforms for transceiver operation. The modulator and demodulator algorithms proposed in this paper can be programmed in FPGA, for processing UWB communication signals. This paper proposes two signaling schemes with one requiring higher complexity in modulation and demodulation, however can increase the range by nearly 4 times without compromising on the bitrate. This is believed to have an interest in its own right, as it corresponds to (or outperforms) todays state of the art. Although the work considers the in-house developed UWB radio (Figure 1) (...truncated)