Spectral efficient IR-UWB communication design for low complexity transceivers
© Yajnanarayana et al.; licensee Springer. 2014
Received: 25 June 2014
Accepted: 23 September 2014
Published: 3 October 2014
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.
KeywordsPulse position modulation Sensor networks Time-to-digital converter Ultra wideband UWB communication
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 license-free UWB operation in the United States of America . 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  or designing the analog transmit and receive structure with non-idealities in . 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) , orthogonal frequency-division multiplexing (OFDM) , direct-sequence spread spectrum (DS-SS) , and time-hopping impulse radio (TH-IR) . 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 .
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) today’s state of the art. Although the work considers the in-house developed UWB radio (Figure 1) for demonstrating the techniques, the results are of general importance which can enable engineers to follow similar methodology to exploit the hardware and spectrum to achieve a higher possible range and bitrate.
In this context, through this paper we propose a method for communication using low-cost and low complexity hardware architecture which can perform ranging, localization, and communication. The main contributions of this paper are summarized below:
A method to design a custom physical layer signaling to maximize the data rate by optimal choice of modulation parameters, given the constraints from hardware and regulatory bodies, is illustrated. An algorithm for such a modulator with no memory between symbols called a no-memory modulator is proposed.
A spectrally more efficient modulator, which can improve the range of the communication by introducing the memory between symbols called with-memory modulator, is proposed. This has a marginal increase in complexity of modulator and demodulator algorithms with an increase in range by 4 times.
A no-memory modulator is analyzed by deriving an expression for the symbol error rate (SER) performance. A detector algorithm for the no-memory modulator is proposed, and its SER performance is verified through simulation.
The detector algorithm for the with-memory modulator is proposed and detector performance of no-memory and with-memory signaling are compared in simulation.
This paper is organized as follows: In Section 2, we discuss the system model and pulse shapes used in the impulse radio. Section 3 discusses UWB constraints. Section 4 is on low complexity UWB hardware platform for localization and communication, and Section 5 details the design of physical layer signal construction and modulator algorithms. Section 6 describes detectors and their performance in terms of symbol error rates. Finally, Section 7 details the conclusions from the design and results demonstrated.
2 Pulse construction and system model
A wide range of pulse shapes have been explored for UWB communication from rectangular to Gaussian . Gaussian pulses and their derivatives, usually called monopulses, are effective due to the ease of construction and good resolution in both time and frequency. In many cost-effective hardware designs, these shapes are generated without any dedicated circuits. A simple transistor or diode, which is turned ‘on’ and ‘off’ to generate a narrow rectangular pulse, will form an approximate Gaussian shape due to the imperfections in micro-electronic design .
Several modulation techniques are proposed in the literature using narrow pulses [12, 32]. Primarily, they are variants of pulse position modulation (PPM), binary phase shift keying (BPSK) or on-off keying (OOK). Since our objective is to employ low complexity hardware structure to perform synchronized ranging and communication, a variant of PPM-based signaling for communication is used. This can reuse the hardware structure (having round trip time (RTT) calculation logic for ranging) for detection and demodulation of physical layer signal. This is further illustrated in Section 4.
where TS is the symbol period, d n is the n-th symbol value d n ∈[0,..,M-1], p(t) is the normalized pulse such that , E p is the energy of the pulse, γ n is a parameter coming from the constraints of the typical UWB hardware which will be explained later, Δ is the modulation index, and log2M is the modulation order. Though in our model each symbol is defined by one pulse, it can be easily extended to multiple pulses for each symbol so that communication rate and detectability can be traded.
where E r is the energy of the received pulse, is the random jitter in the received signal, and n(t) is the receiver noise on the received signal.
The third part of the system model is the detector/demodulator, which demodulates the signal represented in (7). In the signal model, a single-user UWB system with no multiple access interference is assumed. However, it is straightforward to extend the techniques proposed in this paper for multi-user system.
In the subsequent sections, we will show how to optimally design the modulator and detector to the constraints of hardware and regulatory bodies. We will also evaluate the theoretical performance of the demodulator for the chosen modulator. In the next section, we will discuss some of the challenges in transmission and detection of UWB pulses from the regulatory bodies and hardware perspective.
3 UWB constraints
The pulse repetition rate (PRF) (that is, 1/TS in (6)) of the IR-UWB signal plays a significant role on how the UWB device impacts other narrowband receivers in its range; these receivers are called victim receivers . If the PRF is larger than the bandwidth of the victim receiver, then the emission may appear as noise like to the victim receiver. This effect is proportional to the average power of the UWB signal within the receiver’s bandwidth. If pulse rate is smaller than the victim receiver’s bandwidth, then UWB signal would appear like impulse noise to the victim receiver and the effect is proportional to the peak power of the UWB signal. Thus, at low PRF the output levels are constrained by the limit on peak emission levels and at high PRF by the limit on average emission levels .
In , the authors specify the existence of two distinct regimes where only one of the two power constraints are active for the IR-UWB signal. For PRF less than 1 MHz, peak constraints are active; for PRF greater than 1 MHz, average power constraints are active. The signaling proposed in this paper is optimized for high-rate data communication and hence, high PRF is assumed as ≫ 1 MHz yielding only the average power constraint relevant.
4 UWB hardware
Larger bandwidth of ultra wideband signals also enables suboptimal receiver designs, which are more efficient from cost and complexity perspective. Some of these low-cost designs can be found at [18, 37]. In this correspondence, we propose the hardware architecture platform of  for communication. The modulator algorithm on FPGA will generate control signals required to trigger the step recovery diode to generate UWB pulses. On the receive side, the transceiver has an energy detector; whenever the signal energy crosses crosses a certain threshold, it sends a ‘Start/Stop’ signal to time-to-digital converter (TDC). TDC measures the interval between the pulses, which carries information in the PPM variant physical layer signaling. This information is further processed by the demodulator algorithm on FPGA to demodulate the symbol. Estimating the round-trip time on unmodulated signal can be used to localize objects as discussed in . Thus, this low complexity transceiver can be used for both localization and communication .
In general, in a low complexity UWB transmitter, it is not possible to transmit arbitrary close pulses because of the recovery time required for the micro-electronic devices used in them. This creates a constraint on the signaling that the pulses need to be separated by at least a minimum distance equal to the recovery time. This is the reason for having the γ n in (6). Also, at the detector it is not possible to resolve between arbitrarily close pulses. Thus, the modulation index Δ in (6) cannot be arbitrarily small.
The bitrate peak to around 160 Mbps at M=8 when the typical parameter values of Tms = 10 ns and Δ = 1 ns for the in-house UWB platform is considered.
There is better detectability due to an increase in the bin widths (modulation index) as explained before. Later in the paper, we compare the performance (in terms of symbol error rate (SER)) of this signaling with no-memory signaling.
There is better randomization of the pulses. In contrast to no-memory signaling, there are no deterministic gaps and pulses spread to all regions of the symbol interval.
Eq. 15 means that the range can be increased by 4 times compared to the no-memory signaling. This increased range comes with a cost of increased complexity in the modulator, as shown in Algorithm 2.
In the next section, we will evaluate the performance of the demodulator for the proposed signaling scheme.
6 Detector performance
Time intervals used for hard decision demodulation
Quantized time intervals for hard decision demodulation
[(i Δ-((i+1)Δ)] for i∈[1,..,6]
The intervals for two corner bins are larger because the detector can exploit the dead time Tms left for the diode recovery. The algorithm for the hard decision demodulation is shown in Algorithm 3. The algorithm demodulates the received vector, r, into a vector D e m o d u l a t e d S y m b o l. The PeakPosition in line 3 of the algorithm returns the peak position of the received pulse.
where β=Δ/σ and α=Tms/Δ.
A better detectability due to an increase in the bin widths (modulation index), as explained before.
Better randomization of the pulses. Unlike in the no-memory signaling, there are no deterministic gaps and pulses are spread to all regions of the symbol interval.
The presented symbol error rate (SER) performance for the proposed signaling schemes are valid for the transceivers which are in line of sight (LOS) with short distance between them and having highly directional antennas as discussed in Section 2. For these systems, we can assume a simple AWGN channel model. However, for systems having fading channels with multi-path, channel equalization and time of arrival (TOA) estimation needs to be performed prior to demodulation. The SER performance of the proposed signaling schemes in such systems depends on the performance of channel equalization and TOA estimation algorithms.
In this paper, we proposed a custom signaling which is a variant of PPM signaling for IR-UWB communication. We also proposed an alternative signaling called with-memory signaling, which requires memory in the modulator and demodulator, however, can further smoothen PSD compared to no-memory signaling. The result of this is illustrated in Figure 9b. We showed that range can be increased by 4 times compared to no-memory signaling, without violating the regulatory body constraints. This gain comes with a cost of increased complexity in the modulator and demodulator, as discussed in the modulator and demodulator Algorithms 2 and 4 for with-memory signaling. We also derived the theoretical closed-form expression for a hard decision demodulator with no-memory signaling. We compared the performance with the derived theoretical result; this is illustrated in Figure 11. We implemented a detector for the with-memory signaling proposed in this paper. The detector performance of with-memory signaling is compared with the detector performance for no-memory signaling. We showed that with-memory signaling can improve the detector performance by approximately 1 dB at 10-2 SER. Results are illustrated in Figure 12.
The performance of the proposed signaling methods of the paper are demonstrated in simulations in order to assess the performance gains without many platform dependencies. The proposed schemes are valid for any low-cost UWB hardware platform employing a step recovery diode at the transmitter and having a need for minimum time resolution between pulses at the detector for detection. The transceiver in Figure 1 will be an integral part of our next generation infrastructure free indoor position system , the radio here should be used not only for ranging but also for the wireless communication and thus fulfill a need for the proposed method. Today, we can use commercial UWB ranging like Time Domain for this purpose; however, these systems do not have the high bitrate communication capabilities. The results of the proposed signaling methods indicate the possibility of achieving a higher bitrate in excess of 150 Mbps with low probability of error in detection as suggested by the performance curves, using the parameters from our transceiver hardware. With these findings, we intend to further develop the work to implement the proposed algorithms into our transceiver system and evaluate the performance of in-house transceiver hardware. The proposed algorithms and methods are explained in the context of in-house UWB hardware; however, the results are of general importance which could enable engineers to apply similar methods and algorithms toward the design of UWB communication system.
Parts of the work have been funded by The Swedish Agency for Innovation Systems (VINNOVA).
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