A TR-UWB Downconversion Autocorrelation Receiver for Wireless Body Area Network
© S. M. Riazul Islam et al. 2009
Received: 20 March 2009
Accepted: 21 July 2009
Published: 10 September 2009
Low power UWB receiver architecture is proposed for a Wireless Body Area Network (WBAN). This receiving technology is a synergy of existing downconversion-based narrowband rejection mechanism in RF front end and signal processing in frequency domain. Frequency components of converted and filtered UWB pulses are separated into real and imaginary parts, independently correlated and effectively combined to achieve an improved output Signal to noise ratio (SNR). An extensive mathematical analysis has been performed to formulate the close-form expressions for SNRs in order to compare system performances toward favorable BER under BPSK modulation scheme. Analysis shows that optimal rotation of coordination plays an important role for the enhancement of receiving SNR which is further confirmed by computer simulation. A wide range of link level simulation (LLS) urges that the proposed system is more power efficient in higher-order modulation (HOM) schemes. Transmitted Reference (TR) scheme has been considered as the basis for wideband communication.
Cardiovascular disease is the leading cause of death [1–4] and it accounts for approximately 30% of all deaths worldwide  requiring giant healthcare expenditure . A Wireless Body Area Network (WBAN) allows the integration of intelligent, miniaturized, low-power, invasive, and noninvasive sensor nodes to monitor body function and the surrounding environment . Each intelligent node has enough capability to process and forward information to a base station for diagnosis and prescription. A WBAN provides long-term health monitoring of patients under natural physiological states without constraining their normal activities. It can be used to develop a smart and affordable health care system and can be part of diagnostic procedure, maintenance of chronic condition, supervised recovery from a surgical procedure, and handling emergency events. One of the major components of WBAN is low-power miniaturized sensor . This paper aims to develop an extremely low power receiver that can be employed for WBAN.
The transmitted data is encoded by relative phases of two pulses. The first pulse acts as a reference and the second pulse is the modulated one. The receiver delays the first pulse by the delay D, multiplies it with the second pulse, and integrates the result over one delay length, which in fact correlates the two pulses. When using polar Non Return to Zero (NRZ) modulation, for a logical zero, a pulse is transmitted, subsequently followed by a polarity reversed pulse. To send a logical one, two pulses with the same polarity are sent sequentially. The absolute value of the output after integration is in fact the energy of the pulse while the polarity of the output contains the data. If the output is negative, this corresponds to a logical zero, while a positive output corresponds to a logical one. Thus, the information is in the relative polarity of the two pulses and the delay between them acts as a synchronization mechanism. As long as the two consecutive pulses have corresponding waveforms except for their polarity, the autocorrelation receiver can detect them properly. Yet, directly processing at UWB frequencies, due to the inherent influence of on-chip parasitic reactances at these frequencies, requires relatively large currents, or the use of expensive high speed technologies (SiGe, GaAr).
The work in  suggests a new receiver architecture based on the TR scheme  for its mentioned advantages with the rake receiver. It deals with interference rejection by filtering and avoids high frequency on-chip processing by using frequency conversion. Frequency conversion in this receiver is done differently from the traditional narrowband system to reject narrowband interference by making a proper choice for the frequency of the local oscillator (LO). The authors in  show a lot of advantages of their proposed UWB receiver. For example, it does not suffer from timing and template matching problems, and it circumvents processing at high frequencies, thereby reducing the on-chip circuit complexity and power consumption. Recently, many works have been done to digitally implement the TR-UWB system. However, the limited speed and resolution of the current ADC technology have been a major drawback. To lessen the constraints in ADC sampling speed required for digitally implementing the TR-UWB system, a technique using signal expansions into frequency components has been proposed in . Application of proposed technique in  on the receiver architecture suggested in  may develop a new receiving technology in which the front end is for downconversion-based narrowband rejection and there after, signal processing is carried out in frequency domain. In fact, this is the main idea of our work.
The authors in  are dealing with a similar problem where they estimate the joint response of the analysis filter and the propagation channel independently in each subband. However, it does not consider the coordinate rotation; moreover it uses hybrid filtering as subband filters which reduces the design freedom in the aspect of coexistence issues.
2. Analysis of Proposed System
2.1. Interference Rejection
The proposed architecture employs frequency conversion as done in  to coexist with other existing narrowband systems. To achieve this technique, a proper choice for the frequency of the Local Oscillator (LO) is made. Let us investigate this issue in terms of some practical numerical values. If the LO frequency is set to the center frequency of the pulse, which for simplicity is chosen to be in the middle of the band allocated by the FCC for UWB (6.85 GHz), the system acts like a zero IF system and the bandwidth is halved, that is, the band from 3.1 GHz to 10.6 GHz is transformed into a baseband signal from DC to 3.75 GHz. However, the wireless LAN (WLAN) originally residing at around 5.5 GHz is shifted to 1.35 GHz, so it is still in band. Another option is to set the oscillator frequency to this WLAN frequency, such that this interference is shifted to around zero making possible to remove it using a simple high-pass or bandpass filter.
A possible disadvantage is that the downconverted bandwidth of the UWB signal extends up to approximately 5.1 GHz and the converted 2.4 GHz interferer falls in band. A third option is to set the LO to 5.5 GHz and filter out below 0.35 GHz and above 3.1 GHz. By this way, we remove the interferences completely with a simple bandpass filter, albeit at the expense of the loss of part of the incoming frequency band, from GHz to 10.6 GHz and from 5.15 to 5.85 GHz, which is a bandwidth of 2.7 GHz, being only 36% or equivalently 1.9 dB.
2.2. Information Decoding and Performance Analysis
To implement the right most term in (9), we need the frequency components of and at discrete position of frequency domain. This is realized by a series of analog correlators and termed as sampler in Figure 2. Due to Fourier transform properties, the frequency resolution achievable from a time-domain signal confined in duration is calculated by . Thus, the sample values of and at discrete position can be obtained from the sampler.
According to (10), a signal signature resides only on real-axis. Thus a receiver to implement (13) uses the following decision rule:
In the same way, for lower path,
Defining and . Now a message point for is located at in the constellation diagram while a message point for is at and hence a decision boundary is the line to bisect the line at right angle connecting two message points. Two message points are always symmetrical with respect to origin, and the connecting line has a slope of . Let us rotate the coordination by , then two message points are relocated at and where . Then the decision rule becomes
The ratio of SNRs of two systems is
However, (24) implies that the more θ is departed away from , the better the performance of our proposed receiver architecture. Theoretically, two systems are identical regarding performance when . Suppose that as it is intuitionally, the variance of and are same, then yielding a reasonable improvement (double or 3 dB) in SNR performance by new system. Similar approaches can be carried out for other modulation schemes such as QPSK and QAM.
3. Simulation and Results
Gausian 2nd pulse
Over sampling rate
Number of frequency components
BPSK, QPSK, QAM
Number of input data bits
New low power TR-UWB receiver architecture has been introduced. It amalgams existing downconversion-based narrowband rejection mechanism in RF front end and signal processing in frequency domain. A detailed mathematical framework has been formulated for close form expression of SNR that is used to compare the system performance. Theoretical analysis shows that separating the converted UWB pulses into real and imaginary parts, independently correlating and introducing rotation of coordination brings a 3 dB improvement in performance regarding Signal-to-noise-ratio (SNR) using binary modulation. This theoretical conclusion has been verified by performing computer simulation. Additional link level simulations show that the proposed system is also well behaved for higher order adaptive modulation (HOM) by improving 6 dB SNR in every scheme which is really fruitful for WBAN as it requires extremely low power.
This receiver needs perfect tuning while decomposing the received signal to multiple frequency tones within sampler, otherwise, it suffers from interferences and may creates a number of complexities. In future, the performance of this receiver will be investigated introducing synchronization mechanism. Performance evaluation of this proposed system considering fading environments is also a part of our future work.
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