BER Performance of IR-UWB Signal Based WBANs with BPPM

The basic requirement of Wireless Body Area Network (WBAN) is to send physiological signals acquired from implantable or wearable sensor nodes to a remote location. Low-power consumption is required for WBANs since most medical sensor nodes are battery operated. Impulse Radio-Ultra Wideband (IR-UWB) is a suitable wireless technology for the use in WBAN applications due to its inherent properties such as low power consumption,high data irate capability, low complexity hardware implementation, and small form factor.In this paper, the sensor nodes use two mechanisms to ensure that the transmit power is managed effectively. Firstly, the maximum allowable Full Bandwidth (FBW) transmit power is evaluated. Secondly, the number of Pulse Per Bit (PPB) can be dynamically changed according to the Bit Error Rate (BER) required value by using Binary Pulse Position Modulation (BPPM) scheme. These mechanisms enable the sensor nodes to operate at optimum power consumption and dynamic BER while maintaining a reliable data communication link.


II. OVERVIEW of IR-UWB SYSTEM
IR-UWB is a novel wireless short-range technology. According to FederalCommunications Commission (FCC), IR-UWB communications operate in 0-960 MHz and the 3.1-10.6 GHz bands. IR-UWB signals have a fractional bandwidth largerthan 0.2 or at least 500 MHz. Since IR-UWB systems use ultra-wide bandwidth, the transmission irate of IR-UWB systems can go up to 20 Mbps. In the same time, the emission powerofIR-UWB must be kept below −41.25 dBm/MHz. As a result, IR-UWB devices canenjoy a muchlongeroperating time with a battery. On the other hand, low power transmission of signal limits the communication range (usually 0.1-2 m) [3]. Outof the existingwirelessphysicallayer technologies, WirelessLocal Area Network (WLAN) standard is rarely used in WBAN applications because of its large power consumption. Zigbee, Bluetooth, and WLAN operate in the 2.4 GHz unlicensed Industrial, Scientific and Medical (ISM) band; hence create interference issues to each other. Medical Implant Communication Services (MICS)band can only be used for low data irate WBAN applications due to its limited bandwidth capabilities.
It can be concluded that the IR-UWB presents some unique benefits overotherwireless technologies in the designof WBAN sensornodesincluding the low power requirements, high data irate capability, small form factor, and uncomplicated circuit design. In terms ofinterference rejection, IR-UWB spectrumprovides a large bandwidth; hence, a sub-band ofIR-UWB can be selectedfor a particularapplicationsuchthat the interference fromother bands is minimized [4].
IR-UWB transmitter use simple short discrete pulses in order to transmit data. The IR-UWBpulse generation technique is shown in figure 1. IR-UWB pulses are generated by passing a squarewavesignaland its timedelayedversion through an XOR gate. Buffer with 3 v supply voltage isused to introduce delay level to the signal.The data bit generated by the microcontroller is modulated by the IR-UWB pulse stream using a BPPM before entering the Low Pass Filter (LPF). Employing LPF in order to filterout the 0-1 GHz sectionof the IR-UWB pulse spectrum. Thisportionof the spectrum is the highestpowercompared to restof the spectrum. Filtered spectrum is then shifted using a mixerand a Voltage ControlledOscillator (VCO) operating at 4 GHz. A band passfilter is used at the outputof the mixer in order to contain the IR-UWB signalswithin the 3.5-4.5 GHz band.This technique offers the highestpower efficiency for anIR-UWB transmitter.An IR-UWB signalwith a bandwidthof 1 GHz centered at 4 GHz, pulse width of 2 ns and Pulse Repetitive Frequency (PRF) of 100 MHz is shown in figure 2.PRF affects the numberofspectrallinesandtheiramplitudesthat lie within a certain bandwidth. A higher PRF system tends to create a lessernumberofspectral lines that are higher in amplitude.

III. IR-UWB PULSE POSITION MODULATION SCHEME
IR-UWB systems transmit short pulses to transmit data. Pulsed nature of IR-UWB transmitters enables the use of simplest modulation schemessuch as BPPM. BPPM scheme enables less complex hardware systems implementing IR-UWB communicationsystems and reduce the powerconsumption significantly.Also, it provides the best performance in terms of modulation efficiency and spectral performance. It is therefore more suitable forbatteryoperated WBAN applications. BPPM scheme uses the positionof a pulse in two time slots to represent the value of an information bit, i.e. presence of a pulse in the first time slotindicates a '1' and that in the second time slotindicates a '0'. In other words, a binary communication system can be established with a forward or backward shift of the pulse in time. When demodulating a BPPM signal, anIR-UWB receivercompares the energyof the receivedsignal in the twotime slots. If the energy in the first time slot is larger thanthat in the second time slot, a '1' is received; otherwise, a '0' is received. As shown in figure 3. Thus, the keyparameter in pulsepositionmodulation is the time delay ofeach pulse [5].Analytically, the signal can be represented as:

A. Transmission PowerLimitations
According to the FCC regulations, an IR-UWB signal is a powerlimited by measuredFullBandwidth (FBW) peakpowerof 0 dBm (1 mW) andmeasuredaveragepowerdensityof -41.25 dBm/MHz (75 nW/MHz) [7]. The measurementof the average and peak power can be calculated easily using a spectrum analyzer in practice. For the average power measurement, the resolution bandwidth is 1 MHz with an integration time of 1 ms. A resolution bandwidth of between 1 and 50 MHz can be used for the measurement of the peak power. The peak limit is dependent on the resolution bandwidth and varies according to [8]: Peak power 20 log dBm ...
These measuredpower limitations can be converted to maximum allowable FBW transmitpower limits using (3) [9]: Where Ppeak is the actual maximum transmitpowerof the IR-UWB signal, Bp= 1/τ, τ is the pulse width, R is the PRF and δ is the duty cycle of pulse transmissioni slot based on a measurementdurationof 1 ms. For the simulated system, a pulsewidthof 2 ns is used. Hence Bpin (3) is equal to 0.5 GHz for the IR-UWB signals used in the simulations. Figure 6 shows the variation of maximum allowable FBW transmitpowervalueswith the duty cycle (δ) for a sensor node that generatesIR-UWB signalswith a PRF of 100 MHz and a pulsewidthof 2 ns. According to figure 6, the duty cycle of the IR-UWB signal should be kept within 10% in order to transmit at a maximum allowable power of 0.0186 mW(-17.3 dBm), which complies with the FCC limitations.

B. BER Analysis of Multiple PPB Scheme
BER stands for bit error rate. It is the numberofbiterrorsdivided by the total numberof bits itransmitted during a studied time interval. It is usually used as a performancemeasurement in digital communications. The bit errors in a WBAN environmentmainlyoccur due to multipath interference and random fading of the IR-UWB signal that originates from reflection from various surfaces anddifferentabsorption characteristics of objects, such as various body surfaces and indoorequipment [10].
Since the powerrequired to transmit a databit is equal to the summationof the powerof a numberofpulsessent to represent that data bit, a considerablepowersaving can be achieved if the allocationof the numberof PPB can be dynamically changedaccording to the BER required value at the receiver end. Assume that two identical sets ofdata are transmitted using the same transmitpower and same separation distance in a realistic WBAN environment that is susceptible to multipath interference and random fading with onedata set itransmitted using a higher PPB valueand the otherwith a lower PPB value. Thetransmit signal withhigher PPB transmissioniresults in a lower BER than a lower PPB transmissionifor the same separation distance in a realistic environment with fading and multipath interference. Probability of error for single pulse detection of the receiver with BPPM modulationscheme can be derived from [11]: Where Pe is theprobabilityof error, B is the signal bandwidthof 1 GHz, Ts is the integrationperiodwhich is equal to the pulsewidthof 2 ns forthe simulation, Ep is the received signal energyduring the 2 ns integrationperiod and Q represents the Q function. When multiple PPB is sent, it is assumedthat a bit is erroneous when more than halfthe pulses sent per that bit are erroneous. If N pulses are sent per bit, probability that a bitbeingerroneous can be obtainedby: From the above result, we can conclude that the numberof PPB can be dynamicallychanged in order to control the BER value. For best system performance, the optimum BER value is 10 fori all sensor nodes, because a good throughput can be obtained with this value.

VI. CONCLUSION
In this paper, the performance of IR-UWB system based WBAN was studied. A simple BPPM technique is used as the modulationscheme for the IR-UWB transmission.The FBW transmit power and PPB mechanisms are analyzed. Thesemechanisms lead to dynamic BER andpowercontrol at the sensor nodes, which helps to improve the reliability of communicationandpowerefficiencyofsensor nodes underdynamicchannel conditions.It can be concluded that the duty cycle of the IR-UWB signal should be kept within 10% in order to transmit at a maximum allowable power of 0.0186 mW. Furthermore, when sending more number of PPB results in lower BER.