- Research Article
- Open Access
On the Evaluation of MB-OFDM UWB Interference Effects on a WiMAX Receiver
© Eduardo Cano et al. 2010
- Received: 1 November 2009
- Accepted: 6 July 2010
- Published: 26 July 2010
The European Commission has recently adopted specific power spectral density masks for ultra wideband (UWB) devices, with detect and avoid capabilities, for coexistence with licensed standards. Under these regulations, a novel approach for analyzing the UWB interference effects on the WiMAX downlink is provided in this paper by means of a novel theoretical computation of the bit error rate (BER), simulation results, and measurements in a conducted modality. New analytical BER expressions for both uncoded and coded WiMAX systems, impaired by a single multiband-OFDM (MB-OFDM) UWB interference signal, are obtained in this paper for a Rayleigh fading channel. The BER is expressed in terms of the characteristic function of the interference signal. The maximum permissible interference levels and the signal-to-interference (SIR) values, which allow the UWB interference effects to be considered negligible, are estimated in this paper from simulation and measurement results. The analysis considers a WiMAX receiver operating at its minimum sensitivity level. The BER, the symbol error probability (SEP), and the error vector magnitude (EVM) of the WiMAX link are the metrics employed to characterize the interference effects for both frequency hopping and nonfrequency hopping UWB interferers.
- Additive White Gaussian Noise Channel
- Error Vector Magnitude
- Symbol Error Probability
- WiMAX System
- Interference Power Level
The demand for reliable, fast, and low-cost data communications services for all types of wireless applications and environments has increased rapidly in the last few years. Often, different types of wireless networks coexist in the same area and share the communications channel. In such situations, if appropriate mitigation techniques are not applied, wireless signals coming from different sources could interfere with each other causing a considerable degradation in system performance. The coexistence scenario analyzed in this work corresponds to the case of a single ultra wideband (UWB) transmitter operating at the same frequency band as a WiMAX receiver. UWB technology is established as a viable candidate for future wireless personal area networks (WPANs) that require the processing of information with low-power sources at very high speeds across short distances (order of 10 m) . Alternatively, WiMAX systems, which are derived from the IEEE 802.16 air interface standards [2, 3], allow for high-speed broadband connectivity in cellular point-to-multipoint wireless metropolitan area networks (WMAN) of wider range (order of 5 Km).
The Federal Communications Commission (FCC) in the US approved the use of UWB technology for commercial applications under part 15 of its regulations in February 2002 . The FCC report and order defined UWB as a signal with bandwidth to central frequency ratio greater than or, alternatively, with a 10 dB bandwidth exceeding MHz in the frequency range of 3.1–10.6 GHz. The FCC permits UWB devices to operate on an unlicensed basis following restrictive power spectral masks for both indoor and outdoor environments. A maximum mean effective isotropic radiated power (EIRP) spectral density of 41.3 dBm/MHz is established over all the 7.5 GHz operation bandwidth. Under these initial conditions, UWB devices can cause harmful interference to primary services operating simultaneously in their vicinity. This is the scenario under which WiMAX systems operate at GHz in Europe.
The objective of this work is to evaluate the interference effects caused by a UWB transmitter, compliant with the EU DAA regulations and which follows the multiband OFDM (MB-OFDM) approach , on a WiMAX receiver by means of theoretical analysis, simulations, and experimental results.
Several studies that evaluate the coexistence between WiMAX systems and UWB devices with DAA functionality have been carried out in the literature [8–14]. However, there is a lack of published work that validates the theoretical findings in practical implementations and viceversa. In an analytical approach, novel expressions for the bit error rate (BER) for uncoded/coded WiMAX systems are presented in this paper, based on the statistical characterization of the MB-OFDM UWB interference. A similar approach for obtaining the BER in coded systems can be found in [15, 16] and for uncoded systems in . In contrast to the aforementioned works, a novel closed form of the BER for the WiMAX link in the presence of Rayleigh fading is obtained by means of computing the characteristic function of the MB-OFDM interference signal without using numerical integration methods. Furthermore, the analytical BER functions obtained in this paper are expressed in terms of the maximum allowable signal-to-interference (SIR) levels measured at the input of the WiMAX victim receiver. In the measurement study, the impact of the UWB interference on the WiMAX receiver is analyzed in a conducted modality using the error vector magnitude (EVM) and the symbol error probability (SEP) as evaluation metrics.
The remainder of the paper is organized as follows. Section 2 provides a detailed description of the WiMAX communications link and the processing of the received signal, as well as the model of the MB-OFDM UWB interference. In Section 3, novel analytical expressions for the BER for both uncoded and coded WiMAX systems in the presence of a single MB-OFDM UWB interference are presented, along with a link budget analysis to estimate the interference margins. Simulation and experimental results of the most relevant scenarios, in the context of interference, are presented in Sections 4 and 5, respectively. Finally, conclusions are presented in Section 6.
In this paper, , , , , , and denote complex conjugation, statistical expectation, the real part of a complex number, the imaginary part of a complex number, the probability of an event, and the convolution operator, respectively.
The system model consists of a WiMAX base station, transmitting data information to a WiMAX customer-premises equipment (CPE) receiver, and a MB-OFDM UWB transmitter that follows the ECMA-368 standard .
2.1. WiMAX System
The WiMAX system employed in this work follows the specifications of the IEEE 802.16-2004 for fixed wireless access networks . This system is based on OFDM with subcarriers, of which are used for data processing, are nulled for guard band protection and are designated for channel estimation purposes.
A robust forward error control (FEC) technique based on a two-stage process is employed in the standard. This concatenated code is constructed by using an outer Reed-Solomon (RS) code and an inner punctured convolutional code (CC). The CC encoder corrects independent bit errors, while the RS code corrects burst errors at the byte level. Four modulation schemes are specified in the IEEE 802.16-2004 standard for both downlink (DL) and uplink (UL) transmissions. These modulation schemes are binary phase shift keying (BPSK), quaternary phase shift keying (QPSK) and M-ary quadrature amplitude modulation (QAM) with modulation orders and . The PHY specifies seven burst profiles as a result of combining modulations and FEC rates that can be assigned to both CPEs and base stations. The selection of an appropriate modulation-code combination depends on the required performance, taking into consideration tradeoffs between data rate and system robustness. Two modulation-coding formats, QPSK and -QAM with overall coding rates and , respectively, are used in this work.
where is the interference signal contribution measured at the WiMAX receiver.
where represents the frequency-domain channel phase estimated at the coherent WiMAX receiver and it is uniformly distributed on . Perfect channel state information is assumed in this paper.
where is the baseband UWB interference signal and is the channel impulse response of the filtered UWB interference of duration . The parameters and in (5) are the frequency offset of the UWB interference relative to the WiMAX center frequency and the time delay of the UWB interference measured at the input of the WiMAX receiver and uniformly distributed on , respectively.
2.2. MB-OFDM UWB Interference
The interferer system employed in this work is modeled as a MB-OFDM UWB transmitter, which follows the ECMA- standard . In MB-OFDM UWB systems, the available 7.5 GHz bandwidth is divided into fourteen subbands, each having a bandwidth of 528 MHz. These subbands are grouped into six band groups (BG1-BG6) of three subbands each, except BG5 which has two subbands. The center frequency of the th subband is defined as MHz.
The MB-UWB OFDM signal is organized in packets that are sequentially composed of preamble, header, and payload data symbols. The payload data can be transmitted at different data rates. The data rate values fixed by the standard, are , , , , , , , and Mbps. These data rate values are obtained by selecting different combinations of modulation schemes and coding rates. The coding rate value is obtained at the output of a puncturing block with values , , , and . Two different modulation schemes are implemented; a QPSK scheme for data rates of Mbps and below and a dual carrier modulation (DCM) scheme that is used for higher data rate values.
The header and the payload data symbols are generated by using an OFDM technique with subcarriers of which are data subcarriers, are pilots, are for guard protection and the rest are nulled.
The time-domain samples of the preamble, header, and data payload are concatenated to generate the baseband discrete packet and then passed through a digital-to-analog converter (DAC). The continuous signal is up-converted to the RF frequencies by using a time-frequency code (TFC) pattern that allows frequency-hopping capabilities over the different bands that integrate a band group. Among all of the ten different TFC codes, TFC1, and TFC5 applied in BG1 are of particular interest in this paper, since they reflect the effects of the hopping and nonhopping MB-OFDM UWB interference signal, respectively, on the WiMAX band.
where is the modulation value of the symbol mapped into the subcarrier and is the transmitted power of the interference signal. Similarly, the function in (6) is obtained as , where is the basis function modeled as a rectangular pulse of unitary energy with duration equal to the symbol time . The following parameters , and are the subcarrier spacing, the bandwidth of the UWB signal, and the cyclic prefix duration, respectively.
where is the symbol duration of the MB-OFDM UWB signal without appending the cyclic prefix, and .
In this section, analytical BER expressions for the WiMAX link, impaired by MB-OFDM UWB interference, are provided for uncoded (Section 3.1) and coded (Section 3.2) systems using QPSK and -QAM modulation formats. Subsequently, the minimum required SIR values, which allow the interference to be considered negligible, and the minimum distance among DAA protection zones are estimated in Section 3.3.
3.1. BER Performance for Uncoded WiMAX Systems
where , , and are independent variables. Note that accounts for the interference-free situation.
In the following analysis, the CF of the decision variable is obtained by calculating the CF of the individual contributions which are fading of the primary signal, noise, and MB-OFDM UWB interference terms.
where , and is the variance of .
where is the variance of , which is independent of , and is the noise power spectral density.
where the following relationship is applied.
where and are the probability density functions (pdf) of the Gaussian random variables and , respectively.
where , , and is the energy of a transmitted bit.
where in the case of TFC5 and and for TFC1.
respectively. The index in (23) accounts for the data WiMAX subcarriers, is the mean received power of the WiMAX signal, is the noise power and the parameter takes values and for TFC1 and TFC5 interference modes, respectively.
3.2. BER Performance for Coded WiMAX Systems
where is the free distance of the convolutional code, is the truncating order, is the weight spectrum of the code and is the pairwise error probability, defined as the probability that the decoder erroneously selects a code sequence other than the transmitted one. The values of and are tabulated in [23, 24] for all the punctured codes.
where and is the code rate of the RS encoder .
where is the error correction capability of the code.
3.3. Estimation of Interference Margins
where is the nominal bandwidth of the WiMAX signal. The values of and are commonly set to 7 dB and 5 dB, respectively, and these values are used in this work.
where is the received power of the MB-OFDM UWB interference signal and models the increase of the receiver sensitivity due to the addition of the interference signal.
where and are the antenna gains of the UWB transmitter and the WiMAX receiver, respectively, and is the path loss with value . The parameters and are the speed of light and the distance between the UWB interferer and the WiMAX receiver.
Furthermore, the distance values, that delimit the zones in the DAA mechanism of Figure 2, can be calculated by using (34). As an example of this application, a WiMAX system with -QAM scheme, nominal bandwidth of 2 MHz and dBi is considered. In this situation, the two threshold areas of the DAA algorithm are established by setting m and m for dB.
WiMAX and MB-OFDM main parameters.
( -QAM )
( -QAM )
( -QAM )
32,24,4 (QPSK )
120,108,6 ( -QAM )
MHz; (TFC5), (TFC1)
4.1. Validation of Analytical BER Expressions
4.2. Simulation Results: Evaluation of Interference Effects
Initially, it is noticeable that the BER of the TFC5 interference systems degrades by approximately dB with respect to the TFC1 systems. This is due to the fact that only one third of the UWB interference symbols with TFC1 frequency hopping pattern cause interference to the WiMAX link. The Gaussian behavior of the interference can also be observed in this analysis. The BER waterfall curves of the TFC5 interference systems are almost identical to the noninterference coded BER curves, represented in Figure 9, but shifted approximately dB. This is due to the larger value of the interference variance.
Two WiMAX systems with transmission bandwidth values MHz and MHz are used in this initial analysis. The BER performances of these systems, plotted in Figure 10 for the case of TFC5, are shown to be practically identical, leading to the conclusion that the MB-OFDM UWB interference effects on an IEEE 802.16-2004 WiMAX system in an AWGN channel is independent of its subcarrier spacing. It was shown in  that the BER performance of a WiMAX system degrades as the subcarrier separation of the UWB interferer decreases. However, in the inverse situation, in which the subcarrier separation of the interference is fixed to MHz, the interference distortion on WiMAX systems with MHz ( KHz) and MHz ( KHz) behaves the same since only very few UWB subcarriers contribute to the interference component within the narrow WiMAX bandwidth.
The objective of the measurement analysis is to estimate the interference margin levels, and , in order to validate the results previously obtained in the simulation study. Initially, the laboratory test bed, implemented for the measurement campaign, is described in Section 5.1. Subsequently, measurement results given by EVM and SEP metrics are provided in Section 5.2 for different types of interference scenarios.
5.1. Laboratory Test Bed Description
The laboratory test bed for coexistence study between WiMAX and MB-OFDM UWB in the conducted modality is depicted in Figure 14. The instruments employed are listed as follows.
WiMAX baseband vector signal generator (Rohde & Schwarz SMBV100A). Upconverter: Agilent PSG E8267D.
WiMAX Receiver: Tektronix Real-Time Spectrum Analyzer RSA3408B.
WiMAX Demodulator: WiMAX IQSignal software application running on a stand-alone pc.
Two UWB MB-OFDM Sources: ( ) Tektronix AWG7000B UWB Signal Generator. ( ) Wisair DV9110 WiMedia evaluation system operating in the test mode connected to a variable attenuator (0–69?dB).
This test bed has been designed to monitor the errors in the WiMAX channel for any arbitrary values of SNR and SIR. The test bed has the advantage of employing a realtime spectrum analyzer as a programmable WiMAX receiver. Therefore, full control of the receiver parameters, such as center frequency, bandwidth, sampling frequency, and external triggering, is achieved. Also, it allows the use of a WiMAX demodulator software that provides a quantitative estimation of the interference impact on the WiMAX receiver. However, the noise figure of the spectrum analyzer is poorer than the state of the art WiMAX receiver and this difference needs to be taken into account in the measurements. An estimated noise floor of the spectrum analyzer of dBm/MHz is obtained, which is approximately dB poorer than a typical state of the art WiMAX receiver. Furthermore, the analog-to-digital conversion in the spectrum analyzer is made with -bits and, therefore, the receiver has a dynamic above dB. This can be significantly increased using the auto range functionality that sets an adaptive level of the reference signal.
5.2. Measurement Results
An interference scenario with a dominant MB-OFDM UWB interference signal, whose power level is significantly larger than the thermal noise in the WiMAX channel, is considered in the following analysis. The MB-OFDM UWB interference signals, with Mbps and power spectral density (PSD) of dBm/Mhz, are generated from the AWG7112B signal generator for TFC5 frequency-hopping pattern. This PSD value is dB larger than the noise floor. The measurement campaign is carried out in the worst possible interfering scenario, which corresponds to a duty cycle of the interference signal of . The average received EVM performances for the two burst profiles under these interference conditions are shown in Figure 15(b). The results illustrate that the WiMAX receiver with concatenated RS-CC coding is not capable of successfully demodulating the symbols when the SIR level is very low for TFC5 interference signalling. In particular, there are symbol errors when dB and dB for QPSK and -QAM , respectively. For larger values of the SIR, the measured EVM values are the same for both burst profiles and slightly larger than those without interference and AWGN noise.
New EIRP masks released by the European Commission in its Decision regulate the radio spectrum use for UWB equipment in the European Community. In particular, UWB devices are required to use interference mitigation techniques in order to coexist with licensed BWA systems, such as WiMAX at GHz, without causing harmful interference. The DAA mechanism, based on the definition of three zones of operation, dynamically allocates the power of the UWB devices by sensing the presence of WiMAX activity.
The objective of this work is to evaluate the performance of the WiMAX victim receiver under the presence of a single MB-OFDM UWB interferer with DAA capabilities. In the context of interference, a WiMAX receiver, operating in DL at its minimum sensitivity level impaired by an MB-OFDM UWB active interferer located in Zone of the DAA protection area, was identified as the most critical scenario. A comprehensive analysis of these interference effects has been provided in this paper by means of theoretical, simulation and measurement approaches.
Novel analytical expressions of the BER for uncoded and coded WiMAX systems, impaired by a single MB-OFDM UWB interference signal, were provided in this paper for both AWGN and Rayleigh fading channel environments. The BER expressions were obtained by applying the inversion theorem, which expresses the BER as a function of the characteristic function of the decision variable. In this approach, the complexity associated with calculating the exact BER is reduced by first computing the characteristic function of the received interference contribution. Furthermore, the maximum allowable interference levels were analytically obtained.
An extensive simulation analysis has been provided in this paper. Initially, the analytical BER expressions for uncoded QPSK and -QAM WiMAX systems, in the presence of a MB-OFDM UWB interference signals, were validated through simulations for different scenarios. Furthermore, the upper bound analytical BER expressions for coded QPSK and -QAM were also validated through simulations. Subsequently, the simulation results showed that the effect of the nonhopping UWB interference on the WiMAX link is dB larger than the hopping one. This is due to the fact that the frequency-hopped interference is only active one third of the time. The Gaussian behavior of the MB-OFDM UWB interference was also illustrated in the simulation analysis. Furthermore, it was shown that the MB-OFDM UWB interference effects on an IEEE 802.16-2004 WiMAX system in an AWGN channel is independent of its subcarrier spacing.
The simulation results also showed the effects of the intersymbol interference caused by selecting a short cyclic prefix length of the WiMAX signal in a multipath channel environment.
This simulation study allowed the BER values for to be graphically measured. In this situation, the results showed that the BER degrades considerably with respect to the case of noninterference, especially when TFC5 is employed. More restrictive SIR levels are required in order to neglect the UWB interference effects. The criterion was employed on the EVM performance to estimate the levels. It has been demonstrated that the SIR values for noninterference coexistence operability are dB and dB for QPSK with TFC1 and TFC5, respectively. For -QAM , the values are dB and dB for TFC1 and TFC5, respectively.
Measurements in a conducted modality have been carried out to analyze the effects of the UWB interference on the WiMAX link for two defined situations. Firstly, the UWB interference level is larger than the noise floor allowing levels, with no symbol errors in the demodulation process, to be set. Secondly, the UWB interference is of the order of the noise floor and the WiMAX receiver operates at its minimum sensitivity level. In this situation, it is concluded that the effects of the interference signal become negligible when the NIR is larger than dB.
- Porcino D, Hirt W: Ultra-wideband radio technology: potential and challenges ahead. IEEE Communications Magazine 2003, 41(7):66-74. 10.1109/MCOM.2003.1215641View ArticleGoogle Scholar
- IEEE Std. 802.16-2004 : IEEE Standard for Local and Metropolitan Area Networks, Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems. October 2004.Google Scholar
- IEEE Std. 802.16e-2005 : IEEE Standard for Local and Metropolitan Area Networks, Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems, Amendment for Physical and Medium Access Layers for Combined Fixed and Mobile Operation in Licensed Bands. February 2006.Google Scholar
- Federal Communications Commission (FCC) : Revision of Part 15 of the Commissions Rules Regarding Ultra-Wideband Transmission Systems. First Report and Order April 2002., (ET Docket 98-153, FCC 02-48):Google Scholar
- European Commission Decision 2007/131/EC : Allowing the Use of the Radio Spectrum for Equipment Using Ultra-Wideband Technology in a Harmonised Manner in the Community. June 2007.Google Scholar
- ECC Repot 120 : Technical Requirements for UWB DAA (Detect and Avoid) Devices to Ensure the Protection of Radiolocation in the Bands 3.1–3.4 GHz and 8.5–9 GHz and BWA Terminals in the Band 3.4–4.2 GHz. February 2007.Google Scholar
- Batra A, Balakrishnan J, Aiello GR, Foerster JR, Dabak A: Design of a multiband OFDM system for realistic UWB channel environments. IEEE Transactions on Microwave Theory and Techniques 2004, 52(9 I):2123-2138.View ArticleGoogle Scholar
- Mishra SM, ten Brink S, Mahadevappa R, Brodersen RW: Cognitive technology for ultra-wideband/WiMax coexistence. Proceedings of the 2nd IEEE International Symposium on New Frontiers in Dynamic Spectrum Access Networks, April 2007 179-186.View ArticleGoogle Scholar
- Rahim A, Zeisberg S, Finger A: Coexistence study between UWB and WiMax at 3.5 GHz band. Proceedings of IEEE International Conference on Ultra-Wideband (ICUWB '07), September 2007 915-920.Google Scholar
- Facchini F, Giuliano R, Mazzenga F: Ultra-wideband detect and avoid procedure for WiMAX victims. IET Communications 2009, 3(2):268-278. 10.1049/iet-com:20080452View ArticleGoogle Scholar
- Durantini A, Giuliano R, Mazzenga F, Vatalaro F: Performance evaluation of detect and avoid procedures for improving UWB coexistence with UMTS and WiMAX systems. Proceedings of IEEE International Conference on Ultra-Wideband (ICUWB '06), September 2006 501-506.Google Scholar
- Kogane R, Fukao C, Hioki J, Furusawa K, Fuj M II, Itami M, Itoh K: A study on the detection scheme of WiMAX signal for DAA operation in MB-OFDM. Proceedings of IEEE International Conference on Ultra-Wideband (ICUWB '07), September 2007 834-839.Google Scholar
- Perez J, Beltrán M, Morant M, Llorente R, Cavallin L: Protection margins for joint operation of WiMAX 802.16e and WiMedia-defined UWB radio in personal area networks. Proceedings of IEEE International Conference on Ultra-Wideband (ICUWB '09), September 2009, Vancouver, Canada 723-727.Google Scholar
- Kim K-W, Park J, Cho J, Lim K, Razzell CJ, Kim K, Lee C-H, Kim H, Laskar J: Interference analysis and sensing threshold of detect and avoid (DAA) for UWB coexistence with WiMax. Proceedings of the 66th IEEE Vehicular Technology Conference (VTC '07), October 2007 1731-1735.Google Scholar
- Nasri A, Schober R, Lampe L: Performance evaluation of BICM-OFDM systems impaired by UWB interference. Proceedings of IEEE International Conference on Communications (ICC '08), May 2008 3616-3621.Google Scholar
- Snow C, Lampe L, Schober R: Error rate analysis for coded multicarrier systems over quasi-static fading channels. IEEE Transactions on Communications 2007, 55(9):1736-1746.View ArticleGoogle Scholar
- Snow C, Lampe L, Schober R: Analysis of the impact of WiMAX-OFDM interference on multiband OFDM. Proceedings of IEEE International Conference on Ultra-Wideband (ICUWB '07), September 2007 761-766.Google Scholar
- European Computer Manufacters Association (ECMA) : Standard ECMA-368: High Rate Ultra Wideband PHY and MAC Standard. December 2005.Google Scholar
- Edfors O, Sandell M, van de Beek J, Landstrom D, Sjoberg F: An introduction to orthogonal frequency-division multiplexing. Research Report 1996. http://www.sm.luth.se/csee/sp/publications/Google Scholar
- Gil-Pelaez J: Note on the inversion theorem. Biometrika 1951, 38(3/4):481-482. 10.2307/2332598MATHMathSciNetView ArticleGoogle Scholar
- Proakis JG: Digital Communications. 5th edition. McGraw-Hill, New York, NY, USA; 2008.Google Scholar
- Yoon D, Cho K, Lee J: Bit error probability of M-ary quadrature amplitude modulation. Proceedings of the 52nd Vehicular Technology Conference (VTC '00), September 2000 5: 2422-2427.Google Scholar
- Haccoun D, Begin G: High-rate punctured convolutional codes for Viterbi and sequential decoding. IEEE Transactions on Communications 1989, 37(11):1113-1125. 10.1109/26.46505MathSciNetView ArticleGoogle Scholar
- Frenger P, Orten P, Ottosson T: Convolutional codes with optimum distance spectrum. IEEE Communications Letters 1999, 3(11):317-319. 10.1109/4234.803468View ArticleGoogle Scholar
- Wessel RD: Convolutional codes. In Wiley Encyclopedia of Telecommunications. Volume 1. Edited by: Proakis JG. John Wiley & Sons, Hoboken, NJ, USA; 2003:598-606.Google Scholar
- Wessman M-O, Svensson A, Agrell E: Frequency diversity performance of coded multiband-OFDM systems on IEEE UWB channels. Proceedings of the 60th IEEE Vehicular Technology Conference (VTC '04), September 2004 1197-1201.Google Scholar
- Erceg V, et al.: Channel Models for Fixed Wireless Applications. IEEE 802.16.3c-01/29rl, February 2001Google Scholar
- ECC Report 64 : The Protection Requirements of Radiocommunications Systems Below 10.6 GHz from Generic UWB Applications. February 2005.Google Scholar
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.