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Design and Implementation of A Real-Time ERP-OFDM SDR Receiver on the USRP2 Platform Kerem Kucuk
Department of Computer Engineering Kocaeli University Kocaeli, Turkey Email: [email protected]
Abstract-In this paper, the real time ERP-OFDM wireless lo cal area network (WLAN) receiver prototype and its performance analysis based on IEEE 802.11g physical layer requirements are presented. We design and implement a software defined radio (SDR) receiver for all ERP-OFDM WLAN functionalities. The receiver prototype setup based on the universal software radio peripheral 2 called USRP2 with National Instruments (NI) LabVIEW software. It consists of a dipole antenna, the USRP2, and a computer setup USRP2 driver. In this prototype implementation,
estimation, channel estimation, and demodulation over the real wireless channel conditions. To validate the real-time prototype receiver, the spectrum of 2.4 MHz band is measured by IEEE 802.11g supported Wi-Fi card. Moreover, the prototype receiver and Wi-Fi card performances are in very closed agreement with a
frame error rate of 1 %. Hence, this prototype has been designed successfully to implement all PHY functions of ERP-OFDM.
In the literature, there are some SDR based receiver designs to measure specific characteristics in the desired network. Digital beacon receiver presented in  uses USRP to measure high frequency beacon measurement. In , test method is ap plied to receiver and transceiver I1Q imbalance measurements on the USRP equipped with FLEXRF1800 and XCVR2450 daughter boards. Chen et al.  examined two hardware platforms USRP2 and small-form-factor development platform to quantify minimum response delay. Shah et al.  studied positioning algorithm using RF signals. To demonstrate the ability of their technique, GNU radio and its hardware com panion, USRP are used. The SDR IEEE 802.11 b receiver in  enables practical channel impulse response estimation. The receiver is built on GNU radio and USRP hardware.
Keywords-IEEE 802.11g, OFDM, SDR, USRP2, LabVIEW,
Wireless local area networks (WLAN) have been used with the increasing capacity and wide range applications  in the last decade. Rapid prototyping and verification of the WLAN receiver are important to determine the WLAN system performance. To adapt and to meet requirements of these systems, software defined radio (SDR) implementations is relied. Prototyping WLAN receivers on SDR platform that is universal software radio peripheral 2 (USRP2) is becoming a common technology for research to education . The one of the most popular PRY in IEEE standards is ERP-OFDM . The OFDM technique achieves the robust ness against narrow-band interference. To use the overlapping multi-carrier technique, however it requires reducing crosstalk among sub-carriers, which means that orthogonality between the ditlerent modulated carriers is provided. In the IEEE 802.11g standard, the OFDM based transmission technique provides a WLAN with data payload communication capabil ities of 6-54 Mbitls. The IEEE 802.11g  uses mainly OFDM technique but can promote spread spectrum modulation if any one component of the system has older equipment. This study has been supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under project grant I059BI91000372 and Scientific Research Project Units of Kocaeli University under project grant 2013/13.
In this paper, we have focused on the real-time imple mentation of ERP-OFDM WLAN protocol baseband receiver prototype and validation of it with uncontrolled wireless traffic. This baseband prototype has been developed to be a valuable research and education tool in ERP-OFDM channel measurements. The implementation is built upon National Instruments's (NI) LabVIEW software tool  and Ettus Research's universal software radio peripheral 2 (USRP2)  equipped with XCVR2450 daughter board. At the receiver, the USRP2 motherboard is placed between the RF front-end and the host computer. In general, conventional methods for receiver design are prototyping on the board or programming an field programmable gate array (FPGA) chip. These methods may take a longer time than to design the SDR. This prototype design is practical to be used complete WLAN receiver. Additionally, the baseband receiver prototype measurements are compared to the produced Wi-Fi card measurements in terms of frame error rate. The paper is organized as follows. Section II revIsits transmit modulation of ERP-OFDM. Section III explains the theoretical background the ERP-OFDM receiver design. The implementation of the ERP-OFDM receiver on LabVIEW with USRP2 platform is discussed in Section IV. Section V describes the evaluation of the prototype receiver and performance comparison, and finally the conclusion of the paper is presented in Section VI.
TR ANSMIT MODULATIONS FOR
In the ERP-OFDM transmitter, the transmitted OFDM frame is consists of several OFDM symbols. The frame con tains a physical layer convergence protocols (PLCP) OFDM preamble, signal field and data portion. The OFDM preamble has 12 symbols used to synchronize the receiver. The signal field express the parameters of the following data packet of the physical layer service data unit (PSDU) payload. Data part contains a ditlerent number of symbols. The OFDM training structure has short training (ST) sym bols and long training (LT) symbols 10 times and 2 times respectively. While the ST symbols are used for detection of signal, gain control and estimation of coarse frequency, the LT symbols are used for frequency and channel estimation stages. After the IFFT and multiplied by the window function, short preamble signal is obtained. Two periods of the LT symbols are transmitted for fine frequency and the channel estimation. After the IFFT, adding guard interval (GI) and multiplied by the window function, time domain long preamble signal is constituted. The OFDM preamble is followed by the signal field PRY of PLCP section. It consists of the rate and length of the transmitted data. The signal field encoding is performed with the BPSK modulation and convolutional encoding (CE) at R 1/2. The signal field encoding procedure includes CE, interleaving, modulation mapping, pilot insertion, and OFDM technique. The OFDM signal field is followed by the data field. Data field contains two octets zero for service field that initializes data scrambler, variable length of data octets from medium access control (MAC) layer, encoder is reset by 6 tail bits and required pad bits. The data field encoding procedure includes scrambling, CE, puncturing, interleaving, modulation mapping process, pilot insertion, and OFDM modulation. The data field is scrambled by the frame synchronous scrambler. The convolutional encode of the transmitter of the ERP-OFDM uses the standard generator polynomials. The higher data rates are achieved by using other rates and puncturing patterns. The data interleaving of ERP-OFDM is defined by two step permutation. While the first step provides coded bits with non adjacent sub-carriers, the second step provides coded bits with the less and more significant bits of the constellation. The output of the modulation mapping process is translated time domain by 64-point IFFT. The transmitter output for one OFDM symbol can be expressed as; =
Pkej27rkf:;.F(t-TGJ) , (1)
where, WTS is the windowing function of duration time, To! is the GI duration, NSD and NST are the number of data and total sub-carriers, dk,n is the complex number corresponding to the kth sub-carrier of the nth OFDM symbol, the frequency spacing is D..F, Pn is the scrambling sequence, Pk is the
pilot sub-carrier, and M(k) expresses the mapping sub-carrier number into frequency otlset index. The OFDM transmitter output can be written as; NsyM-1 SDATA(t)
SDATA,n(t - nTsyM),
where NSYM is the number of transmitted OFDM symbols and TsyM is the symbol interval. III. ERP-OFDM
In the OFDM based systems such as IEEE 802.11 g, the discrete time OFDM signal with N sub-carriers is expressed as  ; N-1
where X(k) is the vector of complex modulation. The received data is retrieved by FFT on the received signal as; X(k)
1 k -Z;::0 x(n)ej27rN
If additive white Gaussian noise (AWGN) is considered, the received OFDM signal is expressed by y(n)
x(n) @ h(n) + w(n),
where hen) is the channel impulse response (CIR). The mul tipath CIR is described; L-1 h(n)
L hzej27rin,TN 6(>, z=o
where h(Z) is the CIR of Zth path, L is the number of the total propagation paths,fD, is the Doppler frequency of the lth path, A is the index of the delay spread and TZ is the delay of the Zth path. After discarding GI from yen), FFT processing is done on the received signals. Then the time domain received signals are transformed to the frequency domain signals. Y(k)
1 n tr -Z;::0 y(n)e j27rf,y
Assume that the GI is longer than the length of CIR, that is, there is no lSI between OFDM symbols, the transformed signals Y(k) can be expressed by: Y ( k)
X(k)H(k) + W(k)k
0,1, ... , N
where W(k) is AWGN Fourier transform, the discrete fre quency impulse response of multi-path channel H(k) is written as; H(k)
in T hZej27r ,.
sin(nln T) -J' 2nT, k nIn � e ., N
The transmitted signals X(k) can be estimated; X(k)
( k) � , H(k)
where X(k) is the estimation of CIR. The basic OFDM re ceiver used in this project is shown in Fig. 1. After the received signal down converted to the baseband, the receiver performs t...