Performance Analysis of MIMO-OFDM Using NCT ... Performance Analysis of MIMO-OFDM Using NCT Companding

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  • International Journal of Electronics Engineering Research.

    ISSN 0975-6450 Volume 9, Number 3 (2017) pp. 409-427

    © Research India Publications

    http://www.ripublication.com

    Performance Analysis of MIMO-OFDM Using NCT Companding Transform

    Dr. K. Srinivasa Rao1 and Mrs. N. Roopa Vathi2

    1G. V. P. College of Engineering for Women, Visakhapatnam

    2G. V. P. College of Engineering for Women, Visakhapatnam

    Abstract In this paper a new non-linear companding transform (NCT) technique based on

    inverse hyperbolic cosine function is proposed and implemented along with Carrier

    Frequency Estimation (CFO) techniques in order to improve the performance of

    Multi Input and Multi Output Orthogonal Frequency Division Multiplexing (MIMO-

    OFDM) systems. In this proposal, significant PAPR reduction and an improved bit

    error rate (BER) performance can be achieved simultaneously, since the power

    distribution of the companded signal can be reallocated more reasonably while

    maintaining the average power level unchanged. An effective trade-off between the

    PAPR and Bit Error Rate (BER) performance is achieved by introducing an inflection

    point and variable companding parameters in the companding function, such that it

    enables more design flexibility and freedom in companding form. Desired PAPR

    threshold can be achieved by proposing technique, since it introduces the companding

    distortion as small as possible with appropriate selection of the companding

    parameters and forms. Different CFO estimation techniques are implemented to

    estimate ICI along with proposed methods to analyze the performance of MIMO-

    OFDM systems. The analytical expressions regarding the achievable PAPR reduction

    transform gain G, and signal attenuation factor are derived. Simulation results

    demonstrate that NCT and CFO estimation techniques substantially improve

    performance of MIMO-OFDM system.

    Keywords: MIMO-OFDM, PAPR, ICI, Nonlinear companding transform (NCT), Carrier Frequency 0ffset (CFO), high power amplifiers (HPA).

    1. INTRODUCTION Robust, high spectral efficiency and immunity to the interference caused by multipath

    fading of OFDM and very high data rate transmission of MIMO highlights the

    combination of MIMO-OFDM systems [1, 2]. MIMO-OFDM has been adopted in

    mailto:ksrinivas.ece@gmail.com mailto:roopa479@gmail.com

  • 410 Dr. K. Srinivasa Rao and Mrs. N. Roopa Vathi

    3GPP-LTE and IEEE 802.16 (WI-MAX) standards [3]. However, this system has two

    major limitations viz., High Peak-to-Average Power Ratio (PAPR) of the transmitted

    signals and Intercarrier interference (ICI). High PAPR leads to in-band distortion

    across subcarriers and undesired spectral growth if the linear range of higher power

    amplifier (HPA) is not sufficient at the OFDM transmitter [4]. ICI leads to distortion

    in received OFDM signal as orthogonality is destroyed among subcarriers of OFDM

    signal

    Many different types of PAPR reduction methods have been proposed in the literature

    [5-18], e.g. Clipping and filtering, selective mapping (SLM), partial transmit sequence

    (PTS), block coding, active constellation extension (ACE), and companding transform

    techniques. Among existing methods, clipping and filtering is a simple procedure to

    obtain the desired PAPR reduction, but it yields additional clipping noise, resulting in

    significant out-of-band interference (OBI) and other methods SLM, PTS are complex

    and require Side Information (SI) to recover the OFDM signal at the receiver. An

    approach called non-linear companding transform (NCT) was proposed to overcome

    the drawback of OBI. This technique was first introduced in [11], in which mu-law

    companding was implemented which efficiently outer performs the clipping.

    However, after implementing such logarithmic based compression on OFDM symbol,

    its average power increases, leads to more sensitive to the HPA. Since then, many

    different proposals have been made to refine NCT schemes in which two careful

    designs are considered to improve its overall performance. The first design focuses on

    optimizing the nonlinear characteristics of the companding profiles. The second one is

    to employ the statistical distribution of the OFDM signal. To achieve more significant

    PAPR reduction of OFDM signal, a novel technique based on the inverse hyperbolic

    cosine function is proposed. In this proposal, significant PAPR reduction and an

    improved bit error rate (BER) performance can be achieved simultaneously, since the

    power distribution of the companded signal can be reallocated more reasonably while

    maintain the unchanged average power level. Moreover, an effective trade-off

    between the PAPR and BER performance can be achieved by adjusting the variable

    companding parameters; also it allows more flexibility and freedom in the

    companding form. Both theoretical analysis and simulation results confirm the

    effectiveness of this technique.

    In addition to the high PAPR, OFDM is more sensitive to Inter Carrier Interference

    (ICI). It occurs due to mainly two reasons, viz. One is a mismatch in carrier

    frequencies due to Doppler shift. Because of this effect, orthogonality among

    subcarrier fails, in turn distortion occurs in recovered signal and hence BER

    performance degraded significantly. In the literature [19-21], several ICI mitigation

    techniques have been proposed, and mainly classified into two types, time and

    frequency doamain mitigation techniques. Time domain mitigation techniques

    include time doamin windowing and ICI cancellation schemes. Time doamin

    windowing technique utilizes a sharping filter to mitigate the ICI effect.The frequency

  • Performance Analysis of MIMO-OFDM Using NCT Companding Transform 411

    domain include CFO estimation and detection which utilizes estimated pilot symbol.

    In this subsection discusson some CFO estimation and detection techniques are

    analyzed.

    The remainder of this paper is organized as follows: section 2 reviews a typical

    MIMO-OFDM system. Proposed technique, its corresponding theoretical analysis,

    and CFO estimation techniques are described in section 3. Section 4 present

    simulation results and compared with existing NCT techniques. Final conclusions are

    given in section 5.

    2. PAPR of OFDM signals in MIMO-OFDM. Consider full rate Alamouti STBC-OFDM system with two transmitter and receiver

    antennas and as shown in Fig: 1. The bit stream from information source is mapped

    by digital modulator either M-PSK or M-QAM. Then these modulated symbols are

    encoded by Space Time Block encoder and output is fed to OFDM modulator. The

    following steps are performed to each of the parallel output streams corresponding to

    a particular transmit antenna to get OFDM signals.

     Pilot symbol insertion, IFFT operation.

     Addition of CP.

    After adding CP, proposed NCT compression is performed on OFDM symbols and

    up converted to RF frequeucy, then propagated through the MIMO channels. At

    receiver, CP is removed from the received signals and decompanding is perfromed by

    proposed NCT decompanding function, then reverse of OFDM operation (FFT) is

    performed on decompanded signals then passed through the channel estimator, in this

    stage CFO estiamtion is carried out and subsequently optimal ML detection is

    performed to detect the transmitted signal.

    MTx Antenn

    a

    S/P

    Sou

    rce

    S/P

    Decompanding

    & FFT

    Source

    Channel

    Estimation

    Source

    MIMO

    Detector

    Demodulator

    CP

    CP

    P/S

    Pilot allocation

    IFFT & CP

    Source

    NCT

    Compandin

    g

    Source

    NCT

    Compandin

    g

    Source

    P/S

    Sou

    rce

    P/S

    Sou

    rce

    Pilot allocation

    IFFT & CP

    S/P

    Alamouti ST

    Encoder

    Symbol

    Mapping

    Decompanding

    & FFT

    Source

    I/P

    Bits

    O/P Bits MRx

    Antenna

    Figure 1: Block diagram of MIMO-OFDM system using proposed NCT techniques

  • 412 Dr. K. Srinivasa Rao and Mrs. N. Roopa Vathi

    The frames 1mx and 2 mx are generated from xm as follows:

     

      

      

     

    

     

    )( *

    )1( *

    )1()(

    )1( 2

    )( 2

    )1( 1

    )( 1

    vmxvmx

    vmxvmx

    vmxvmx

    vmxvmx (1)

    where v = 1, 2, …, Ns. Rows represents antenna indices and columns represents time intervals i.e. xm(v) and xm(v+1) are generated from antenna 1, and -xm*(v+1) and xm*(v) are generated from antenna 2 during first and second time intervals. In the following subsections antenna index and time index are omitted from MIMO-

    OFDM signal for the sake of analysis convenience.

    Symbols )1(),0( XX and )0(),1( ** XX are tra