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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