12-04-2012, 05:09 PM
OFDM Link Performance with Companding for PAPR Reduction in the Presence of Non-Linear Amplification
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Introduction1
Orthogonal Frequency Division Multiplexing (OFDM) is a firmly established air
interface for fixed wireless, WLAN, and mobile wireless applications. Because OFDM is
a multicarrier waveform, it may exhibit large peak-to-average power ratios (PAPR),
which can result in saturation of the amplifier or alternatively encourage use of large
back-offs that impact power efficiency and subsequently SNR at the receiver. Hence,
methods to reduce PAPR are of interest, since they potentially can yield performance
gains in these systems.
System Description
The system that is analyzed in this work involves an OFDM air interface with system
functions and impairments as illustrated in Figure 1. The transmitter section maps a
random data bit sequence, btx, into a sequence of QAM symbols, Stx. The QAM symbols
are partitioned into N-length blocks and modulated onto the subcarriers of an OFDM
modulator via the inverse Discrete Fourier Transform (IDFT). After guard interval
insertion, the resulting OFDM signal is integrated into a frame that includes a preamble
and training sequences for synchronization and channel estimation, respectively. Prior to
transmission, the signal is companded with a u-law compander, and then scaled for power
normalization. The scaled, companded signal is then passed through the transmit
amplifier, which distorts the signal according to non-linear solid state power amplifier
models [9].
Companding Transform
Our strategy in this work involves applying u-law companding at the transmitter to
reduce the PAPR of the transmitted waveform so as to reduce distortion through the
transmit amplifier and allow operation closer to amplifier saturation. Values of u ranging
between 0.125 and 64 were used in the study since the optimal performance was found to
reside within this range of operation.
Simulation Results
The symbol distance error metric with and without companding are presented in
Figures 7 through 9 below for SNRs of 25 db, 30 db, 35 db, and 40 db. In each figure,
the abscissa represents log2(u), and the ordinate represents the backoff value in db.
Constant error contours are shown, where the contours correspond to the error values
associated with each backoff when companding is not used (i.e., when u = 0). For low
backoff values, the transmit power is highest, but the performance at the receiver exhibits
high error rates due to amplifier nonlinearities. The error rate falls as the backoff
decreases but then climbs as the error rates begin to be dominated by low SNR at the
receiver.
Conclusions
We have investigated the link performance of OFDM with companding as a PAPR
mitigation strategy. Impairments from nonlinear distortion at the transmitter, AWGN
noise from the channel, and noise amplification due to the expansion transform at the
receiver were considered. Simulation and hardware testing were employed to investigate
performance trends as a function of companding parameters, amplifier nonlinearities,
backoff, and SNR. The simulation tests utilized a symbol error distance metric, while
hardware testing utilized bit error metrics. The results from the study show that at low
backoffs, impairments from nonlinear amplification can be significant. Increasing the
compression (via the companding parameter, u) reduces the non-linear distortion and
suggests that moving the operating point towards saturation might be beneficial.