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【应用笔记】OFDMA系统的波峰因子降低(Crest Factor Reduction for OFDMA Systems)
波峰因子降低(Crest factor reduction,CFR)是一个用来降低正交频分复用(orthogonal frequency division multiplexing,OFDM)波形峰均比(peak-to-average ratio,PAR,峰值对平均功率比)的技术。一个OFDM信号是由在频域的一系列正交载波构成的，这些正交载波每个都被一个星座符号调制。
Crest factor reduction (CFR) is a technique for reducing the peak-to-average
ratio (PAR) of an orthogonal frequency division multiplexing
(OFDM) waveform. An OFDM signal is made up in the frequency
domain as a set of orthogonal carriers that are each modulated by a
constellation symbol. The main disadvantage of OFDM modulation is
that the time domain representation approximates a Gaussian
distribution and therefore exhibits large envelope variations. Power
amplifiers usually only have a limited linear region, making it necessary
to back off the amplifier so that the transmit signal never drives the
amplifier into the non-linear region, which causes spectral regrowth and
inefficient amplifier power.
By reducing the PAR of the input signal, you can achieve greater power
efficiency from the amplifier or use a less expensive power amplifier with
a more limited linear region. The CFR module reduces the PAR of the
signal and ultimately reduces the overall basestation cost or power
requirements.
CFR is ideal for implementation on Altera® FPGA families such as
Stratix® III devices, where the capability for supporting high sampling
frequencies, high throughput and low latency are essential. This
document demonstrates a block-based constraint-driven design entry
point that makes it easy for system engineers to develop optimal
hardware that is scalable for a variety of operating scenarios.
The CFR reference design includes the following key features:
■ Support for OFDM based systems, including Worldwide
Interoperability for Microwave Access (WiMAX) and 3GPP Long
Term Evolution (LTE)
■ Support for additional functionality including orthogonal frequency
division multiple access (OFDMA) symbol modulation, digital up
conversion (4× interpolation) and cyclic prefix insertion
■ Programmable mask for any environment
■ Programmable error vector magnitude (EVM) budget
■ Support for 10 and 20 MHz channel bandwidth, and is easy to
modify for other channel bandwidths
■ Up to 5dB PAR improvement
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