Signals And Systems By Sanjay Sharma Pdf 11
Hayley Sweigard
Digital predistortion (DPD) techniques are widely used to linearize of RF power amplifiers. In this article, a memory polynomial-based power has been modeled with memory order of 5 and nonlinearity order of 9. These specifications had been found suitable for advanced long-term evolution (LTE) wireless systems. A suitable DPD model was created based on the least square error minimization. Results for the complementary cumulative distribution function and normalized mean squared error were also computed. The adjacent channel leakage power ratio and error vector magnitude calculations showed that the proposed DPD for LTE system performed within the required limits as desired.
There are two classes (memoryless model and model with memory) for DPD. Memoryless models focus on the PA that has a memoryless nonlinearity, i.e., the current output depends only on the current input through a nonlinear mechanism [6]. This instantaneous nonlinearity is usually characterized by the AM/AM and AM/PM responses of the PA, where the output signal amplitude and phase deviation of the PA output are given as functions of the amplitude of its current input. Both memoryless polynomial algorithm and look-up table-based algorithm are two key algorithms for memoryless models.
As the signal bandwidth gets wider, such as in WCDMA, mobile WiMAX and 3GPP LTE and LTE-advanced (up to 100 MHz bandwidth, five component carriers of carrier aggregation), PAs begin to exhibit memory effects. This is especially true for those high PAs used in wireless base stations. The causes of the memory effects can be attributed to thermal constants of the active devices or components in the biasing network that have frequency-dependent behaviors. As a result, the current output of the PA depends not only on the current input, but also on past input values. In other words, the PA becomes a nonlinear system with memory. For such a PA, memoryless predistortion can achieve only very limited linearization performance. Therefore, digital predistorters must need to have memory structures [7].
The average input power should be kept to the level that the peak output power of the PA does not exceed 3-dB gain compression point. Otherwise, the extraction error will be large. The system was designed for an LTE bandwidth of 20 MHz, base DFT size of 2048, and transmission bandwidth configuration, expressed in units of resource blocks is chosen as 100. The total power dynamic range is the difference between the maximum and the minimum transmit power of an OFDM symbol for a specified reference condition and is set to 20 dB, the cyclic prefix used is normal and the modulation scheme used is 64-QAM.
The EVM is a measure of the difference between the ideal symbols and the measured symbols after the equalization. This difference is called the error vector. The value of EVM recommended for LTE systems for 64-QAM is 8%. Adjacent channel leakage power ratio (ACLR) is the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency and the minimum requirement of ACLR in LTE systems is set to 45 dB.
The PA model is used to adjust the value of the baseband signal and the input signal of the extracted DPD model to the same range. The PA output normalized by small-signal gain is taken as the input of DPD model. When the extracted DPD model is implemented, the actual input signal is the baseband signal. To make the DPD model work correctly, the baseband signal should be adjusted to the same range as the normalized output of the PA. The gain is the adjustment factor. Typically, the gain adjustment is implemented between the signal source and digital predistorter model as shown below. The value of gain is the adjustment factor calculated in this model.
Engineers migrating to 4G require a solution that makes implementing DPD fast and practical for 4G communications systems, i.e., one that can be used by engineers at all levels of expertise and requires minimal equipment. In modern communication systems, spectrally efficient wideband RF signals have a PAPR as high as 13 dB. CFR preconditions the signal to reduce signal peaks without significant signal distortion. By reducing PAPR, CFR allows the PA to operate more efficiently at higher power levels, without impacting compliance with spectral mask and EVM specifications. CFR acts on the signal itself, whereas DPD corrects for the PA nonlinearity, allowing the signal to be run even higher.
Figure 6 shows an LTE-advanced signal with 2 20 MHz carrier aggregation (40 MHz total signal bandwidth). The PA output spectrum is shown with and without DPD. Note the use of oversampling for an actual measurement bandwidth that is wider than the original signal. Without this additional bandwidth to quantify the out-of-band energy (distortion products), it is not possible to correct for this spectral regrowth later.
Table 1 shows the ACLR measurements at three stages which involve the stage before PA, after the PA, and then finally after the PA and after the DPD was employed. In LTE systems, the minimum requirement of ACLR has been set to 45 dB. Table 1 clearly shows the deterioration in the ACLR values after the PA stage and when DPD is employed, the ACLR values crosses beyond the threshold limits.
As engineers migrate to the advanced wireless communication systems such as LTE-advanced or 802.11ac, choosing and designing the right PA to meet design goals at the lowest possible cost becomes more difficult, both for base stations and mobile devices. Because DPD enables the PA to be operated with high efficiency, near saturation, and without significant signal distortion, it allows engineers to address many base stations/mobile device PA design challenges. This article considered the design of DPD systems to linearize PAs with memory effects. By adding a digital predistorter in the baseband, the PA is allowed to operate into its nonlinear region, thereby significantly increasing its efficiency. The efficiency gain translates into electricity and cooling cost savings for service providers and longer battery life for mobile terminal users. The challenge here is to address the memory effects exhibited by the higher PAs or the PAs for wideband signals. In addition, analog components in the transmitter have imperfections that need to be compensated as well.
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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