Orthogonal frequency division multiplexing (OFDM) is widely regarded as the key underlying air interface technology for wireless systems such as WiMAX and 3GPP long term evolution (LTE). Due to the inherent nature of these technologies, OFDM signals have high peak-to-average power ratio (PAR) that adversely impacts the efficiency of power amplifiers (PAs) used in wireless basestations.
Crest factor reduction (CFR) schemes help reduce PAR and are widely implemented for code division multiple access (CDMA) systems. When used in conjunction with OFDM signals, CFR schemes developed primarily for CDMA signals exhibit poor performance given the stringent error vector magnitude (EVM) requirements specified in a standard such as WiMAX.
Altera’s CFR for OFDM reference design is a low-latency, high-performance design that significantly reduces the PAR of the output signal while being standards compliant, leading to improved power amplifier efficiency and reduced cost. Although WiMAX is taken as an example, the design can be applied to a wide variety of OFDM-based wireless systems including LTE and future 4G systems.
Algorithm Overview
The CFR algorithm described is based on a modified version of the algorithm in Constrained Clipping for Crest Factor Reduction in Multiple-User OFDM [1]. This algorithm offers these key advantages:
- No receiver-side modifications needed
- Never violates the spectral mask
- Always meets the error vector magnitude specification
- Allows good PAR reductions
Figure 1 shows the CFR algorithm block diagram.
Figure 1. CFR Algorithm Block Diagram
Despite the original algorithm [1] suggesting an optimal method for achieving the target output EVM, the hardware resources and latency required to implement such a large sorting network is not feasible. Instead, you can use a reduced complexity technique that requires minimal resources. Due to the statistical distribution of the errors introduced by the polar clipping block, the output PAR of the reduced complexity technique is almost identical to the output PAR of the original algorithm.
Hardware Implementation
Emerging wireless technologies such as WiMAX and LTE increasingly employ the use of multiple antennas for techniques such as multiple-input multiple-output (MIMO). As a result, the hardware platform must have a very high throughput processing capability to perform complex functions such as CFR that would be required for each antenna separately.
The combination of Altera’s high-performance, lowest power Stratix® series FPGAs and HardCopy® ASICs provide a unique opportunity to design for volume from inception, while avoiding time consuming and risky ASIC conversions. Stratix series FPGAs provide an effective mix of on-chip memory and high-performance signal processing capability to achieve the lowest cost-per-channel in next-generation LTE RF card and remote radio head (RRH) designs. With unique low power consumption features such as Programmable Power Technology and selectable core voltage, Stratix series FPGAs meet these allowable power consumption levels. The high density of Stratix series FPGAs also provides unprecedented levels of integration, enabling single-chip solutions for multiple antenna-based systems, with further cost, size, and power consumption reduction via risk-free seamless migration to HardCopy ASICs.
Figure 2 shows an example of the CFR design integrated into a 10-MHz LTE wireless system. The CFR module processes three baseband channels where each channel has a sampling rate of 15.36 MSPS. The baseband channels are multiplexed together such that each frequency domain OFDM symbol is presented to the CFR module in a sequential fashion.
Figure 2. Example System Integration of CFR in 10-MHz LTE System
At the output of the CFR, the symbols are in the time domain and have been interpolated by a factor of four. The CFR uses an intermediate frequency of 61.44 MSPS, which requires no further upconversion. If an IF sampling frequency of 122.88 MSPS is required, just one single stage of interpolation by two filters is required.
Related Links
[1] C. Zhao, R. J. Baxley, G. T. Zhou, D. Boppana, and J. S. Kenney, Constrained Clipping for Crest Factor Reduction in Multiple-user OFDM, in Proc. IEEE Radio and Wireless Symposium, pp. 341-344, Jan. 2007.
