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Aiming to address the challenges of the 230 MHz band, which suffers from a discrete frequency distribution, narrow bandwidth, and limited capability for high-speed data transmission, this paper presents a low-cost, high-reliability, and high-performance digital intermediate frequency (IF) receiver. The proposed receiver employs a two-stage digital down-conversion, decimation, and filtering structure to aggregate 40 discrete frequency points within the 230 MHz band, enabling broadband communication. Following simulation and FPGA verification, the receiver has been successfully integrated into the LTE230 wireless communication baseband chip and has operated stably in pilot projects over an extended period, confirming the effectiveness of the design.
**Keywords**: LTE230; digital intermediate frequency receiver; digital down conversion; downsampling; filter
**CLC number**: TN851
**Document identification code**: A
**DOI**: 10.16157/j.issn.0258-7998.170880
**Chinese citation format**: Zhou Chunliang, Zhou Zhimei, Wang Liancheng, et al. Design of LTE230 digital intermediate frequency receiver [J]. Electronic Technology Application, 2017, 43(9): 46-49.
**English citation format**: Zhou Chunliang, Zhou Zhimei, Wang Liancheng, et al. Design of a digital IF receiver for LTE230 [J]. Application of Electronic Technique, 2017, 43 (9): 46-49.
**0 Preface**
LTE230 is a specialized wireless communication system based on TD-LTE advanced technology, developed specifically for the power industry. It operates in the 223–235 MHz band (known as the 230 band). This frequency band exhibits a random, comb-like structure with discrete frequency distribution, narrow bandwidth per channel, and limited transmission capacity. To support high-rate data transmission in power applications, it is necessary to aggregate multiple frequency points, overcoming the limitations of the available spectrum. However, existing LTE-Advanced carrier aggregation techniques are not suitable for the 230 MHz band due to differences in frequency spacing, bandwidth, and adjacent channel suppression. Additionally, the analog-based IF receivers used in LTE-Advanced systems are complex and lack reliability, making them unsuitable for LTE230 terminal modules. This paper introduces a novel digital IF receiver designed for the 230-band carrier aggregation scenario, offering cost-effectiveness, reliability, and performance.
**1 230 Band Spectrum Characteristics and LTE 230 Frame Structure**
The 230 MHz band is primarily used by industries such as energy, military, water resources, geology, and mining. It spans a total bandwidth of 12 MHz, divided into 480 frequency points, each 25 kHz wide. Out of these, 40 points are allocated for power load monitoring. These 40 points are grouped into three clusters, with 10 simplex and 15 duplex pairs on each side. LTE230 shares similarities with TD-LTE in network architecture and air interface technology but differs significantly in physical layer frame structure due to its unique frequency allocation and bandwidth configuration. Each radio frame consists of five subframes, each containing nine OFDM symbols, with a 64-point FFT and a 2 kHz subcarrier spacing, resulting in 10 active carriers.
**2 Receiver Overall Design**
**2.1 Receiver Design Principle**
Despite the discrete nature of the 230 MHz band, the overall 12 MHz bandwidth is relatively small compared to LTE-Advanced. Therefore, a non-continuous carrier aggregation approach can be applied. By using one RF unit, the entire 12 MHz signal is moved to zero frequency, then converted to a baseband signal through digital processing. This allows efficient handling of the dispersed 25 kHz signals within the 12 MHz bandwidth.
**2.2 Receiver Architecture**
The receiver comprises a radio frequency (RF) chip and a baseband chip. The RF chip, AD9361, supports TDD mode and provides a 12-bit parallel digital interface. The baseband chip processes the received data, converting it from the RF clock domain to the IF domain. The IF link extracts the desired baseband signal, sends it to memory via DMA, and notifies the DSP for further processing like FFT, demodulation, and decoding.
**2.3 IF Receive Link Design Specifications**
The IF receive link must maintain the time-frequency characteristics of the in-band signal while suppressing out-of-band noise. Based on LTE230 specifications, the design includes parameters such as sampling rates, filter orders, and decimation factors to ensure optimal performance.
**2.4 IF Receive Link Structure**
The IF receiver uses a two-stage digital down-conversion and filtering approach. The first stage moves each cluster to zero frequency using NCOs and half-band filters, while the second stage isolates individual frequency points using additional NCOs and CIC filters followed by a low-pass filter to suppress out-of-band interference.
**3 IF Receive Link Module Design**
**3.1 Module Design Method**
MATLAB's fdatool was used to design and analyze the IF receiver filters. Parameters such as filter order, coefficients, and bit width were determined based on the system requirements. The design allows flexible configuration of frequency points, NCOs, HBF, and LPF through software.
**3.2 Numerically Controlled Oscillator (NCO)**
The NCO generates local oscillator signals for down-conversion. It uses lookup tables for sine and cosine values, with different word widths and sampling rates for NCO1 and NCO2. Time division multiplexing reduces the need for multiple multipliers.
**3.3 Half Band Filter (HBF)**
The HBF is an efficient FIR filter with symmetric coefficients, reducing computational load by approximately 50%. It is configured to have a passband of 2 MHz, 25th-order, and 78 dB stopband rejection.
**3.4 Cascade Integrator Comb Filter (CIC)**
The CIC filter is used for decimation, with a 4-level cascade and a decimation factor of 50. It is simple, requiring only adders and delay elements, and no multiplication operations.
**3.5 Low Pass Filter (LPF)**
The LPF minimizes the sidelobe amplitude of the CIC and enhances stopband attenuation. It is configured with a 255-order filter, 18-bit quantization, and 65 dB stopband rejection to meet the requirements of coexistence with traditional 230-band systems.
**4 Simulation and FPGA Verification**
**4.1 Simulation Verification**
MATLAB simulations confirmed the feasibility of the design. Sine and cosine waves were used instead of actual LTE230 IF signals, and the results matched theoretical expectations. The RTL code was verified against the fixed-point algorithm to ensure accuracy.
**4.2 FPGA Verification**
Due to FPGA speed limitations, full system testing was not possible. Instead, tone signals and LTE230 camping processes were tested. The measured sensitivity of -124 dBm met system requirements, proving the receiver’s robustness.
**5 Conclusion**
This paper presents a fully digital IF receiver tailored for the 230 MHz band and LTE230 system. Using two-stage digital down-conversion and high-order filters, the design achieves efficient aggregation of 40 frequency points, meeting high-rate data transmission needs. After MATLAB simulation, RTL verification, and FPGA testing, the receiver has been successfully implemented in the LTE230 baseband chip. It has operated stably for over two years in real-world applications, demonstrating the success of the design.