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Aiming to address the issue of discrete frequency distribution in the 230 MHz band, which is characterized by narrow bandwidth and limitations in 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 architecture to aggregate 40 discrete frequency points within the 230 MHz band, thereby enabling broadband communication capabilities. Following simulation and FPGA-based verification, the receiver has been successfully integrated into the LTE230 wireless communication baseband chip and has demonstrated stable operation in pilot projects over an extended period. This confirms the effectiveness and success of the digital IF receiver 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 dedicated wireless communication system based on TD-LTE advanced technology, specifically developed for the power industry. It operates in the 223–235 MHz band (commonly referred to as the 230 MHz band). However, the 230 MHz band has a random, comb-like structure, with discrete frequency points and limited bandwidth per point, making it unsuitable for high-rate data transmission. To overcome these challenges, it is necessary to aggregate multiple frequency points to enhance transmission capacity. While existing LTE-Advanced systems use carrier aggregation techniques, they are designed for wide-bandwidth public networks and cannot be directly applied to the 230 MHz band due to differences in frequency spacing, bandwidth, and adjacent channel suppression. Additionally, traditional analog IF receivers used in LTE-Advanced suffer from complex hardware structures and poor reliability, making them unsuitable for LTE230 terminal modules. In response, this paper proposes a low-cost, high-reliability, and high-performance digital IF receiver tailored for the 230 MHz band.
**1 230 Band Spectrum Characteristics and LTE230 Frame Structure**
The 230 MHz band is primarily used in sectors such as energy, military, water resources, geology, and mining. It spans a total bandwidth of 12 MHz, divided into 480 frequency points, each spaced 25 kHz apart. Out of these, 40 points are allocated for power load monitoring. These 40 points are grouped into three clusters, with 10 simplex frequencies in the center and 15 duplex pairs on either side. LTE230 shares similarities with TD-LTE in terms of network structure and air interface technology but differs in physical layer frame structure due to its unique frequency band and customizable bandwidth. Each LTE230 radio frame consists of five subframes, each containing nine OFDM symbols with a 64-point FFT and a subcarrier spacing of 2 kHz. The system supports a total of 10 subcarriers per frequency point.
**2 Receiver Overall Design**
**2.1 Receiver Design Principle**
Despite the discrete nature of the 230 MHz band, its total bandwidth of 12 MHz is much smaller than that of LTE-Advanced. Therefore, a single-band non-continuous carrier aggregation approach can be adopted. By using one RF unit, the entire 12 MHz signal is moved to zero frequency, converted to digital, and processed in the baseband domain. This allows the extraction of low-speed baseband signals from high-speed IF signals spread across the 12 MHz bandwidth.
**2.2 Overall Structure of the Receiver**
The receiver comprises an RF chip (AD9361) and a baseband chip. The RF chip operates in TDD mode and uses a JESD207 interface for communication with the baseband. During reception, the RF chip generates a 12.8 MHz clock, with I and Q channels transmitted on the rising and falling edges, respectively. The RF interface converts the DDR I/Q data into parallel I/Q data and synchronizes the clock domains from 12.8 MHz to 51.2 MHz. The IF receiving link then extracts the desired baseband signal, sends it to memory via DMA, and notifies the DSP for further processing.
**2.3 IF Receive Link Design Indicators**
The IF receive link performs digital down-conversion and decimation filtering to convert IF signals to baseband. Key performance metrics include maintaining in-band signal integrity and suppressing out-of-band noise. Based on LTE230 specifications, the design requirements are defined and implemented accordingly.
**2.4 IF Receiving Link Structure**
The IF receiver uses a two-stage digital down-conversion and filtering structure. The first stage uses three NCOs to shift each cluster to zero frequency, followed by a half-band filter. The second stage uses 40 NCOs to extract individual frequency points, with cascaded integrator comb filters (CIC) and a low-pass filter (LPF) for further decimation and noise suppression.
**3 IF Receiving Link Module Design**
**3.1 Module Design Method**
MATLAB’s fdatool is used to design and analyze the IF receiver filters. The design includes determining filter order, coefficients, and bit width. The system allows flexible configuration of frequency points, NCO carrier frequency, HBF, and LPF through software to accommodate different application scenarios.
**3.2 Numerically Controlled Oscillator (NCO)**
The NCO generates sine and cosine signals for down-conversion. The NCO1 and NCO2 have sampling rates of 12.8 MS/s and 6.4 MS/s, respectively, with phase accumulation word widths of 9 and 8 bits. Lookup tables store sine and cosine values, and time division multiplexing reduces the need for multiple multipliers.
**3.3 Half-Band Filter (HBF)**
The HBF is a special FIR filter with symmetric passband and stopband. It reduces the number of operations by approximately 50% due to its zero-coefficient pattern. It is configured with a 2 MHz passband, 78 dB stopband rejection, and 16-bit quantization.
**3.4 Cascaded Integrator Comb Filter (CIC)**
The CIC filter is used for decimation. A 4-level CIC with a decimation factor of 50 is employed, reducing the sampling rate from 6.4 MS/s to 128 KS/s. It is implemented without multiplication, using adders, subtractors, and delay registers.
**3.5 Low-Pass Filter (LPF)**
The LPF is the final stage of decimation, ensuring flat passband and sharp stopband. It meets the requirement of 65 dB out-of-band rejection for coexistence with traditional 230 MHz digital radio stations.
**4 Simulation and FPGA Verification**
**4.1 Simulation Verification**
MATLAB simulations were conducted using sine and cosine waves instead of actual LTE230 IF signals. The results showed consistent performance with theoretical calculations, confirming the feasibility of the design.
**4.2 FPGA Verification**
FPGA testing confirmed the receiver's functionality under real-world conditions. Single-tone tests and LTE230 camping processes were performed, demonstrating excellent sensitivity and compliance with system requirements.
**5 Conclusion**
This paper presents a fully digital IF receiver tailored for the 230 MHz band and LTE230 system. By employing two-stage digital down-conversion and high-order digital filters, the design achieves efficient signal processing with reduced circuit area. The receiver successfully aggregates 40 frequency points, meeting the high-rate data transmission needs of power applications. After algorithm design, simulation, RTL verification, and FPGA testing, the receiver has been successfully integrated into the LTE230 baseband chip and has operated stably in field trials for over two years. Its performance meets all practical application requirements, proving the success of the digital IF receiver design.