Advances in wireless communications have enabled various technologies for wireless digital communication. In the field of digital radio broadcasting, several specifications have been proposed, such as Eureka-147 and digital radio mondiale (DRM). These systems require a new spectrum assignment, which incurs heavy cost due to the depletion of the available spectrum. Therefore, the in-band on-channel (IBOC) system has been developed to work in the same band with the conventional analog radio and to provide digital broadcasting services. This paper discusses the function and algorithm of the high definition (HD) radio frequency modulation (FM) digital radio broadcasting system. Content includes data format allocation, constellation mapping, orthogonal frequency division multiplexing (OFDM) modulation of the transmitter, timing synchronization, OFDM demodulation, integer and fraction carrier frequency (integer carrier frequency offset (ICFO) and fractional CFO (FCFO)) estimation, and channel estimation of the receiver. When we implement this system to the field programmable gate array (FPGA) based on a hardware platform, both theoretical and practical aspects have been considered to accommodate the available hardware resources.
With advances in wireless communication, many schemes have been developed and are currently available. In the digital radio broadcasting area, several specifications are currently in use, for example, the Eureka-147 in Europe [
IBOC has been adopted by the United States (US) as its digital radio broadcasting standard, including two sets of specifications: high definition (HD) radio frequency modulation (FM) and HD radio amplitude modulation (AM), both of which share identical working frequencies as conventional FM and AM spectrums [
This system specifies two broadcasting modes: the hybrid mode and the all-digital mode. A station occupies 400 kHz bandwidth to transmit in parallel its analog and digital signals to the hybrid mode; meanwhile it can utilize the whole 400 kHz bandwidth to transmit the pure digital signal in the all-digital mode [
The structure of this paper is as follows. Section
The technical document released by iBiquity details the specification of the HD radio FM. The subcarrier spacing is 363.4 Hz, the cyclic prefix (CP) width is 7/128, and the FFT size is 4096. These three parameters result in the OFDM symbol duration of 2.902 ms. In the 400 kHz bandwidth assigned to each station, the central 200 kHz is for analog use only, while the other two remaining sidebands of total 200 kHz are for digital use. The spectrum allocation is shown in Figure
HD radio FM spectrum.
In this subsection, the transmitted block structure defined in the Spec. will be introduced. The physical layer architecture of the HD Radio FM system is shown in Figure
HD radio FM transceiver block diagram.
The HD radio FM system utilizes quadrature phase shift keying (QPSK) mapping in the data subcarriers and BPSK mapping in the reference subcarriers. OFDM subcarrier mapping converts the interleaved data to the frequency domain. Pairs of adjacent columns within an interleaver partition are mapped to individual complex, QPSK-modulated data subcarriers in the frequency partition. The transmission subsystem formats the baseband IBOC FM waveform for transmission through the very high frequency (VHF) channel. The concatenation functions include symbol concatenation and frequency upconversion. The concatenation functions are executed in the IFFT block. In this block, frequency domain data is transformed to time domain, and the transformed data is concatenated into OFDM symbols. Afterward, CP is added to the OFDM symbols. The frequency upconversion is executed in the upconversion block after the CP block. When transmitting the hybrid or extended hybrid waveforms, this function modulates the baseband analog signal before combining it with the digital waveforms. Figure
Hybrid transmission subsystem functional block diagram.
The analog signal
The CP delay correlation is designed in this system to detect the symbolic timing offset (STO) and the fractional carrier frequency offset (FCFO) [
The peak value in the autocorrelation indicates that the starting point of the OFDM is a symbol with the high autocorrelation feature in one symbol. Correction functions must be added to compensate for the influence induced by the multipath channel. Nevertheless, it is still difficult to distinguish the peak value from the surrounding noises, such as AWGN noise. AWGN noise is a basic noise model that presents random noise in nature. Except for AWGN, the simulation channels considered in this paper are shown in Table
FM band channel models.
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Delay (ms) | Attenu. (dB) | Delay | Attenu. | Delay | Attenu. | |
1 | 0.0 | 2.0 | 0.0 | 4.0 | 0.0 | 10.0 |
2 | 0.2 | 0.0 | 0.3 | 8.0 | 1.0 | 4.0 |
3 | 0.5 | 3.0 | 0.5 | 0.0 | 2.5 | 2.0 |
4 | 0.9 | 4.0 | 0.9 | 5.0 | 3.5 | 3.0 |
5 | 1.2 | 2.0 | 1.2 | 16.0 | 5.0 | 4.0 |
6 | 1.4 | 0.0 | 1.9 | 18.0 | 8.0 | 5.0 |
7 | 2.0 | 3.0 | 2.1 | 14.0 | 12.0 | 2.0 |
8 | 2.4 | 5.0 | 2.5 | 20.0 | 14.0 | 8.0 |
9 | 3.0 | 10.0 | 3.0 | 25.0 | 16.0 | 5.0 |
There are two main sources of the CFO. First, the relative speed between the transmitter and the receiver results in the Doppler shift. The second source is the mismatch between the oscillator of the transmitter and that of the receiver. The working carrier frequency of the HD radio FM can reach as high as 108 MHz. The channel model defined in [
As discussed above, the CFO which affects this system can be divided into the FCFO and the ICFO. When a CFO is greater than a half of subcarrier spacing, it is customary to express it as an ICFO plus/minus an FCFO. The FCFO can be estimated via the CP delay correlation method mentioned previously, as shown in
As for the ICFO, it can be resolved by using the evenly spacing reference subcarriers in the frequency domain. According to the Spec., the length of the system control data sequence is 32 bits, and each time 1 bit is transmitted in one OFDM symbol at a time. In this sequence, there exists an 11-bit synchronization pattern which is designed for the purpose of frame synchronization.
Based on this property, the receiver is able to cross-correlate the received subcarriers to the 11-bit synchronized pattern upon receiving the OFDM signal. If a highly correlated subcarrier combination in each 19 subcarriers apart is found, then the position of the reference subcarrier is reached and the ICFO is acquired. The frame and synchronization methodology is shown in Figure
HD radio FM frame and ICFO synchronization.
The frequency response of CM1.
In designing the receiver, attention should be paid to the timing/frequency synchronization as well as the channel effects. In the receiver, these harmful effects should be estimated and compensated. The pilot signals, which are located in the reference subcarriers, can be used in the channel estimation. In each transmission, all subcarriers except for bit 20 and bit 21 are transmitted as the same data in one OFDM symbol. These two bits are transmitted in a predefined order. Upon receiving these two signals, the system control data sequence can be decoded correctly by using the signals’ redundancy feature. The decoded signal is then used as the known pilot signal to assist the channel estimation. The channel response of each reference subcarriers is estimated by using the least squares (LS) channel estimation method. Consequently, the channel response of each data subcarrier is estimated by using the linear interpolation, as shown in (
The FM band channel model proposed by Electronics Industries Association (EIA) in 1993 [
The frequency response of CM2.
The frequency response of CM3.
The frequency response of CM4.
The robustness of different coding schemes under these channel models is shown in Figure
BER performance under FM band channel models.
Note that a convolutional code with the code rate 2/5 is implemented in this system. Besides, a three-element sliding window is applied to execute the averaging operation. That is, the final outcome of each symbol in each subcarrier is the weighted sum of the current symbol and the two most adjacent ones, as shown in Figure
Time domain averaging by using a 3-element sliding window.
After careful examination of the parameters defined in the Spec. and the available hardware resources, the following parameters are chosen in the hardware implementation: FFT size: 2048, CP length: 112, sampling rate: 781.25 kHz, subcarrier spacing: 381.5 Hz, symbol rate: 361.9 Hz, transmission rate: 104.2 kbps (code rate 2/5).
The built-in CP2102 USB-UART bridging chip on the FPGA board is used as the communication interface between the board and the computer, as shown in Figure
Diagram of the communication interface between the computer and the FPGA.
The Agilent 89600 vector signal analyzer is used to monitor the QPSK constellation map generated by the transmitter, as shown in Figure
Using the Agilent 89600 vector signal analyzer to monitor the constellation points of the transmitter.
The OFDM waveforms generated by the transmitter are fed into the Agilent 89600 vector signal analyzer to analyze its spectrum map. The result is shown in Figure
Real transmitting spectrum of the HD radio FM implementation.
Simulated transmitting spectrum of the HD radio FM implementation.
This system utilizes the OFDM technology to achieve symbol synchronization through the autocorrelation property. First, upon receiving the signal, it uses a FIFO memory to temporarily store the signal so as to extract the two sampling points of the intervals OFDM length of symbol for autocorrelation calculations. This autocorrelation calculation value will be stored in another FIFO memory having the CP length in order to sum up the entire CP length actions. The summed value will be stored in another FIFO memory having the length of an entire OFDM length of symbol, adding this to the autocorrelated value having the length of a symbol. The position of the largest value in the memory storage will be the standard time for time synchronization, which is then fed into the FFT. The flow chart is shown in Figure
Flow chart of time synchronization and fractional frequency offset.
The channel estimation scheme contains two main steps. The first one is to detect the channel response of the subcarrier, while the second one is the division of complex numbers signals needed to be processed in this area. Through (
After that, the channel response will be estimated. This step is derived by interpolation from the preamble. The circuit of this matter is shown in Figure
The circuit diagram of channel estimation.
In this paper, the HD radio FM structure and algorithms of their transmitter and receiver are presented. The transmitter design is fully complied with the HD radio FM specification. In the receiver algorithms, the timing and frequency synchronization issues are studied and discussed. As for the channel impairments, a channel estimation and compensation algorithm is presented and performed in different channel models. From the simulation results, it can be seen that the system performance still maintains in a descent range under various channel environments. A hardware prototype system is realized on FPGA and a PC platform. The system is capable of processing signals at a fast pace due to the nature of FPGA and also adding flexibility for the future algorithm configuration.
The authors declare that there is no conflict of interests regarding the publication of this paper.