Design and Implementation of Smart Antenna in GPS

Smart antenna is the combination of array antenna and advanced signal processing technology, extending signal processing from time domain and frequency domain to space domain, using the spatial information of the received signal to complete the space domain filtering and positioning, thus forming a space-time processing capability Antenna. Compared with the traditional FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access) methods, the smart antenna introduces the SDMA (Space Division Multiple Access) method. SDMA breaks through the traditional three-dimensional way of thinking and greatly improves the utilization of wireless spectrum resources in the fourth dimension. Smart antennas have many advantages such as adaptive adjustment of antenna patterns, tracking of desired signals, cancellation of interference signals, and improvement of signal-to-interference ratio. Smart antenna technology is an important field in signal processing.

The core technology of adaptive smart antenna research is the adaptive algorithm, which has a crucial impact on the performance and structure of the smart antenna. After years of development, the theory of smart antennas has matured. The main work in theoretical research is to study new algorithms for fast and efficient smart antennas. At present, there have been many achievements in the application research of smart antennas, such as the application in mobile communications. This article will design a smart antenna system for GPS (Global Positioning System) based on power inversion calculation [1].

1 GPs smart antenna system based on power inversion algorithm

1.1 Smart antenna structure

Smart antennas are generally composed of multiple antenna units, each antenna is followed by a complex weighter, and finally combined and output with an adder.

The main meaning of intelligence is that these weighting coefficients can be adaptively adjusted according to a certain adaptive algorithm. The smart antenna uses modern digital signal processing technology to select an appropriate adaptive algorithm to dynamically form a spatial directional beam to make the main lobe of the antenna array pattern. Align the user signal arrival direction, and the side lobe or null to align the interference signal arrival direction, so as to fully utilize the mobile user signal and cancel or minimize the interference signal. Given a set of weighted values, a certain incident signal strength, and signals of different incident angles, due to the different phase differences between the antennas, the combined output signal strength will also be different. Ideally, the main lobe of the antenna pattern can be aligned with the useful signal, and the side lobe or null can be aligned with interference.

1.2 The composition of the smart antenna system in GPS

The radio frequency signal modulated on the L1 carrier (1 575.42 MHz) is filtered out of the out-of-band interference through the BPF (band pass filter), amplified by the LNA (low noise amplifier), and the signal is divided in a 1: 1 ratio through the power divider For two ways. The traditional analog method is used in I / Q separation, which is realized by a 90 ° power splitter. In the beamforming network, the I / O component and the weight are multiplied and output by superposition. In the superposition, the superposition coefficient of the first array element is 1, and the rest will be superimposed in reverse phase. This process uses 180 ° power division器 Realize.

The GPS signal received by the antenna is very weak, and the signal power is only -130 dBm (-160 dBW). After receiving the antenna array element, the gain will be 6 dBm, but because the BPF is placed between the antenna and the LNA, a 2 dBm insertion will be introduced. The loss makes the signal power become -126 dBm. After the LNA is amplified, the signal gains 24 dBm. Passing the power splitter will cause a signal loss of 4 dBm. The power divider has a very important role in this system, which can 1: 1 multiplex the signal.

1.3 Hardware implementation of digital power inversion algorithm

The signal from the power divider undergoes 1: 1 in-phase distribution, one channel is output by the beamforming network filter, and the other channel is used to update the weights in the power inversion algorithm. Because the higher the A / D sampling rate, the lower the sampling accuracy. To make the algorithm transplanted to DSP (Digital Signal Processor) still effective, you must ensure the number of data bits obtained by sampling; and the sampling frequency is too close to radio frequency, The speed requirements for DSP will also be very high. Therefore, in specific processing, it is necessary to down-convert the GPS radio frequency signal to the intermediate frequency, and then band-pass sample the intermediate frequency signal.

1.3.1 Separation of down conversion and I / Q

Although the carrier frequency of GPS satellite signal transmission is public, the Doppler frequency shift will cause deviation between the actual carrier and the known transmitted carrier. The satellite signal is down-converted to zero intermediate frequency using the known carrier, around zero frequency Aliasing will occur, the solution is to down-convert to a suitable intermediate frequency. For the down conversion of GPS signals, the available chips are CXA1951, CXA3355ER, RF2498, MAX2680 / 2681/2682, MAX2740 / 2742/2745/2744, etc.

1.3.2 Quantization and sampling accuracy

In the process of A / D quantization, there is inevitably signal loss. For the selected A / D device, the sampling accuracy is determined, but the electromagnetic environment faced by the receiver changes at any time. The dynamic range of D is too small, which will cause the weak useful signal to be basically quantized. When the sampling rate is sufficiently high, an A / D converter with a sampling accuracy of at least 12 hits should be selected, so that in the case of gain processing in advance, the distortion of the data can be ensured as small as possible.

1.3.3 Selection of DSP module

In the design of the antenna anti-interference module, in addition to the algorithm used to suppress interference, as an auxiliary function module of the antenna part, it must always consider the real-time problem of processing, and it cannot be caused by this part of the digital signal processing work. Some additional difficulties. This is also a difficulty in this design, so in the selection of the core chip used to complete the anti-jamming algorithm, the processing speed must be fast enough, while also ensuring the accuracy of the algorithm.

At present, the high-performance general-purpose DSP chips produced by TI and AD in the chip market are popular chips, but the performance of TI's TMS320C6000 series is more superior, and the TMS320C64X series is the leader in fixed-point computing in the TMS320C6000 series. The main characteristics of this chip structure are: the main frequency is 400 MHz / 5 00 MHz / 600 MHz, corresponding to the instruction cycle of 2.512 ns / 1.67 ns (optional in 3 levels); each cycle can execute up to 8 32-bit instructions, the maximum processing Speed ​​up to 4 800 MIPs (million instructions per second); 8 highly independent functional units, including 6 ALUs (32 bit / 40 bit), 2 multipliers, 64 32-bit general registers, dedicated access Structure; L1 / L2 two-level memory structure, which has independent L1P program cache and LID data cache each 128 kbit, L2 (8 Mbit) is a unified program / data space, can be flexibly allocated according to needs; 2 external memory interfaces , One of which is 64-bit EMIFA and the other is 16-bit EMIFB; DMA controller has 64 independent channels; the host interface of 32 bit / 16 bit bus width can be set by the user, through which the entire storage of the DSP can be accessed space.

Compared with other DSPs of the TMS320C6000 series, TMS320C64x has improved processing capabilities in many aspects, has more functional units, adds cross-channel and double-word read and store instructions, and can also pack and unpack data. , Greatly enhanced the ability of high-precision data processing.

2 Software implementation and simulation of power inversion algorithm

The real-time updating of antenna weights is realized by DSP. Currently, the development of DSP application programs is mostly written in high-level languages ​​and high-level languages.

There are simulation results of 3 interfering signals. It can be seen that the interfering signals in 3 directions are all attenuated to a small extent, which also shows the effectiveness of the algorithm.

3 Conclusion

The application of smart antennas is bound to be more and more extensive, and the application of smart antennas in GPS will effectively improve the anti-interference characteristics of the system. Of course, how to design a smart antenna system with a more reasonable structure and superior performance remains to be discussed and studied.