Development and application of mobile phone smart antenna test system
This article describes a research project initiated by Texas Instruments, a collaboration between the Virginia Tech Antenna Group (VTAG) and the Mobile Portable Wireless Research Group (MPRG) at Virginia Tech and State University.
The project focuses on determining the feasibility of smart transmitting and receiving mobile phone antennas, and its purpose is to demonstrate that this antenna has lower power consumption, greater capacity, and better link reliability. Research topics include developing new smart antenna algorithms and evaluating link reliability and capacity improvement. In order to evaluate the performance of the smart antenna in the actual application environment, the researchers collected a comprehensive set of space-time vector channel measurement methods. Data collection is completed by four array hardware test platforms developed by VTAG, which are handheld antenna array test platform (HAAT), MPRG antenna array test platform (MAAT), loss-of-impulse response (VIPER) and transmit diversity test platform (TDT) .
Figure 1: Typical test using HAAT in a multipath environment. One transmitter is used for diversity combination test, and the second transmitter can be used for anti-interference test using adaptive beamforming algorithm.
Smart antennas can greatly improve the performance of third-generation handheld wireless devices. Two research teams, MPRG and VTAG, formed a joint team to study the key characteristics of TI ’s smartphone antennas, including collecting antennas and transmitting measurement data, evaluating diversity and adaptive algorithms, simulating overall system performance, and quantifying the paired smart antenna The basic phenomenon of the impact of mobile phones. Since the project was launched in July 1998, we have developed three tools: a handheld antenna array test platform (HAAT), a vector multipath propagation simulator (VMPS), and a broadband VIPER measurement system. We have used these tools and the MPRG antenna array test platform (MAAT) to understand the transmission environment of mobile phone antenna arrays. This information has been used to predict the performance of mobile phone smart antennas.
Extensive 2.05GHz measurements show that at 99% reliability, a 7-9 dB link gain budget is achieved on narrowband systems in outdoor and indoor non-linear visual environments. These gains can be obtained using cell phone diversity and adaptive small antenna arrays with an isolation spacing between antennas of 0.15 wavelength or greater. Other measurements have shown that the use of adaptive beamforming (beamforming) algorithm can reduce a single interfering signal by 25-40dB. Therefore, reliability, system capacity and transmission power performance can be greatly improved.
System Development
1. Handheld antenna array test platform
The HAAT system can be used to evaluate the performance of various antenna configurations in diversity combining and adaptive beamforming experiments (typical applications are shown in Figure 1). Figure 2 shows a typical test scenario using the HAAT system. The receiver down-converts signals from two or more receive channels to baseband. These signals are recorded on digital audio cassettes for offline processing using appropriate algorithms. The receiver moves on a 2.8-meter long track at a constant speed that mimics human walking. A small handheld radio supports two antennas, and the spacing and direction of the antennas are variable. The system has the following characteristics: 2.05GHz CW signal; two transmitters; one receiver (two channels, can be expanded to 4); 2.8 meters linear track can continuously collect data and offline processing; highly portable battery-powered system The real working environment of the handheld receiver.
Figure 2: MAAT consists of 8 Harris 40214 programmable direct digital downconverters and 8 C54x DSPs
2. MPRG antenna array test platform
The MAAT in Figure 2 has many of the same characteristics as HAAT, but has more channels and can accommodate more bandwidth. However, MAAT is a bit bulky and it is not easy to change positions. Its operating frequency is 2.05GHz and the signal is a sine wave or a modulated signal. The bandwidth is set to 100kHz, but it can be extended to 1MHz by adjustment. MAAT can perform digital real-time beamforming and angle-of-arrival estimation.
3. Vector impulse response measurement system
VIPER is a software-defined wideband vector channel measurement receiver that supports transmit and receive diversity measurements. The VIPER receiver can receive signals with a bandwidth of up to 400MHz and process these signals in software. As a test platform for smart antenna algorithms, the receiver can perform the functions of a multi-path measurement system to compare the performance of antenna algorithms in multiple wireless channel environments. Figure 3 shows a photo of the VIPER RF front-end part. A four-channel oscilloscope is used as the sampling system, and the computer obtains all signal information from the oscilloscope.
VIPER is designed to implement processing functions in software with minimal RF hardware. Figure 4 shows the block diagram of the receiver hardware. After performing single-stage down conversion, the IF signal in each of the four channels is sampled at a sampling rate of 1G per second. The collected sample signals are stored in RAM and processed by the computer.
The VIPER software is responsible for collecting, processing, and recording the received signals and displaying the measurement or algorithm results. The software has been improved in the past year, and now includes the following modules: antenna diversity and diversity gain processing; time-discrete feature (multipath) measurement of wireless channels; implementation of smart antenna algorithms developed by MATLAB; power consumption, time domain and spectrum measurement ; Collection and recording of the original received signal; playback of the recorded signal for the development and testing of new algorithms.
4. Broadband transmit diversity test platform
Figure 3: VIPER RF front-end components
The broadband transmitter is designed for broadband diversity and channel measurement experiments. The transmitter is based on an FPGA with on-chip EEPROM, where the PN and data sequence are defined. Current transmitters allow PN chip sequences to run at speeds up to 25Mcps, but in the future, the performance of FPGA chips can be fully utilized to make PN sequences run at speeds up to 100Mcps. Detailed measurement of multipath wireless channels requires high chip rates, but in diversity experiments, low chip rates are used so that the resulting signal bandwidth is similar to that of 3G wireless systems.
5. Vector multipath propagation simulator
VMPS is used in conjunction with experimental measurements in narrowband or wideband signal environments. The simulator can model a complete wireless channel, including antennas and propagation effects. The test results can be used to optimize the model implemented by VMPS. The purpose is to study and isolate the effects of various parameters, such as antenna pattern and spacing, multipath, interference, algorithm performance, and other factors.
The VMPS simulator can be used to model a receiving system with 8 antennas. Six transmitters can be activated and placed anywhere around the receiver. Multipath propagation can be simulated by inserting a scatter at a location selected by the user or determined by the built-in model. The transmission power and reflection coefficient of the diffuser are variable, and the linear transmission environment conditions can be turned off or on. These characteristics can simulate multiple channel states.
The simulator can mimic the performance of several diversity configuration schemes, such as spatial, polarization, mode, and angle diversity. For the two antenna elements in the non-linear visual urban propagation environment, the maximum ratio combination is adopted, and the VMPS can obtain a diversity gain of 7-11 dB at the 99% level. These simulation results are consistent with the measurement results under similar propagation conditions using the HAAT system. VMPS can also evaluate the performance of broadband communication systems under different interference and multipath conditions, such as the use of space-time arrays, space arrays, tapped delay line equalizers, or single antenna receivers.
Figure 4: VIPER system block diagram
System measurement
A wide range of measurements were made using the developed hardware test platform, including cell phone diversity measurement, antenna spacing and the effect of operator body on diversity, adaptive beamforming, angle of arrival, channel reciprocity verification, and wideband vector channel measurement. Figures 5 and 6 show the sampling diversity measurement of outdoor non-linear visual channels. Figure 5 compares the correlation coefficient with respect to the antenna spacing, noting that when the correlation is far below 0.7, it will be very beneficial to improve the diversity performance. Figure 6 shows the diversity gain as a function of antenna spacing: at 99% reliability, the gain is about 9dB; at 90% reliability, the gain is about 5-dB. When the interval drops to 0.1 wavelength, there is almost no correlation.
We have conducted in-depth research on adaptive beamforming using handheld antenna arrays. The small four-element antenna array used in the investigation was installed on a receiver as small as a mobile phone. The adaptive beamforming study conducted 250 trials in remote areas, suburbs, and urban areas using two interfering transmitters. Using the least squares constant modulus algorithm (LSCMA), controlled experiments can improve performance by 25 to 50dB.
In a multipath channel, if there is no separation between transmitters in the eyes of the receiver, and there is no difference in the directions of the two transmit antennas, the performance improvement is more obvious. Under peer-to-peer and microcell conditions, the performance of the receiver when it was moved at walking speed in the hand was also measured. Under peer-to-peer network conditions, the average SINR increases by approximately 37-41dB, while under microcell conditions, the average SINR after beamforming is 21-27dB. Part of the reason for the lower SINR under microcellular conditions is that the signal has a low SNR due to attenuation on a longer propagation path. In the measured multipath channels, the advantages of dual or multi-polarized antenna arrays over co-polarized arrays are less than 3dB, which indicates that polarization flexibility in these channels is helpful for improving performance, but it is not a key factor.
Figure 5: In urban areas and non-LOS environments, the relationship between envelope correlation coefficient and antenna spacing in spatial diversity measurement
The MAAT system is used for angle-of-arrival measurement, adaptive interference cancellation algorithms for spread spectrum systems (low bandwidth), and multi-spectrum vector channel measurement based on frequency scanning over a 10MHz bandwidth. Multi-spectrum measurements reveal the flat attenuation characteristics of indoor channels and the frequency-selective attenuation characteristics of outdoor to indoor channels.
VIPER is used to initiate a series of wideband vector channel measurements, targeting various channels with similar IMT-2000 bandwidth (such as indoor and outdoor). The initial test was conducted in an indoor environment.
Transmit diversity study
This section describes the research group's recent research activities in mobile phone transmit diversity, which involves the study of different aspects of diversity. Transmit diversity is used when the symbol sequence is transmitted on all antennas of the antenna array on the transmitter. The problem is to maximize the signal-to-noise ratio at the receiver for a constant transmit power. In order to achieve mobile phone transmit diversity on a flat attenuation channel, the researchers used a variety of algorithms and methods. These methods involve the use of complex weight vectors at the transmitter to adjust the symbols passing through different antenna elements. Compare the maximum SNR and signal aggregation characteristics that can be obtained by various methods. These methods include the morning-evening method, the subspace method, the slope-based method, and the Least Square (LS) method.
These methods are tested through simulation, and the results show that the LS method is more suitable for flat attenuation channels. In an indoor environment, compared to a single antenna system, a 2-element antenna array can achieve a performance gain of 2-6 dB, and a 4-element antenna array can achieve a performance gain of 5-12 dB. The feedback and delay issues related to these algorithms are also studied. Simulations show that coarse magnitude and phase quantization of complex weight vectors are possible, with only slight performance degradation. We also studied the applicability of these algorithms in the implementation of WCDMA in IMT-2000. The channel structure and signal format of WCDMA can adapt to these algorithms.
Launch Diversity Demo
Figure 6: Relationship between average diversity gain and antenna spacing in spatial diversity measurement in urban and non-LOS environments
The feasibility of the transmit diversity system is demonstrated through hardware implementation. The hardware device includes a 2-unit broadband transmit diversity test platform and a VIPER as a receiver. The gain of one unit remains constant, while the phase of the other unit changes in a discontinuous manner. By measuring the signal strength at each phase setting, the setting with the highest power can be identified and transferred to the transmitter. Measure the signal strength of each antenna element and compare the performance of the diversity system with a single antenna system. The initial results show that at a level of 1% of the cumulative distribution function (CDF) graph, a performance improvement of 3-4 dB is possible.
Conclusion of this article
This article introduces VTAG's research on smart phone antennas. Through the use of different test platforms developed for various propagation experiments, channel measurements show that the performance of the diversity system is improved over the single antenna system. Narrowband measurements show that the adaptive beamforming technology with a four-element antenna array can achieve up to 40dB of anti-interference performance. Using corresponding algorithms, broadband systems can also achieve similar gains. We used the VIPER system for broadband diversity experiments. We also discuss the transmit diversity for flat attenuation channels, and verify the proposed algorithms by simulation. Transmit diversity was demonstrated in indoor environments with broadband signals. Based on our experience with VIPER, a broadband handheld antenna array test platform with continuous data acquisition can be quickly developed to support various tests to evaluate the adaptive beamforming performance of mobile phone broadband signals.
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