Performance of TD-SCDMA using smart antenna technology in high-speed channels
TD-SCDMA has been formally adopted by the International Telecommunication Union (ITU) and will become an important part of the future third-generation mobile communication system (IMT2000) and will be further standardized by the 3GPP organization [1]. The smart antenna technology, which is one of the key technologies in the TD-SCDMA system, can enable the system to achieve better performance in a high-speed channel environment. In this article, first introduced the TD-SCDMA system model, and then, explained the basic concepts of smart antenna technology and the feasibility of applying the technology in high-speed motion environment. At the same time, the corresponding simulation results in the high-speed movement environment with different speeds are given in the article. It can be seen that the use of smart antenna technology in the TD-SCDMA system can achieve good system performance and can meet the various needs of the third generation mobile communication system.
Abstract: TD-SCDMA (TIme Division Synchronous Code division MulTIple Access) has been accepted by the ITU (InternaTIonal TelecommunicaTIon Union) as a 3G standard and is being standardized in 3GPP (Third Generation Partnership Project) [1]. Smart antennas as one of the key technologies used in the TD-SCDMA can provide good system performance with high mobile velocity. In this paper, the system model of TD-SCDMA is introduced at first. Then the concept of smart antennas and its feasibility under high vehicle channel are described . Simulation results in the uplink with different high speeds are also included in this paper. It can been seen that smart antennas used in TD-SCDMA will get good performance and meet the requirements of the 3rd generation mobile systems.
Keywords: TD-SCDMA; smart antenna technology; joint detection technology
Key Words: TD-SCDMA, smart antenna, joint detection
1 Introduction
In recent years, mobile communication has become a field of rapid development. The second-generation mobile communication systems using digital signal processing technology, time division multiple access and frequency division multiple access technologies, such as GSM and IS54 systems, have been widely used around the world. Compared with the second-generation mobile communication system, one of the biggest challenges for the third-generation mobile communication system is not only to be able to provide symmetric circuit-switched services such as voice and images, but also to provide non-mobile Internet access and other non-linear services. Symmetrical packet switching service. At the same time, some of the frequency bands used for third-generation mobile communications in the future may be asymmetric. In this way, the TDD mode is very important in the future development of mobile communications.
To solve these problems, the TD-SCDMA system incorporates two advanced technologies. It is an advanced TDMA system with adaptive CDMA characteristics that works in synchronous mode. With the development of the mobile communication market, the requirements for mobile communication systems are getting higher and higher. As the future mobile communication system, TD-SCDMA must be able to meet various types of business needs. China Wireless Communications Standards Organization (CWTS) proposed TD-SCDMA and made it one of the third international mobile communication international standards (IMT2000). Smart antenna technology, which is one of the key technologies of TD-SCDMA, can increase the capacity of the system, expand the maximum coverage of the cell, reduce the transmission power of the mobile station, improve the signal quality and increase the data transmission rate. These advantages provide mobile network operators with great flexibility.
2 System model
In this part, we introduce the low-pass equivalent model of the TD-SCDMA mobile communication system using smart antenna technology, including forward error correction coding. In Figure 1 and Figure 2, we give the basic structure of the transmitter and receiver in the uplink.
Figure 1 Block diagram of the mobile station transmitter
In the same cell in the system, K users can simultaneously communicate on the same frequency band with a bandwidth of B, and users are distinguished by different user spreading code sequences. We assume that each mobile station has only one transmit antenna. The uplink receiver in the base station part has M antennas that can receive the signal transmitted by the mobile station. The Ray Tracing channel model is used in our verification system. This channel model is based on geometric theory and reflection, refraction, and scattering propagation models. By using the location information of the designated location, such as the architectural drawing database, this technology can definitely model the propagation channel, including path loss, angle of incidence, and time delay. It is very suitable for application in the simulation to verify the effectiveness of the smart antenna system, and the performance results obtained through it are very convincing.
Figure 2 Block diagram of a receiver using smart antenna technology in a base station
Figure 3 Basic frame structure of TD-SCDMA system
The frame structure [2] in the TD_SCDMA mobile communication system is very similar to that of GSM (see Figure 3), and they all use Midamble for pulse detection.
The duration of a superframe is 320 ms, and a superframe can be divided into 32 radio frames. A radio frame can be divided into two radio subframes with a duration of 5 ms. Each radio subframe is composed of 7 service time slots with a duration of 675 μs and 3 special time slots: DwPTS (downstream pilot time slot), GP (protection time slot) and UpPTS (upstream pilot time slot) .
TS0 is always used for the downlink, TS1 is always used for the uplink, and other time slots are determined for the uplink or downlink according to the flexible configuration of the switching point. The pulse structure of each service time slot is composed of two data symbol areas, a Midamble with a length of 144 chips, and a protection area with a length of 16 chips. In the TD-SCDMA system, there are up to 16 different spread spectrum subscriber units in each service slot at the same time. We use the midamble training sequence to do decorrelation operations to get an estimate of the mobile channel, and then perform joint detection on all signals in each time slot. By eliminating multiple access interference, the dynamic range of the received signal reaches approximately 20 dB. Joint detection technology is an algorithm that enables weaker signals to be demodulated in the presence of other stronger signals. Therefore, the use of this technology can reduce the requirements for power control, that is to say, it can eliminate the impact of average power fluctuations caused by slow fading.
3 Smart antenna
In this part, we explain the basic concepts of smart antenna technology [3] and the feasibility of using the technology in a high-speed motion channel environment.
3.1 Smart antenna technology concept
As shown in Figure 4, an antenna array system is composed of some space-separated independent antenna elements. The output of this array is combined with a set of multiple inputs of the transceiver. These multiple antenna elements are combined to provide a comprehensive spatio-temporal signal. Compared with a receiver that uses a single antenna to combine signals from antenna ports in a fixed manner, the antenna array system can dynamically adjust the signal combining method to improve the performance of the system. For this reason, the antenna array is often referred to as a smart antenna, which is regarded as equivalent to an antenna whose characteristics can be automatically adjusted as needed.
Figure 4 Antenna array of M antenna elements with spatial diversity
People often use loop or linear antenna arrays. In the TD-SCDMA mobile communication system, we use 8 identical antenna elements to place them evenly on a circle with radius R to form the loop antenna array we need. This array is particularly effective for eliminating interference [4]. The distance between each two antennas is half of the carrier wavelength. Since each antenna is in a different position in space, the amplitude and phase of the signals of different antenna elements are different. In this way, many independent directional high-gain beams can be generated without reducing the signal-to-noise ratio. Different beams are allocated to different users, ensuring the maximum gain on all links. Using adaptive beamforming can effectively eliminate interference and increase the capacity of the system. Various algorithms that can be expressed in mathematical formulas can be implemented.
3.2 Feasibility of using smart antenna technology in high-speed motion environment under TDD mode
With the development of transportation and communication, the demand for high-speed data services in high-speed movements becomes more and more important. In the vehicle speed environment, there is generally no line-of-sight signal, which means that the received signal is composed of reflected waves, refracted waves and scattered waves. The average power of the received signal decreases with increasing distance. The TD-SCDMA mobile communication system using smart antenna technology is not only suitable for indoor environments, but also for outdoor vehicle speed environments. According to our analysis and simulation, the system can work normally even when the mobile station has a high speed. On the uplink, the receiver at the base station can determine the beam structure characteristics of the received signal in real time to demodulate the signal without requiring any storage unit to store the beam information of the past frame. So no matter how high the speed of the mobile station, the uplink receiver can quickly adapt to the new beam characteristics in each frame. In TDD mode, the uplink and downlink use the same frequency band, and the transmitter at the base station can learn about the fast fading characteristics of the downlink multipath channel based on the received signal on the uplink. In this way, the transceiver of the base station can use the channel estimation information obtained on the uplink to implement downlink beamforming. Only in a TDD system like TD-SCDMA can the coordination of uplink and downlink reach such a good level. In the TD-SCDMA system, since the length of the wireless subframe is 5 ms, the maximum response time allowed from the downlink to the uplink is 5 ms. According to the uplink and downlink channel allocation in the radio frame, this reaction time can be shorter. As the speed of the mobile station increases, the correlation between the channel characteristics of the uplink and the downlink becomes stronger and stronger. There is a deviation between the characteristics of the downlink channel and the uplink, but this deviation is very small, so using the beam information obtained from the uplink to perform downlink beamforming can still work normally.
For example, assuming that the speed of the mobile station is 250 km / h, and the response time of using downlink beamforming is 2.5 ms; thus, when the minimum distance between the mobile station and the base station is 10 m, as shown in FIG. 5, The angle between beamforming and actual deviation is about 2.
Figure 5 Examples of deviations in downlink beamforming at high speed
4 Simulation results
The results of the link-level simulation are given in this section. The main features of the simulation environment are as follows:
· Using smart antenna technology;
· Joint space-time processing is used on the uplink;
· Channel model with spatial information (Ray Tracing channel model).
According to the proposal of CWTS, we chose the mapping method of 12.2 kbit / s and 2.4 bit / s. In Table 1 we list the basic simulation parameters. At the base station side, a loop antenna array consisting of 8 antennas is used, while the mobile station uses only a single antenna.
We use the COSSAP simulation platform to obtain different simulation results at different speeds as shown in the following figure:
(a) 1 user / slot (b) 8 users / slot Figure 6 Bit error rate performance at 120 km / h vehicle speed
(a) 1 user / slot (b) 8 users / slot Figure 7 Bit error rate performance at 250 km / h vehicle speed
Through careful and meticulous simulation work, we can see that the TD-SCDMA mobile communication system can effectively work in the high-speed motion channel environment.
5 Conclusion
In this article, we propose a TD-SCDMA mobile communication system that combines TDMA and CDMA multiple access and uses joint detection technology, and discusses the performance of the system under high-speed motion channel environments. The use of smart antenna technology in the base station not only brings diversity gain, but also can eliminate interference. The current simulation work shows that the TD-SCDMA mobile communication system can work effectively in the high-speed motion channel environment.
If the speed of the mobile station is very high, the traditional channel estimation information on the uplink cannot be accurately used in downlink beamforming. This is not because of the change in the position of the mobile station, but because of the time-varying nature of the wireless channel itself. As far as the high-speed motion channel environment is concerned, the application prospect of the TD-SCDMA mobile communication system is still very optimistic. At present, we do not use adaptive channel estimation technology, which will result in better performance. In the future, we will study how to use adaptive channel estimation techniques in high-speed motion channel environments.
references:
[1] 3GPP TSG RAN WG1. 1.28 Mcps Functionality for UTRA TDD Physical Layer. Meeting AH213G TR 25.928 V0.3.0, June 2000
[2] Mr. Kammerlander. Benefits and Implementation of TD-SCDMA. ICCT 2000, Beijing
[3] Liberti Joseph C, Theodore JR, Rappaport S. Smart Antennas for Wireless Communications. Prentice Hall, 1999
[4] Bar-Ness, Haimovich A M. Usage of smart antenna for cancelling neighboring base-station interferences in wireless CDMA communications. IEEE Signals, Systems & Computers, 1997 Conference Record of the Thirty-first Asilomar Conference, Vol 1, 1998.
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