TD-SCDMA

TD-SCDMA is the abbreviation of Time Division-Synchronized Code Division Multiple Access, which is what we call Time Division Synchronous Code Division Multiple Access technology. TD-SCDMA mainly includes the following features and core technologies:

Time Division Duplex In TDD (Time Division Synchronous) mode, TD-SCDMA uses the mode of transmitting basic TDMA bursts in the periodically repeating time frame (same as GSM), by periodically changing the transmission direction, on the same carrier. Alternate uplink and downlink transmissions. The advantages of this program are:

· According to different services, the position of the transition point between uplink and downlink can be arbitrarily adjusted;
TD-SCDMA adopts asymmetric frequency bands, and does not require paired frequency bands. It flexibly meets different data transmission rates required by 3G.
A single carrier frequency bandwidth of 1.6MHz, frame length of 5ms, each frame contains 7 different bursts of different code transmission at the same time, because it occupies narrow bandwidth, so there is great flexibility in the spectrum arrangement;
· TDD uplink and downlink work at the same frequency, symmetrical wave propagation characteristics make it easy to use new technologies such as smart antennas, can achieve the purpose of improving performance and reducing costs;
· Low cost of TDD system equipment, no need to send and receive isolation, can use a single chip IC to achieve RF transceiver, its cost is 20% to 50% lower than FDD system.

At the same time, this time division duplex technology also has certain flaws:

Multi-slot discontinuous transmission method is adopted, and the capability of anti-fast fading and Doppler effect is worse than that of continuous transmission FDD mode. Therefore, ITU requires that the mobile terminal speed of TDD system users is 120km/h, which is far lower than frequency division duplex (FDD). )Level;
· The ratio of the average power to the peak power of a TDD system increases with the number of gaps at a given time. Considering the power consumption and cost factors, the transmit power of the user terminal cannot be large, so the communication distance (cell radius) is small and generally does not exceed 10 km. The FDD system has a cell radius of up to 10 km.

The Smart Antenna Smart Antenna System consists of a set of antennas and connected transceivers and advanced digital signal processing algorithms. It can effectively generate multi-beam forming, each beam pointing to a specific terminal, and can automatically track the mobile terminal.

At the receiving end, through spatially selective diversity, the receiver sensitivity can be greatly improved, interference of co-channel users at different locations can be reduced, multipath components can be effectively combined, multipath fading can be cancelled, and uplink capacity can be improved; at the transmitting end, intelligent spatial selective beamforming can be performed. Transmission, reducing output power requirements, reducing co-channel interference and increasing downlink capacity.

The smart antenna improves cell coverage, and the radiation pattern of the smart antenna array can be completely controlled by software. When the network coverage needs to be adjusted and the original coverage is changed, the network optimization can be performed very simply through software. In addition, the smart antenna reduces the cost of the wireless base station. The smart antenna increases the equivalent transmit power. Using multiple low-power amplifiers instead of a single high-power amplifier can greatly reduce the cost, reduce the power requirements and increase the reliability.

The problem that cannot be solved by the smart antenna is the multi-path interference whose delay exceeds the chip width and the channel deterioration caused by the high-speed moving Doppler effect. Therefore, in a high-speed mobile environment with serious multipath interference, smart antennas must be used together with other anti-interference digital signal processing techniques to achieve the best results. These digital signal processing technologies include joint detection, interference cancellation, and Rake reception.

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Multi-user detection Multi-user detection mainly refers to using a plurality of user symbols, time, signal amplitude and phase information to jointly detect a single user's signal in order to achieve better receiving results.

The goal of optimal multiuser detection is to find the largest input sequence of the output sequence. For synchronous systems, it is to find the largest input sequence of the function. The spectrum utilization of the joint detection is improved and the power control part of the base station and the user terminal is simpler. It is worth mentioning that in the case of different smart antennas, the joint detection can be used in the existing GSM infrastructure equipment through the C= The ultimate result of TD-SCDMA in the cellular re-multiplexing mode of 3 is that TD-SCDMA can pass in the low carrier frequency band of 1.6MHZ.

Software Radio Software Radio is a technology that uses digital signal processing software to implement wireless functions. It can use software to process baseband signals on the same hardware platform. Different software can be used to achieve different service performance. Its advantages are:

·Flexibly complete hardware functions through software;
Good flexibility and programmability
Can replace expensive hardware circuits to implement complex functions;
· Good adaptability to the environment, no aging;
·Easy system upgrade, reduce user equipment costs.
For TD-SCDMA systems, software radios can be used to implement smart antennas, synchronization detection and carrier recovery.

The relay switching mobile communication system adopts a cellular structure. When the cells are divided across spaces, handover must be performed, that is, the air interface conversion from the mobile station to the base station is completed, and the corresponding transfer from the base station to the network entrance and the network entrance to the exchange center.

Since the smart antenna can be used to roughly locate the user's position and distance, the base station and the base station controller of the TD-SCDMA system can adopt the relay switching method. According to the user's position and distance information, it can be determined whether the mobile phone user should now be switched to another one. Base station's immediate area. If you enter the handover area, you can notify the other base station through the base station controller to prepare for the handover and achieve the purpose of relay switching.

Relay switching can increase the handover success rate and reduce the occupation of channel resources of adjacent base stations during handover. The base station controller (BSC) obtains the location information of the mobile terminal in real time and informs the mobile terminal about the same-frequency base station information. The mobile terminal establishes contact with two base stations at the same time. The handover is determined by the BSC and initiated, and the mobile terminal is switched from one cell to another. Community. TD-SCDMA system not only supports intra-frequency handover, but also supports inter-frequency handover. It has high accuracy and short switching time. It can dynamically allocate the capacity of the entire network, and can also switch between different systems.