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Mobile data traffic is growing quickly, and Long-Term-Evolution (LTE) cellular communications technology offers a low-cost means of satisfying this growing appetite for wireless data. LTE systems provide spectrum flexibility, allowing bandwidth to be selected between 1.4 and 20.0 MHz depending upon available spectrum, to meet the needs of network operators with different bandwidth allocations. One of the two variants of LTE, time-division LTE (TD-LTE) technology, has become a standard fourth-generation (4G) mobile telecommunications technology for China. Co-developers include China Mobile, Alcatel-Lucent Shanghai Bell, ZTE, ST-Ericsson, and Qualcomm. Improved performance in these TD-LTE systems is possible by using compact multiple-input and multiple-output (MIMO) transceivers.1,2
The compact MIMO transceiver design presented in this article includes the analog baseband processor and RF transceiver. It can support MIMO configurations as large as 8 x 8 flexibly for high-speed capacity. Data transmission between the subsystem and the digital baseband unit of the base station can be implemented by either the fiber-optic or local-area-network (LAN) cable. The MIMO subsystem occupies eight channels from 3.411 to 3.551 GHz in 20-MHz steps, which can be configured dynamically. The subsystem has been used successfully in a TD-LTE trial network established by Alcatel Shanghai Bell Co. Ltd.
The TD-LTE base-station MIMO transceiver subsystem supports two types of interface. The optical Common Public Radio Interface (CPR™) is suitable for long-distance data-link applications such as the Radio Remote Unit (RRU), while the Gigabit Ethernet (GE) interface can support short-distance data-link applications such as microcells and picocells. The MIMO transceiver subsystem allows multiple baseband and RF circuit boards to be integrated into a miniature 4U high subrack. The transceiver subsystem works in 20-MHz channels, from 3.411 to 3.551 GHz. Excellent performance is possible with different modulation types and a 20-MHz TD-LTE signal.
Figure 1 shows the overall configuration of a TD-LTE base station. The function of the transceiver subsystem is to transfer digital signals by means of optical fiber or LAN cable links using the base station controller (BSC). As an alternative approach to the configuration presented in ref. 3, a 2 x 2 MIMO unit can provide improved performance using one baseband circuit board and two adjacent RF transceiver circuit boards. This configuration enables the design of a transceiver subsystem that is compact and flexible. To better understand this transceiver design, it will be examined more closely in terms of its baseband processing unit and its RF transceivers.
As a block diagram of the baseband processing unit shows (Fig. 2), it can be divided into several parts. These include the field-programmable-gate-array (FPGA) processor; the optical-fiber interface circuit; the clock management circuit; and the data-converter circuits with analog-to-digital converters (ADCs) and digital-to-analog converters (DACs). The FPGA is the core of the baseband circuit board, responsible for data transmission with base-station controllers by LAN cable or optical fiber and timing logic control with ADC and DACs. A Virtex-5 XC5VSX95T FPGA from Xilinx is used in the baseband processing unit in part for its logic resources and processing capabilities, and also because of its low cost. The on-board FPGA can also be used for some signal processing, such as calibration and digital predistortion (DPD),4,5 to make the baseband unit even more flexible.
This transceiver subsystem incorporates multiple synchronized clocks in addition to data converters, so the clock management scheme is necessarily complex. Compared to other clocks in the system, the converter sample clock must have very low jitter because of the effect of jitter on converter signal-to-noise ratio (SNR). Figure 3 shows the transceiver subsystem’s system clock distribution diagram.