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Modern communications systems are often judged by their data rates. Organizations such as the International Telecommunication Union (ITU) suggest by their definitions that next-generation mobile-communications systems should operate at rates to 100 Mb/s for high-mobility operation and 1 Gb/s for low-mobility operation. So far, third-generation (3G) communications networks and their more advanced successors, such as 3.5G networks, can support data rates to several megabits per second.1-4

Of course, modern communications users will want even faster data rates as part of fourth-generation (4G) wireless networks. In China, for example, time-division, long-term-evolution (TD-LTE) wireless technology in the 2.6-GHz spectrum (2500 to 2690 MHz) using time-division-duplex (TDD) techniques offers the promise of high data rates in wireless communications.  

In many ways, with its application of orthogonal-frequency-division-multiplexing/frequency-division-multiple-access (OFDM/FDMA) technology, 3GPP LTE networks have been considered “Quasi 4G” systems. In other words, they apply 4G technologies to 3G systems for improved performance, including the use of multiple-input, multiple-output (MIMO) antenna techniques. The use of these advanced technologies allow 3G networks to achieve data rates of 100 Mb/s on downlinks and 50 Mb/s on uplinks.5,6

Transceiver Supports 8 × 8 MIMO Systems, Fig. 1

RF/microwave engineers are concerned with achieving the highest performance possible using TDD-LTE technology in these wireless networks. To aid those efforts, the current transceiver was developed as part of a project for a next-generation wireless communications system for use in radio-over-fiber (RoF) applications. The transceiver supports 8 × 8 MIMO use in user-equipment (UE) and base-transceiver-station (BTS) equipment. The transceiver’s output is designed for connection to an optical transceiver and front amplifier unit (FAU) (not included in this report). Figure 1 shows a block diagram of the full system.

Transceiver architectures typically rely on either zero-intermediate-frequency (zero-IF) or superheterodyne configurations. Zero-IF approaches have been widely used in personal mobile devices due to their low cost and practical application as single-chip devices.7,8 The current transceiver design is based on the superheterodyne approach so as to achieve maximum performance, even though it is somewhat bulky and costly.

Transceiver Supports 8 × 8 MIMO Systems, Fig. 2

Each transceiver consists of several main parts, including a transmitter circuit, receiver circuit, local-oscillator (LO) module, and control circuit. Figure 2 shows a simplified block diagram. The whole system operates in the TDD scheme. Therefore, a RF switch controlled by transmit/receive (T/R) switch signal ensures that the system can be switched between transmit and receive states.

The switching time must be short enough to meet the standards of TDD-LTE systems, or it may affect the first few symbols used for channel estimation. The decoupling capacitors of some key devices and the resistive-capacitive (RC) network of the RF switch should be designed carefully. The T/R switch signal is also used to control four other switches used in the transmitter and receiver circuits, maintaining high isolation between the two circuits.

In contrast to many conventional superheterodyne systems, the baseband output of this system is a 185-MHz digital IF signal. At baseband, the signal is sampled at digital IF with a higher sample rate to help avoid potential problems. A baseband anti-aliasing filter can provide much better performance in the digital domain compared to an analog filter. Of course, as digital-signal-processing (DSP) techniques have become more widely used, this superheterodyne transceiver architecture with digital IF has also grown in popularity.

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