New-Tech Europe Magazine | October 2018

Improve Signal Reception with Antenna Diversity Using Multiple High Speed DACs

Clarence Mayott, Linear Technology Corp.

Introduction In modern wireless systems, numerous diversity techniques are used to maximize data throughput and improve transmission reliability. Time, frequency and code diversity techniques are used to transmit signals to multiple users simultaneously and to maximize the amount of data transmitted. By transmitting at different times, on different frequencies, or with different binary sequences known as Gold codes, signals are produced so that they can be differentiated from each other and can be received error free. These techniques are widely known and have been perfected over decades of use. These techniques can also be used with antenna diversity or spatial diversity. Antenna diversity involves using multiple antennas to transmit or receive a signal. Simple antenna diversity uses the antenna combination with the best performance to decode the signal. More

complex versions of antenna diversity include MIMO (multiple input, multiple output) systems and beamforming applications, where multiple antennas are deployed at the transmitter and receiver to increase spatial diversity. With multiple broadcasting antennas it is critical to have timing variation in the picosecond range between the digital to analog converters (DACs) on each channel. This requires DACs that can be synchronized together so that the transmitted data is broadcast simultaneously. With this criteria met, the system will be able to transmit identical data over multiple antennas to a common receiver, maximizing the likelihood of proper signal reception and minimizing the likelihood of dropping a signal from a transmitter. TDMA, FDMA, CDMA Communication systems can handle multiple users by means of some kind of

diversity. The most basic version of this is time division multiple access (TDMA), which essentially involves transmitting data to multiple users at different times. The receiver then simply waits for its time slot and decodes the appropriate data. Frequency division multiple access (FDMA) works in much the same way but in the frequency domain. Data is transmitted to different users on specified frequencies and the receiver decodes only the data on those frequencies. More modern systems incorporate code division multiple access (CDMA) into wireless systems where the transmitted data is convolved with a specific code before it is transmitted. This code is then used at the receiver to decode the data specifically transmitted to that user. Since these techniques are implemented in different domains, they can be used together in the same system for maximum diversity.

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