New-Tech Europe Magazine | October 2018

These techniques have been used in several generations of wireless protocols and in each iteration they have improved the data throughput of the system. These techniques address an unavoidable effect when transmitting data over any distance, known as fading. Fading occurs when a signal decreases in amplitude due to cancellation in the channel in which it is transmitted. From a broadcasting antenna, the signal will undoubtedly take several different paths on its way to the receiver. The transmitted signal will change in phase on the path to the receiver. These differences in phase can potentially cancel the signal at the receiver, causing fading. In order to counteract this fading, multiple transmit or receive antennas can be used. It is very unlikely that when a signal is transmitted or received on multiple antennas that cancellation will occur in all cases. Using multiple antennas is referred to as antenna diversity and it can further improve data throughput in a wireless system. Antenna Diversity There are several ways to implement antenna diversity in a wireless system. Antenna diversity is one way of implementing spatial division multiple access (SDMA), since the space between antennas is used to distinguish between signals. There can be multiple antennas on the transmit side and a single antenna on the receive side (MISO), a single transmitting antennas and multiple receiving antennas (SIMO) or multiple transmitting antennas and multiple receiving antennas (MIMO). MIMO based systems offer the best results in terms of antenna diversity, but the complexity of the decoding requires a sophisticated transmitter and receiver. In an environment that is constantly changing, constant channel characterization is required. Also, as the distance between the transmitter and receiver increases, the

Figure 1: System with multiple LTC2000A DACs synchronized

complexity of the channel between the transmitting and receiving antennas becomes unstable and difficult to differentiate, making the benefits of MIMO less pronounced. Systems with multiple transmitting antennas and a single antenna for reception are quite common in wireless communications and take advantage of antenna diversity to improve performance. In a MISO wireless system with antenna diversity, multiple DACs transmit data on multiple antennas simultaneously. Since transmitting antennas are arranged at physically different locations, on a tower for instance, the signals will propagate to the receiver in different ways. The

path from the transmitting antenna to the receiving antenna will be different. The received signal will be different from each of the antennas due to the specific multipath affects in each of the channels. The characterization of each individual channel with transmitted pilot tones will give the receiver valuable information on the performance of each channel. This information can be used to digitally modify the data before transmission, maximizing the likelihood of reception at the receiver. Since each channel requires specific modification and correction, separate DACs and a dedicated digital signal processor (DSP) are required for each transmitting antenna. If the transmitting DACs

Figure 2: Frequency domain plot showing 16 channels of CDMA produced by the LTC2000A with a single gap channel.

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