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EETE MAY 2015

Test & Measurement Testing multi-antenna beamforming transmission systems TBy Sheri DeTomasi he rapid growth of consumer demand for wireless communications services continues to push the boundaries of the current wireless standards. The number of devices being added to the networks is growing exponentially and consumers are requiring higher data rates, improved quality of service, and robust connections from anywhere in the world. There’s a lot of talk about 5G and IoT and how these will continue to tax the existing infrastructure. 5G is in its definition phase and still years away from commercial implementation. Today, LTE 4G cellular networks are becoming mainstream in leading markets, and next generation standards Fig. 1: Summary of multi-antenna techniques. like LTE-Advanced will be the next phase of implementation. LTE-A and wireless LAN standards like 802.11n, 802.11ac allow for multi-antenna techniques to improve data rate, capacity and quality of service. While adding multiple antennas in the RF design provides significant improvements for the user, the verification of these design becomes quite complicated. This article provides a quick overview of multi-antenna techniques and describes some of the key challenges and critical tests needed to verify multi-antenna designs. Multi-antenna designs used in cellular and wireless LAN applications Multi-antenna designs are found in many different industries. In cellular and wireless LAN applications, multi-antenna designs are used to increase peak data rate, capacity and improve the quality of service. How is this achieved? • In cellular and WLAN, multi-antenna techniques such as diversity, spatial multiplexing MIMO (Multiple Input, Multiple Output), beamforming and Multi-User MIMO (MU-MIMO) use multiple antennas to transmit and/or receive data allowing theoretical peak data rates up to 1 Gbps in LTE-A downlink and 6.93 Gbps in 802.11ac systems. Future enhancements to LTE-A, 802.11ac and even 5G will allow for higher order modulation schemes, denser multi-antenna techniques and wider transmission bandwidths. • In cellular networks, carrier aggregation combines multiple component carriers (CC) through wider transmission bandwidths up to 100 MHz. When CCs reside in different frequency bands, called inter-band carrier aggregation, multiple antennas are used to transmit the data in each frequency band. 3GPP Release 12 allows CCs to be combined in up to 3 different frequency bands. Enhanced multi-antenna techniques Figure 1 provides a quick summary of multi-antenna techniques used in cellular and WLAN communication systems. Path diversity uses multiple antennas at either the transmitter or the receiver to improve the robustness of the signal or the ability for the receiver to correctly receive the transmitted data. Transmit diversity uses multiple transmitters with a single receiver (MISO) and Receive diversity uses multiple receive antennas and a single transmitter (SIMO). These techniques are used to improve the signal quality under channel fading conditions. Spatial multiplexing is a MIMO technique to improve the spectral efficiency, increasing either the data throughput for a single user or the system capacity for multiple users. MIMO techniques communicate using two or more transmit and receive chains and include multiple antennas on both the transmitter and receiver. Separate portions of user data are transmitted simultaneously to multiple receivers. MU-MIMO allows data to be sent to multiple users simultaneously through spatially distributed transmissions in the same frequency spectrum to communicate with multiple devices. Beamforming uses multiple antennas offsets in phase and magnitude to add directional transmission to the RF signal. The same signal is transmitted from two or more spatially separated transmission points simultaneously. T The constructive in-phase signal summation results in a coherent power gain at the receiver. Beamforming has an effect similar to diversity in terms of increasing the signal robustness and improving the SNR at the receiver while minimizing interference with other devices in the system. This is very attractive in modern wireless communication systems due to the combined advantages of beam selectivity, interference management and coherent signal gain. Multi-antenna design test challenges Validation of multi-antenna designs can be challenging. Designers need to consider the analysis of the multiple transmit or receive chains and channel-to-channel performance in the measurement systems. Adding beamforming complicates these tests even further, typically adding the requirement for phase-coherent test systems to achieve accurate inter-channel magnitude and phase signal generation and measurements. Key test challenges include complicated test set up for higher order MIMO including beamforming applications, the ability to verify and visualize the multi-antenna performance at the RF antenna, cost and footprint of the multi-channel test system Sheri DeTomasi is Program Manager for the modular solutions at Keysight Technologies, Loveland, USA – www.keysight.com - She can be reached at Sheri_Detomasi@keysight.com 24 Electronic Engineering Times Europe May 2015 www.electronics-eetimes.com


EETE MAY 2015
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