Design and Verification Challenges: Beamforming

Keysight Technologies

Design and Verification

Challenges: Beamforming

Reprinted from

 LTE and the Evolution to 4G Wireless Design and Measurement Challenges

 Second Edition

LTE and the Evolution to 4G Wireless

Design and Measurement Challenges, Second Edition

Chapter 6 Design and Verification Challenges

6.9 Beamforming

This section briefly recaps multi-antenna techniques before introducing the concept of beamforming, its advantages, and its use within modern wireless communications systems such as LTE.

Multi-antenna beamforming measurement challenges are discussed from an eNB test perspective, and the importance of calibration is highlighted when it comes to testing the performance of a beamforming transmission system.

6.9.1 Multi-Antenna Techniques Summary

Various multi-antenna techniques employed by LTE are introduced in Section 2.4. These radio access techniques are illustrated in Figure 2.4-1, which captures the concept of single input single output (SISO), single input multiple output (SIMO), multiple input single output (MISO), and multiple input multiple output (MIMO).

SISO is the most basic radio channel access mode and sets the baseline for minimum transmission performance. However, SISO does not provide any diversity protection against channel fading.

SIMO provides additional receive antenna redundancy compared to the SISO baseline, which allows the use of receive diversity techniques such as maximum ratio combining in the receiver. This flexibility improves signal to interference noise ratio (SINR) observed at the device receiver and can help improve robustness under channel fading conditions.

MISO provides additional transmit antenna redundancy, allowing the use of transmit diversity techniques such as Alamouti symbol coding, or space frequency block coding (SFBC) as is the case for LTE. Similar to SIMO, MISO also provides an improvement in the observed SINR at the device receiver which helps protect against channel fading.

In contrast MIMO provides both additional transmit and receive antenna redundancy. This redundancy can be used to improve the SINR at the device receiver using transmit and receive diversity techniques. Alternatively some or all of the potential SINR performance improvement can instead be traded off to obtain an improved spectral efficiency by employing spatial multiplexing transmission techniques. This improved spectral efficiency can be realized either in the form of increased data rate throughput for a single user device using single-user MIMO (SU-MIMO) techniques, or alternatively in the form of increased system cell capacity using multi-user MIMO (MU-MIMO) techniques. In addition to the diversity and spatial multiplexing techniques summarized above, it is possible to use the multi-antenna path redundancy to support transmit and receive beamforming techniques to improve system performance, as will be introduced next.

6.9.2 Introduction to Beamforming

The basic principles of transmit diversity, spatial multiplexing, and beamforming are compared in the context of a multi-antenna transmitter. Figure 6.9-1 illustrates the basic concepts.

In the case of transmit diversity, orthogonally modified redundant copies of an information symbol pair, S0 and S1, are transmitted simultaneously in time across the multiple antenna elements. The example at the far left of Figure 6.9-1 shows SFBC transmit diversity for a subcarrier pair. The symbols S0 and S1 are transmitted on different subcarriers on Tx0 and the complex subjugate of the symbols (denoted by *) is transmitted on Tx1, with S1 also being negated. The benefit is improved SINR, observed at the device receiver, plus improved robustness to channel fading.

n the case of spatial multiplexing, separate and unique information symbols, S0 and S1, are transmitted simultaneously in time across the multiple antenna elements, as shown in the center example of Figure 6.9-1. The benefit is improved spectral efficiency observed as either increased individual user throughput or increased cell capacity.

In the case of beamforming, weighted copies of an information symbol, S0, are transmitted simultaneously in time across the multiple antenna elements, as shown in the example at the far right of Figure 6.9-1. In this example w0 and w1 represent the applied complex per antenna weightings. The benefits are improved SINR observed at the receiver of the primary target device, resulting from a coherent signal gain, plus the ability to minimize interference to other devices within the system.