Agilent Technologies recently introduced what it claims is the industry’s first signal generator and analysis product for LTE-Advanced 8×8 MIMO.
LTE-Advanced is designed to enable peak data rates of up to 1 gigabit per second on the downlink and up to 500 megabits per second on the uplink, depending on the amount of spectrum used. To achieve those speeds, LTE-Advanced supports carrier aggregation of up to five component carriers, each up to 20 megahertz wide, and better mult-antenna techniques in both the uplink and downlink. LTE-Advanced includes the use of multiple-input, multiple-output technology with up to eight spatial streams and antennas. The evolution to 8×8 MIMO from 2×2 and 4×4 is still very much in a developmental phase, and having the ability to test 8×8 MIMO enables the ecosystem to advance.
RCR Wireless News asked Randall Becker, applications engineer at Agilent, to explain the need for testing and analysis of LTE-Advanced 8×8 MIMO.
RCR Wireless News: How are the signal generation and analysis different with 8×8 MIMO than with previous generations (2×2, 4×4)? What are some of the factors at work?
Randall Becker: Agilent Technologies is actively engaged in developing LTE analysis and signal generation solutions that will help the industry move forward with development and deployment of LTE systems as the technology evolves from 2×2 to 4×4 to 8×8.
The 8×8 MIMO scheme involves more than just adding additional antennas to 2×2 or 4×4 LTE systems. The LTE-Advanced specification, in Release 10, added Transmission Mode 9 to enable the transmission of higher order MIMO, which is significantly different than the previous transmission modes. … One of the main differences is that Transmission Mode 9 is based on non-codebook based precoding. This concept was introduced in previous LTE releases in Transmission Mode 7 for single layer transmissions and Transmission Mode 8 for two-layer (2×2 MIMO), commonly known as single-layer and dual-layer beamforming respectively.
Transmission Mode 9 is really an extension of the dual-layer beamforming technique, adding up to eight layers, and one of the obvious enhancements is the definition of additional antenna ports to handle the eight-layer transmissions. There will be some new challenges associated with operating eight radios simultaneously in a small space. For example, consider the potential interference and cross-talk issues that could degrade system performance. This means that the test equipment will need to support up to eight ports for complete testing. Agilent’s Signal Studio software automatically generates the eight-layer LTE signals and synchronizes multiple signal generators for accurate testing at the receiver.
What is less obvious in Transmission Mode 9 is the inclusion of the UE-specific reference signals, which are transmitted among the sub-carriers used for the transmission of the physical downlink shared channel. They follow the same signal processing (precoding) as the downlink shared channel, and this is what enables the use of non-codebook-based precoding where the mobile handset demodulates the downlink shared channel without knowledge of the precoding that was used, and thus the base station avoids the overhead of specifically signaling this information to the mobile handset. These UE-specific reference signals are already supported by Agilent’s signal creation and analysis solutions to enable testing of this new functionality.
In the codebook based precoding scheme, the cell-specific reference signals are added after the precoding and therefore the mobile handset needs to know explicitly how the precoding is performed since the downlink shared channel will have undergone some additional amplitude and phase changes that the cell-specific reference signals did not. The cell-specific RS definitions from Release 8 were not required to be repeated on the new port definitions since the UE-specific RS are used for demodulation in Transmission Mode 9, which saves resources for transmitting real user data.
However, another function of the cell-specific reference signals is to enable the mobile handset to make measurements of the channel conditions and report them back to the base station, regardless of where or whether a downlink shared channel was scheduled for transmission. To meet the need for reporting the channel conditions, the standard introduced new channel state information reference signals, or CSI-RS, which, for this purpose, thereby saving resources because they do not need to be transmitted as frequently as the cell-specific RS. Agilent’s signal analysis solutions enable testing of the CSI-RS signal quality as well as the proper placement in time and frequency. From the receiver perspective, Agilent’s signal creation software enables the designer to determine if the UE is reporting the correct channel conditions under different propagation conditions.
Other aspects of Transmission Mode 9 include the ability to do beamforming for up to eight-layers and the ability to seamlessly switch from single-user MIMO to multi-user MIMO, facilitated through the use of a new DCI format in the control channel (PDCCH). Again, Agilent’s solutions also support these new aspects of Transmission 9 to measure and evaluate device under testing (DUT) performance.
So, in terms of things that are different for 8×8 MIMO compared to 4×4 MIMO test solutions, it is clear that the test equipment now needs to support up to eight channels for signal generation and analysis. Additionally the way these Transmission Mode 9 signals are constructed is different from the 4×4 MIMO in Release 8, including the use of some new reference signals, and thus it is important to verify proper operation of these new techniques in the DUT.
RCRWN: How far along is the market for testing 8×8 MIMO, in your view? Do you see antenna/device manufacturers even close to 4×4, much less 8×8? How will devices like yours advance LTE-Advanced development?
Becker: It is still very early in the development cycle for 8×8 MIMO and deployment of 4×4 MIMO. Most of the 8×8 MIMO development work is still in the research stage since there are a lot of practical issues that need to be resolved. For example, the physical space on a mobile handset is very limited, considering a DUT would need to have eight antennas in order to receive eight-layers or 8×8 MIMO transmissions. Additionally, these eight antennas would need to be spatially separated enough to ensure that they are uncorrelated from each other to enable the MIMO scheme to work adequately. Now, considering that most mobile handsets support multiple bands, and depending on which bands are used, this could also require additional sets of antennas. And, finally, Release 10 introduced the carrier aggregation concept where up to five carriers can be theoretically transmitted and received simultaneously. Agilent’s solutions are specifically designed to mitigate the complexity of the challenge facing 8×8 MIMO device developers by offering standard-compliant solutions to measure device performance, yet are flexible enough to enable troubleshooting of systems and components.
Today, most people are only looking at aggregating two separate bands, each with a single component carrier. Since each of these component carriers could theoretically be configured for 8×8 MIMO capability, this would result in 16 antennas. You also have to consider the battery life required to simultaneously run this many radios in a single mobile handset. We’ll first start to see the industry using additional antenna systems on devices that have the resources to support them, i.e. laptops.
However, with regard to antenna systems, what is more realistic is using transmission mode 9 for the beamforming and multi-user MIMO capabilities. The base station has the resources to support eight antenna systems while the mobile handset does not and these eight antennas could be used to focus a single layer at a particular user location and this only requires one antenna at the mobile handset to receive or perhaps we take two layers for four layers, each directed at different users, again only required one antenna per mobile handset.
Agilent’s solutions are definitely advancing LTE. We already support Transmission Mode 9 with up to eight-layers, carrier aggregation, and the uplink enhancements like clustered SC-FDMA and the simultaneous transmission of the PUSCH and PDCCH (which was not available in Release 8 or 9 of the standard). In fact, today we can already generate carrier aggregation scenarios with two component carriers, including cross carrier scheduling, each simultaneously generating an eight-layer MIMO signal using Transmission Mode 9. This 8×8 MIMO solution provides early researchers access to test solutions to verify and troubleshoot early designs and monitor system performance. These solutions are scalable, so they can adapt as technology moves into product development and manufacturing. Agilent is monitoring, tracking, and participating in LTE standard development and we strive to bring solutions to market as early as possible to help those working on the newest generation of 8×8 MIMO devices get their products to market quickly.
RCRWN: What type of companies are you targeting with this product at this stage in LTE-Advanced network/device development?
Becker: Agilent targets the entire LTE ecosystem with our solutions for early LTE device simulation to LTE system deployment. From signal analyzers capable of characterizing transmitter performance of the newest enhancements of the LTE standard to signal generators that generate benchmark reference signals for to verify receiver performance to the simulation and analysis software that enable modeling of DUT performance before the next generation of the standards are completed. Agilent provides solutions for all stages of LTE development. The particular solutions we have been discussing here are focused on testing the transmitter and receiver physical layer of the radio, so they are perfect for anyone needing to test actual devices. These solutions also work with our simulation software as well.
To see some demos of this equipment, check out:Â http://www.youtube.com/user/agilentlte
Read more about the LTE device ecosystem, and the challenges of MIMO antenna proliferation, in RCR’s new special report.Â
