SRG tests T-Mo’s FDD-TDD CA: ‘You can have your cake and eat it too’

The aggregation of 600 MHz + 2.5 GHz uses low-band, FDD spectrum to improve performance

The aggregation of low and mid-band carriers in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) spectrum improves cell edge performance in T-Mobile US’ network, according to recent testing by Signals Research Group.

SRG tested the FDD+TDD carrier aggregation (CA) in T-Mobile US’ commercial network. The testing was conducted during the first week of December, at a large cluster of Ericsson sites in a 180-square-kilometer swath of the suburban Maryland/Washington, D.C. area.

“A few mobile operators have already launched TDD-TDD CA in other parts of the world but to the best of our knowledge T-Mobile is the first operator in the world to deploy FDD-TDD CA functionality in a commercial network,” SRG noted.

SRG said that T-Mobile US provided the test SIMs and insight into the cluster where the testing was conducted, as well as two Galaxy S21 smartphones — one with pre-commercial software on it that allowed the consulting group to test some of the features it was interested in. Intriguingly, however, SRG found that both devices appeared to support much the same functionality, with a bit of finessing of “hidden” menu options to force the smartphones into the modes of operation that it wanted to test.

SRG used test tools from Accuver Americas and Spirent Communications to conduct the study.

The firm said that the cell cluster supported a “wide range of different technologies,” including both NSA and SA 5G, using T-Mo’s 600 MHz (FDD) and/or 2.5 GHz (TDD) spectrum. In terms of 5G New Radio CA, the sites either used two 5G NR carriers at 2.5 GHz, leveraging T-Mo’s deep midband holdings for TDD-TDD carrier aggregation; or they paired a single radio channel of 600 MHz spectrum with a single channel of 2.5 GHz — the FDD-TDD CA that SRG sought to test.

SRG found that with the TDD-TDD CA (NSA 5G, using 100 megahertz + 20 megahertz), the peak throughput exceeded 1.7 Gbps, with 85% of that throughput attributable to 5G NR. (Two LTE carriers contributed around 260 Mbps when that measurement was taken, SRG noted.)

FDD-TDD CA didn’t quite achieve that much speed, in part due to less total bandwidth in the FDD channel of 600 MHz (for a total of 100 megahertz aggregated, rather than 120). But, SRG commented, “the true benefit of FDD-TDD CA has very little to do with total throughput and everything to do with [2.5 GHz] coverage extension, not to mention making the transition to a complete 5G NR SA network architecture possible.” The use of 600 MHz as the primary cell “results in better performance of 5G NR in Band n41 (2.5 GHz), including higher data speeds close to the cell site, and, most importantly, higher data speeds and extended [2.5 GHz] coverage at the edge of the cell.”

“5G NR FDD-TDD CA demonstrates you can have your cake and eat it too,” SRG explained. The primary cell, or anchor carrier, in this case is using the low-band FDD channel and the secondary cell uses the midband spectrum. T-Mo’s 600 MHz is used for the uplink control and data channels, and “the uplink is always the limiting factor with respect to coverage,” the testing firm said. TDD is also coverage-limited compared to FDD, SRG said, so the use of a low-band, FDD anchor bolsters coverage and cell-edge performance of the mid-band TDD spectrum.

Part of SRG’s analysis also specifically looked at performance comparisons between NSA and SA 5G in the network, and concluded that Standalone 5G mode enables a “dramatic reduction in handover times”, as well as fewer handovers.

Read more from SRG here.

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