Startup PHAZR is asking the FCC for an experimental license to test its Quadplex technology that pairs high-band millimeter wave spectrum for downlink with midband spectrum for the uplink to enable 5G systems.
PHAZR, which works with 3 of the 4 biggest U.S. carriers but doesn’t identify them, is seeing an increased interest on pairing millimeter wave spectrum for downlink with sub-6 GHz spectrum for the uplink.
PHAZR CEO Farooq Khan said the industry over the past two months or so has come to the realization that uplink on millimeter wave is going to be cost-prohibitive because the links need to be close together and there needs to be more of them—perhaps 10 times more.
“We realized earlier, that’s why we took this approach,” he said. But it took industry a little bit of time to come to this conclusion, and PHAZR is seeing a lot of interest in its approach.
In the 5G New Radio (NR) standard, it is possible to configure parameters to do millimeter wave on the downlink and sub-6 GHz on the uplink.
The company aims to use the 3.5, 28, 24 and 39 GHz bands for tests at its facilities in Allen, Texas, over the course of about 24 months.
The industry is also realizing the limitations of a hybrid beamforming approach, which is the combination of analog and digital beamforming, currently promoted by big radio network infrastructure vendors, Khan said. The limitations include an inability to frequency-multiplex users, an inability to use power-amplifier efficiency improvement techniques and poor performance in non-line-of-sight (NLOS) scenarios, according to Khan.
“The Hyperdense beamforming that we are evaluating overcome these limitations,” he said.
The recently approved 3GPP 5G-NR standard limits the transmit power for mobile devices operating in the millimeter wave bands to less than 25 dBm.
Basically, both chipset vendors and device/smartphone vendors lobbied the 3GPP for a lower transmit limit. The concerns from both were on power consumption (battery drain) as well as the heating. A millimeter wave transmitter generally operates at much lower efficiencies compared to a sub-6 GHz transmitter and generate a lot of heat, Khan noted.
The duplexing scheme specified in 5G-NR standards for new millimeter wave band and midband frequencies is time division duplexing (TDD). With TDD, a mobile device gets a fraction of the time to transmit on the uplink, which further limits the achievable coverage and data rates on the uplink.
Quadplex enables frequency division duplex (FDD) operation on widely spaced bands such as millimeter wave band for downlink with sub-6 GHz band for the uplink, according to Khan. This way, a mobile device can continually transmit on the uplink providing greater coverage and data speeds. TDD is generally used for short-range communication like in Wi-Fi systems, while FDD has always been preferred for wide area cellular systems due to its inherent coverage advantage.