More than 1 million travelers passing through Denver International Airport over the long holiday weekend will experience better cellular service than nearly anywhere else in Denver. Then they’ll board their planes and leave the city or return to their Denver neighborhood where subpar mobile service is the norm. This is because in a city that ranks as one of the worst nationwide for cellular service, every major mobile carrier has beefed up the number of antennas at DIA so more people can get a good signal and decent data speeds during the busy travel season, which the airport expects to be busier than last Thanksgiving holiday. “Airport upgrades should be up and running” by Tuesday afternoon and are permanent, Suzanne Trantow, a Denver-based AT&T spokeswoman, said in an email.