Wetinvsdny, April 11, /W/
NEW YORK, April 10ÛÓThe stock market posted a modest advance today amid investor wariness about the pace of economic growth and the direction of interest rates.</br></br>The Dow Jones average of 30 industrials, which rose 1.68 points on Monday, finished up 4.40 points today at 1,138.30, the third consec-. utive advance in the blue-chip measure after a string of six straight declines.</br></br>Bell Atlantic Mobile Systems inaugurated its $15 million mobile telephone service yesterday, making Washington the only U.S. city with two competing cellular radio systems.</br></br>Bell AtlanticÛªs cellular system, dubbed ÛÏAlexÛ after Alexander Graham Bell, will compete with Cellular One, a mobile telephone system owned by a group of six nontelephone companies.</br></br>Competition already is fierce, with each company aggressively advertising its service as the best in cellular radioÛÓa new technology that permits thousands of simultaneous calls over car and portable telephones linked by high-speed computers and low-powered transmitters scattered throughout a metropolitan area. Previous mobile-phone systems use a single high-powered transmitter for an area, thereby limiting the number of calls that can be made simultaneously to about two dozen.