A TURBO COMMANDER ATP PLT TAXIES ONTO RWY 36 FOR A BACK TAXI OP WHEN ONLY CLRED TO THE RWY BY TWR AT LEB; NH.

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I WAS CLRED BY GND CTL TO 'TAXI TO RWY 36;' BUT I MISTAKENLY THOUGHT I WAS CLRED TO 'BACK TAXI' ON RWY 36 PRIOR TO TKOF; AND SO I ENTERED THE RWY WITHOUT PROPER CLRNC. THERE WAS NO OTHER ACFT IN THE AIR IN THE CTL ZONE AT THE TIME; SO THERE WAS NO POTENTIAL CONFLICT. THE TWR OPERATOR ADVISED ME OF MY ERROR AS I WAS BACK-TAXIING. HOW THE PROB AROSE: I TAXIED FROM THE HANGAR RAMP TO TXWY A AND STOPPED THERE TO CALL GND CTL FOR MY IFR CLRNC. THIS IS MY HOME BASE AND I FOLLOW THIS PROC 1) TO GET AWAY FROM THE HANGAR OFFICES BECAUSE OF MY NOISY ACFT; 2) I AM THEN VISIBLE FROM THE TWR. IN THIS INSTANCE THERE WAS A NEW (TO ME) VOICE WORKING BOTH GND AND TWR FREQS. I RECEIVED MY IFR CLRNC AND WAS SETTING UP MY RADIOS; XPONDER AND HAVING A LITTLE TROUBLE GETTING THE GPS WAYPOINT WHEN I HEARD (BUT MISINTERPED) THE TAXI CLRNC. I AM WELL AWARE THAT A CLRNC 'TO A RWY' DOES NOT MEAN YOU CAN CROSS IT OR ENTER IT WITHOUT ANOTHER PROPER CLRNC. MY PERCEPTION AND PERFORMANCE WAS WRONG; AND I THINK MY MIND WAS PREOCCUPIED BY MY AVIONICS WHEN I SHOULD HAVE BEEN LISTENING. I DO NOT THINK THAT I READ BACK THE TAXI CLRNC.