An apparatus for wireless communications, comprising: means for generating a frame comprising: a first portion with first blocks of symbols, each first block being surrounded by first guard interval, GI, sequences of a first GI length, each of the first GI sequences having a first half and a second half, wherein the first half is closer towards a beginning of the frame as compared to the second half of said each first GI sequence, and a second portion with second blocks of symbols, each second block being surrounded by second GI sequences of a second GI length shorter than the first GI length, wherein: the second portion of the frame precedes the first portion of the frame, each of the first half of the first GI sequences is formed by one of the said second GI sequences, and the first half of a first GI sequence at a starting first block of the first portion of the frame serves as one of the second GI sequences that surrounds an ending second block of the second portion of the frame; and means for outputting the frame for transmission. means for generating a frame comprising: a first portion with first blocks of symbols, each first block being surrounded by first guard interval, GI, sequences of a first GI length, each of the first GI sequences having a first half and a second half, wherein the first half is closer towards a beginning of the frame as compared to the second half of said each first GI sequence, and a second portion with second blocks of symbols, each second block being surrounded by second GI sequences of a second GI length shorter than the first GI length, wherein: the second portion of the frame precedes the first portion of the frame, each of the first half of the first GI sequences is formed by one of the said second GI sequences, and the first half of a first GI sequence at a starting first block of the first portion of the frame serves as one of the second GI sequences that surrounds an ending second block of the second portion of the frame; and a first portion with first blocks of symbols, each first block being surrounded by first guard interval, GI, sequences of a first GI length, each of the first GI sequences having a first half and a second half, wherein the first half is closer towards a beginning of the frame as compared to the second half of said each first GI sequence, and a second portion with second blocks of symbols, each second block being surrounded by second GI sequences of a second GI length shorter than the first GI length, wherein: the second portion of the frame precedes the first portion of the frame, each of the first half of the first GI sequences is formed by one of the said second GI sequences, and the first half of a first GI sequence at a starting first block of the first portion of the frame serves as one of the second GI sequences that surrounds an ending second block of the second portion of the frame; and the second portion of the frame precedes the first portion of the frame, each of the first half of the first GI sequences is formed by one of the said second GI sequences, and the first half of a first GI sequence at a starting first block of the first portion of the frame serves as one of the second GI sequences that surrounds an ending second block of the second portion of the frame; and means for outputting the frame for transmission.