Orthogonal Frequency Division Multiple Access or with abbreviated version OFDMA is a methodology of digital data, that a signal is splitted into multiple carrier frequencies (subcarriers). OFDMA is a multi-user version of OFDM scheme. OFDMA allows multiple users to transmit simultaneously on different sub-carriers per OFDM symbol.
Subcarriers are orthogonal to each other in the system. OFDMA is used for downlink direction, for uplink direction SC-FDMA is used in LTE.
Characteristics of OFDMA system:
🔘 LTE Radio Frame duration = 10ms
🔘 Sub-frame duration = 1ms
🔘 Time Slot duration = 0.5ms
🔘 Symbol duration = 0.5ms / 7 or 0.5ms / 6
🔘 Symbol count = 7 in normal CP, 6 in extended CP
🔘 Subcarrier bandwidth = 15 KHz
🔘 Subcarrier count in one PRB = 12
LTE Radio Frame consists of = 10*Subframe = 20*TimeSlot = 140*Symbols
• A PRB (Physical Resource Block) consists of 12 subcarriers, 7 symbols in normal CP, resource elements and lasts for one 0.5 ms timeslot. PRB can be imagined as a box where user data, signals and control data are allocated. Number of PRB and number of subcarriers depend on frequency bandwidth:
20 MHz = 100 PRB, 1200 subcarriers
15 MHz = 75 PRB, 900 subcarriers
10 MHz = 50 PRB, 600 subcarriers
5 MHz = 25 PRB, 300 subcarriers
3 MHz = 15 PRB, 180 subcarriers
1.4 MHz = 6 PRB, 72 subcarriers
• Transmission Time Interval (TTI) equals to 1ms and it is minimum scheduling unit in LTE. So two PRB is scheduled for the user at a time.
• Resource Element (RE) is a smallest unit in OFDMA system. It is used to carry user data, signals and control data.