Prb Utilization.docx

On the LTE air interface the unit of allocation is a PRB. PRB is 12 subcarriers by 7 OFDM symbols = 84 modulation symbols. The instantaneous throughput achieved will depend on the modulation scheme used for transmission of the PRB (lets ignore MIMO). Now, given that the PRB is a Unit of allocation the Minimum allocation to a single UE during a subframe (1 ms) are 2 PRBs with one PRB in each slot of the subframe. Hence a UE will get a total of 2 PRBs/subframe = 84 x 2 = 168 modulation symbols/subframe. Now, not all the 168 modulation symbols can be used to transmit user information because one needs to take out the modulation symbols used for Reference Signals, PDCCH, PCFICH, PHICH and PSS, SSS, PBCH if applicable. The amount of overhead will depend on number of MIMO antennas, bandwidth .etc. Once can safely take approximately a 25% overhead out so. About 144 usable modulation symbols per subframe is a reasonable approximation for a "back of the envelope calculation" if you consider a 2x2 MIMO 10 MHz deployment. Please note, I am approximating to motivate the associated concepts. Now one can calculate the raw throughput based on a chosen modulation scheme. Please remember the bits that you calculate includes the FEC Turbo/Convolutional coding overhead. Also on a subframe basis to a UE the MAC and RLC will coordinate to maximize the use of the resource allocation - though rate matching is always an option available while assembling the MAC PDU for transmission.