The configuration & capability of the system: whether it’s 2×2 MIMO, SISO, and the MCS scheme.How a UE communicates ? : Measurements and SignalsĪmong various measurements, the three most important ones are :Īll these are derived from Reference signals.Network loading: number of subscribers in a cell which impacts the overhead.To conclude, the LTE capacity depends on the following: Common channel overhead (adequate to serve 1 UE/subframe) = 10%ĭownlink data rate = 4 x 6 bps/Hz x 20 MHz x (1-14.29%) x (1-10%) x (1-6.66%) x (1-10%) = 298 Mbps.ġ Tx antenna (no MIMO), 64 QAM code rate 1 (Note that typical UEs can support only 16QAM).Pilot overhead (4 Tx antennas) = 14.29%.There is another technique to calculate the peak capacity which I include here as well for a 2×20 MHz LTE system with 4×4 MIMO configuration and 64QAM code rate 1: Note that the uplink would have lower throughput because the modulation scheme for most device classes is 16QAM in SISO mode only. The peak data rate is then 0.75 x 50.4 Mbps = 37.8 Mbps. The total approximate overhead for the 5 MHz channel is 17.86% + 4.76% + 2.6% = 25.22%. The other channels (PSS, SSS, PBCH, PCFICH, PHICH) added together amount to ~2.6% of overhead.Downlink RS signal uses 4 symbols in every third subcarrier resulting in 16/336 = 4.76% overhead for 2×2 MIMO configuration.Assuming that on average it is 2.5 symbols, the amount of overhead due to PDCCH becomes 2.5/14 = 17.86 %. PDCCH channel can take 1 to 3 symbols out of 14 in a subframe.We now have to subtract the overhead related to control signaling such as PDCCH and PBCH channels, reference & synchronization signals, and coding. The MIMO data rate is then 2 x 25.2 = 50.4 Mbps.
Then we calculate the data rate assuming 64 QAM with no coding (64QAM is the highest modulation for downlink LTE):Ħ bits per 64QAM symbol x 4,200 Res / 1 msec = 25.2 Mbps We first calculate the number of resource elements (RE) in a subframe (a subframe is 1 msec):ġ2 Subcarriers x 7 OFDMA Symbols x 25 Resource Blocks x 2 slots = 4,200 REs
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