Throughput analysis of TDD LTE Random Access Channel

TDD version of 3
^rd
Generation Partnership Project (3GPP) Long Term Evolution (LTE) provides a mode of operation in a single block of spectrum with Uplink and Downlink 1ms-subframes multiplexed in time. Limited number of Uplink subframes coupled with unequal number of Physical Random Access Channel (RACH) opportunities per subframe results in TDD LTE RACH resources and the offered load being non-uniform in time. Thus, performance of TDD LTE RACH is different from classical multi-channel slotted ALOHA analyzed in the past. We derive the offered load in each subframe assuming the details of TDD LTE configuration and subsequently compute the normalized throughput under Poisson load. We show that non-uniform RACH resource allocation across subframes does not cause throughput degradation as long as the resources are located in the same half of the 10ms-radio frame. For selected “worst-case” 3GPP configurations we show that the degradation is less than 10%. Our results provide capacity estimates for the RACH in TDD LTE which can be used for sizing under different traffic conditions.