In a previous blog entry I’ve been looking at how WiMAX and HSDPA allow several mobiles to simultaneously use the uplink. This is necessary as the power output of a terminal is much lower than that of a base station. Due to this restriction a single terminal can not use the total uplink bandwidth of a channel. The only way to compensate for this is to allow several mobiles to transmit at the same time. After writing this article a reader asked how LTE (Long Term Evolution), the successor of HSDPA/HSUPA, deals with this. So here we go:
In downlink direction, LTE is based on Orthogonal Frequency Division Multiplexing (OFDM) technology, quite similar to WiMAX (802.16e). While WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access) modulation in uplink direction, it was decided by 3GPP to go a different way for LTE. Here, SC-FDMA (Single Carrier – Frequency Division Multiple Access) will be used. It took me quite a while to figure out the basics of SC-FDMA but I think I’ve finally got the basics right and have posted the results of my research here.
So why doing it differently?
It looks like while OFDMA has many advantages it suffers from bad Peak to Average Power Ratio (PAPR). Again, I didn’t find an easy to understand explanation of PAPR and it’s implications on the web. Therefore I decided to do some of my own research and I am very thankful to a number of readers who have helped in the process. The results are presented here.