The PAPR Problem

I’ve happened to stumble over PAPR (Peak to Average Power Ratio) quite a lot lately as it seems to play a big role in WiMAX and 3GPP LTE mobile devices. Most papers mention that LTE has a better PAPR than WiMAX but fail to explain what it is and why this is so important. After some research and help from a number of experts here’s an intro to PAPR:

When transmitting data from the mobile terminal to the network, a power amplifier is required to boost the outgoing signal to a level high enough to be picked up by the network. The power amplifier is one of the biggest consumers of energy in a device and should thus be as power efficient as possible to increase the operation time of the device on a battery charge. The efficiency of a power amplifier depends on two factors:

  • The amplifier must be able to amplify the highest peak value of the wave. Due to silicon constraints, the peak value decides over the power consumption of the amplifier.
  • The peaks of the wave however do not transport any more information than the average power of the signal over time. The transmission speed therefore doesn’t depend on the peak power output required for the peak values of the wave but rather on the average power level.

As both power consumption and transmission speed are of importance for designers of mobile devices the power amplifier should consume as little energy as possible. Thus, the lower the difference between the peak power to the average power (PAPR) the longer is the operating time of a mobile device at a certain transmission speed compared to devices that use a modulation schemes with a higher PAPR.

Now let’s come back to the beginning of this blog entry in which I said that papers generally say that LTE has a better PAPR than WiMAX. This is because of different modulation schemes used in the uplink. While WiMAX uses OFDMA (Orthogonal Frequency Division Multiple Access) which is fast but has a high PAPR, LTE designers choose to use SC-FDMA (Single Carrier – Frequency Division Multiple Access) which is as fast but is said to have a better PAPR. So what’s OFDMA and SC-FDMA? Well, that’s for another blog entry.