Differential encoder to the Gray mapping is to remove the 90 degree phase variance, e.g. the received constellations are 90 degree rotated. The receiver doesn't reply on the absolute phase of the QAM constellations but uses the transition between consecutive symbols to demodulate the data.  

Why is a precoder necessary for DQPSK and what does it accomplish?
So, I'm having problem downloading apps from Play Store because apparently, I have insufficient space on device. The app I want to download is of only 12.68 MB and my system storage has 113 MB, phone storage has 596 MB, and SD card has 1.46 GB available. 
I tried cache cleaning, uninstalling unwanted apps, moving the movable apps to SD card but it still won't work. 
The device I'm using Micromax Bolt A089 with a 4 GB SD card. 
Please help. Thank you. 
