The new Walking Dead companion series will begin shooting next month, and will be set in Los Angeles, according to E Online.  With shooting to begin in January (and the time period of the spinoff different from the core series), it would make sense for the companion series to air in the break between seasons of the main series next summer.
 
The cast is taking shape, and elements of the story are starting to come out (see "'Walking Dead' Spinoff Has Male Lead").