SIB1 is sent with the periodicity of 80 ms when SFN mod 8=0 and is repeated when SFN mod 2 =0. In other words, a new SIB1 is sent every 80 ms and within the 80 ms period, the same SIB1 is repeated every 20 ms. SIB1 is always sent in subframe 5.
The periodicities of SIB2 and up are configurable, and can be set to 8, 16, 32, 64, 128, 256 or 512 radio frames. These SIBs can be grouped into a set of SI (System Information) messages which are transmitted with different periodicities. All of the SIBs contained within a single SI message should have the same periodicity.
In order to make sure the SIBs are received by the UE, an SI-window is defined to enable multiple transmissions of the SI message within the window. The length of the SI-window can be set to 1, 2, 5 10, 15, 20 or 40 ms. Within an SI-window, only one SI message can be sent, and may be repeated multiple times. When a UE tries to acquire an SI message, it listens from the starting of the SI-window until the SI message is received.
The following figure shows an example of the configuration of the periodicities of SIBs 2, 3, 6, and 7. Here, we use two SI messages, SI1 and SI2. SI1 contains SIB2 and SIB3 with a periodicity of 16 radio frames, while SI2 contains SIB6 and SIB7 with a periodicity of 64 radio frames. Assume that the SI-window length is 10 ms (1 radio frame).
Source: http://lteuniversity.com/get_trained/expert_opinion1/b/hongyanlei/archive/2010/09/21/lte-system-information-part-3.aspx
