Figure 2
From: Bit Level Synchronized MAC Protocol for Multireader RFID Networks
Reader-Reader Collision Problem: tags fall within the read range of multiple readers (called overlapped tags), and the query-response sequence with one reader is interfered with by the simultaneous query-response sequence with other readers. In this example, Tag 1 and Tag 2 are overlapped tags within the read range of both Reader's 1 and 2. When both readers operate at the same time, each interferes with the query-response sequence of the other with any of the overlapped tags. This type of interference is stronger (as compared to that of Figure 1) as the affected tag is also within the read range of the interfering reader and affects both the reception as well as response of signals to/from the tag.