Introduction to Coded Mark Inversion (CMI)
Coded Mark Inversion (CMI) is a type of used in digital transmission systems, particularly in high-speed optical communications. Its name perfectly describes its operation:
- Coded: It transforms the input binary stream into a new, specific waveform.
- Mark: In telecommunication terminology, a "mark" traditionally represents a logical '1'.
- Inversion: The code's key feature is that the polarity representing a '1' is inverted for each consecutive '1' that is transmitted.
CMI is designed to combine the advantages of other codes, such as excellent synchronization properties and the absence of a DC component, making it robust for high-performance links.
The CMI Encoding Rules
The CMI algorithm is a two-part rule that depends on whether the input bit is a '0' or a '1'. It uses a bipolar signal with three levels: positive (+V), negative (-V), and zero.
Rule for Logical '0'
A logical '0' is always encoded as a signal transition from a negative level (-V) to a positive level (+V) occurring in the middle of the bit period (). The signal stays at -V for the first half of the bit duration and at +V for the second half. This guaranteed transition is crucial for clock recovery.
Rule for Logical '1' (Mark)
A logical '1' is encoded as a constant voltage level maintained for the entire bit period (). The key is that the polarity of this level alternatesfor each subsequent '1' transmitted. For instance:
- The first '1' is sent as +V.
- The second '1' is sent as -V.
- The third '1' is sent as +V, and so on.
Analysis of CMI Properties
CMI code exhibits several desirable properties that make it well-suited for high-reliability transmission links.
- Guaranteed Clock Synchronization: CMI ensures frequent signal transitions regardless of the input data sequence. A transition is guaranteed in the middle of every '0', and the level change between consecutive '1's also provides timing information. This makes it very easy for the receiver to perform .
- No DC Component:The code is perfectly DC-balanced. The encoding of a '0' is inherently balanced (half the time at -V, half at +V). The alternating polarity of the '1's ensures that, over time, the average voltage level is zero, regardless of the data pattern. This is a critical advantage for systems coupled via transformers or capacitors.
- Bandwidth Usage: The primary trade-off of CMI is its bandwidth requirement. Due to the fast transition within each '0' bit, the main lobe of its power spectrum extends to twice the clock frequency ( or ). This is twice the bandwidth required by simpler codes like NRZ, making it less spectrally efficient.
- Simple Error Detection:CMI provides inherent error detection capability. If the receiver detects a violation of the coding rules (e.g., two consecutive '1's with the same polarity, or a '0' without a mid-bit transition), it knows a transmission error has occurred.