The ePMP series can transport data over the wireless link using a number of different modulation modes ranging from 64-QAM to QPSK.
For a given channel bandwidth and TDD frame structure, each modulation mode transports data at a fixedrate. Also, the receiver requires a given signal to noise ratio in order to successfully demodulate a given modulation mode. Although the more complex modulations such as 64-QAM will transport data at a much higherratethan the less complex modulation modes, the receiver requires a much higher signal to noise ratio.
The ePMP series provides anadaptivemodulation scheme where the receiver constantly monitors the quality of the received signal and notifies the far end of the link of the optimum modulation mode with which to transmit. In this way, optimum capacity is achieved at all times.
These modulations are employed by the link's Modulation and Coding Scheme. MCS values range for ePMP data links from MCS 15 to MCS 1 (not including MCS 8).
The relationship between MCS and link data transmission
A higher MCS value (e.g. MCS 15) carries more data, but requires more link budget / stronger radio signal. Thus, MCS 15 provides the most data, but it is also the least robust link (i.e. greatest chance data can be lost). MCS 1 provides the least data, but the most robust link (i.e. least change data can be lost).
MCS and spatial streams
MCS 15 – MCS 9 provide two streams of independent data over the two transmit chains while MCS 7 – MCS 1 provide one stream of data over both transmit chains.
Some MCS states share similar properties - MCS 15 is similar to MCS 7, MCS 14 is similar to MCS 6, and so on down to MCS 9 and MCS 1 in that data is transmitted in the same manner, but using two streams (applicable to MCS 15 - MCS 9)) versus one stream (MCS 7 - MCS 1).
Setting the SM configuration parameters Uplink Max Rate to an MCS 15 – MCS 9 value will allow this MCS and all lower "two-stream" MCS values to be used along with all similar or lower "single-stream" MCS values. For example, choosing MCS 14 means that MCS 14 – MCS 9 and MCS 6 – MCS 1 will be used in the Rate Adapt algorithm. Setting Uplink Max Rate to an MCS 7 – MCS 1 value will allow this and all lower single stream MCS values.
Each MCS level has a corresponding Coding Rate, which indicates the proportion of the data stream that is useful data. For example, in MCS 15 with corresponding coding rate 5/6, for every 5 bits of useful data 6 bits of data are generated by the coder (with 1 bit of redundant data generated). For MCS 1 with corresponding coding rate 1/2, for every 1 bit of useful data 2 bits of data are generated by the coder (with 1 bit of redundant data generated). This example exhibits the efficiency gains represented by MCS states with higher coding rates.