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Hi,
I see the CC1101 transceiver can be configured for several modulation formats, of which the more interesting one for me are :
2-FSK
GFSK
MSK
My question relates to the statement in the CC1101 documentation that "MSK is only supported for data rates above 26kBaud''.
I am currently trying out the CC1101 with a modulation configuration of GFSK with a frequency deviation (fd) which is 1/2 the bit-rate (rb), which conforms to the condition for coherent FSK/MSK, where fd = 1/2 x rb. The bit rate of my CC1101 modem is configured so that I essentially transmit an MSK signal by setting the fd = 1/2 x rb but with the GFSK modulation configured in the MDMCFG2 register.
So my question relates to this settings - can I essentially obtain an MSK modulation by using the GFSK configuration, but with a setting of fd = 1/2 x rb for the deviation and data rate settings ?? Will the receiver modem be capable of coherent FSK demodulation in this mode ? or do I really need to configure both the modems (Tx & Rx) for MSK (with the 26kb/s data rate limit) ??
Essentially, the spectral efficiency that I seek is important, and I would like to obtain best sensitivity with the best spectral efficiency, but it is not entirely clear which modem configurations would yield these two optimal conditions I seek, and if my above settings would give me GMSK communication for instance ??
Can somebody please clear this up for me ? Can I "emulate" GMSK modulation with the fd = 1/2 x rb settings with GFSK modem configuration in the MDMCFG2 register, or not, and why not if so, in a nutshell ?
thanks in advance.
Regards, M
You can use a modulation index of 0.5 and "emulate" GMSK modulation in MDMCFG2 and DEVIATN registers. Please note though that the sensitivity will be degraded as you reduce the modulation index (you can of course scale down the Rx filter BW if possible when reducing the deviation). Theoretically, there is an optimum deviation/datarate setting if you simultaneously minimize the receiver filter bandwidth. Every halving of receiver filter bandwidth increases sensitivity with 3 dB whereas sensitivity vs deviation/datarate decreases with about 1.5-2.5 dB per halving down to a certain limit where the loss increases very fast.
None of our transceivers are coherent. Although we need the preamble/sync word for initial symbol timing recovery, we generally track symbol timing during the entire packet. The sync word is also needed for identifying the start of the payload. To put it simple, each 2FSK bit is detected as the sign of the phase change over one symbol period.
Is there an app note about how the constellation diagram works depending on the DEVIATION_M setting, when using MSK? I always thought that common MSK used a pi/4 (45 degree) constellation offset. I assume this would mean to use 4/8=011. Am I correct? Thanks!