|
|
|
|
|
|
To know total above overhead, we need to know the number of code blocks. Smallest TTI (maximum data rate, so smallest TTI) is 1 ms i.e. one subframe. As seen earlier, in one subframe, 99000 bits of DL-SCH data can be transmitted. Maximum code block on which coding can work is 6144 bits [36.212:5.1.2]. So roughly we can say there would be (99k/6144=) 13 code blocks.
The total number of input bits to coding would be 6144 x 13. Since this is now known, we can easily estimate maximum size of transport block that can be passed to PHY. Maximum TB size divided by smallest TTI would give us maximum data rate for DTCH. Note that we are not requied to estimate overhead added by coding/rate matching.
Each code block will have 24 bit CRC; 6144 includes this CRC. So the nunber of bits input to code block segmentation would be (6144 - 24) x 13 = 79560. The transport block size then be 79560 - 24 (bit CRC) = 79536. Maximum data rate for DTCH would be (transport block size/TTI=) 79.536 Mbps.
Thus percentage loss in data rate due to PHY processing is (99 - 79.536)/99 = 19.6 % !
Maximum TB size as per specifications
In above section, we estimated maximum TB size. Various possible TB sizes are specified in "Transport block size table" in 3GPP TS 36.213 [36.213:7.1.7.2.1]. As per this table, maximum TB size is 75376 bits. For this TB size, data rate would be 75.376 Mbps !
Maximum TB sizes are also specified in "DL PHY values as per UE category table" in 3GPP TS 36.306 [36.306:4.1]. Per antenna maximum TB size is mentioned as 75376 (which of course is same as specified in 36.213).
With Multi-antenna
So far our discussion was about single antenna (port) configuration. LTE specfies multi-antenna or spatial multiplexing configuration of 2 and 4 antenna ports. Easier way to calculate maximum data rates for multi-antenna configuration would be to check maximum TB size for it and calculate rates for 1 ms TTI. For 2x2 configuration, "DL PHY values as per UE category table", max TB size is 150752, giving DTCH rate of 150.752 Mbps. For 4x4, DTCH rate comes out to be 299.552 Mbps.
Next step would be to account for L2/L3 signaling overhead.
|
|
|
|
|
|
|
|
|
|
|
