Altera CPRI IP Core - Manual (Page 175)

Appendix D: Advanced AxC Mapping Modes D–3
Advanced Mapping Mode Similarities and Differences
December 2013 Altera Corporation CPRI MegaCore Function
User Guide
1 Some table entries are not available, depending on the CPRI line rate and on K. In the
example illustrated in
Figure D–2 , the table entries 7 and 15 are not available.
In 16-bit mode in all advanced mapping modes, and in 15-bit mode in advanced
mapping mode 2’b01, you can use the
width field to specify the size of the sample that
starts in the bit position indicated in the position field, allowing you to pack a second
sample immediately following the first sample in the timeslot, or to specify a sample
width larger than the timeslot. In the case of a sample that spills into the following
timeslot, you must enable the following timeslot in the Rx or Tx mapping table.
In 15-bit width mode in advanced mapping modes 2’b10 and 2’b11, you must set
width to the value of 15 (indicating a 30-bit IQ sample), and you must set position to
specify the offset of the next available bit in the current 32-bit timeslot, because the IQ
samples are packed in the timeslots with no intervening spare bits.
You can calculate the number of timeslots that correspond to a CPRI frame. Only the
data bytes pass through the AxC interface; the control bytes in a CPRI frame do not
pass through the AxC interface. Refer to the Number of Bits column in
Table 4–5 on
page 4–17 or Table 4–6 on page 4–17 for the number of data bits in a CPRI frame
at each CPRI line data rate. The calculation depends on the presence and values of any
position offsets, on whether the CPRI IP core is in 15-bit width mode or in 16-bit
width mode, and on how remainder bytes are handled. The following discussion
focuses on the cases with
position fields all set to zero. You can increment the
timeslot counts as needed to accommodate unused leading timeslot bits specified
with
position offsets.
Fifteen-Bit Width Mode
In 15-bit width mode, you either pack the 30-bit data samples in the 32-bit words (in
advanced mapping modes Advanced 2 (2’b10) and Advanced 3 (2’b11)), or you
selectively allow gaps, specifying them with the
position and width fields of the table
entry (in the new Advanced 1 mapping mode (2’b01)). In 15-bit width mode,
advanced AxC mapping modes 2’b10 and 2’b11 act identically, packing the data into
consecutive bits. Because the number of bits in the IQ data block of every CPRI frame
is a multiple of 30, packed 15-bit I- and Q-samples fill an AxC container—and one or
more CPRI frames—with no spare bytes remaining. However, in the Advanced 1
mapping mode, you can specify an offset in the
position field, potentially leaving
spare bytes in the IQ data block.
Figure D–1 shows the contrast between these advanced mapping modes. In this 15-bit
mode example, the CPRI data rate is 1228.8 Gbps and the value of K is two. For a
CPRI IP core running at CPRI data rate 1228.8 Gbps, the number of data bits in a CPRI
basic frame is 240. (Refer to
Table 4–6 on page 4–17 ). If K (specified in the K field of the