ms: Reduce DL CS level if only a few LLC bytes are left

If just a few bytes are left to send to the MS, it makes sense to
reduce the coding scheme level to increase the throughput. This
has been shown by Chen and Goodman in their paper "Theoretical
Analysis of GPRS Throughput and Delay". See their throughput over C/I
measurement graphs (figures 4 and 5 in the paper) for details.

This commit implements a simplified CS downgrade feature for the
downlink. The coding scheme will be downgraded if there are only a
few octets are left to be send over the TBF (see the
downgrade-threshold command below) and the NACK rate is not low (the
CS will not get degraded on a high quality RF link). As an exception,
CS-3 will be degraded to CS-1, since CS-2 does not improve the
throughput in general when a few small packets are sent and the
signal fades slowly (see Chen/Goodman).

The following VTY command is added to the config-pcu node:

- cs downgrade-threshold <1-10000>
- cs no downgrade-threshold

to set the threshold of the number of remaining bytes to be RLC/MAC
encoded. The CS will only be reduced, if the number is below the
threshold. The 'no' command disables this feature completely. The
default value is 200 octets.

Sponsored-by: On-Waves ehf

1 files changed, 33 insertions, 0 deletions

diff --git a/src/gprs_ms.cpp b/src/gprs_ms.cpp
index b40e1eac..2cfedc3b 100644
--- a/src/gprs_ms.cpp
+++ b/src/gprs_ms.cpp
@@ -485,3 +485,36 @@ void GprsMs::update_l1_meas(const pcu_l1_meas *meas)
 		}
 	}
 }
+
+uint8_t GprsMs::current_cs_dl() const
+{
+	uint8_t cs = m_current_cs_dl;
+	size_t unencoded_octets;
+
+	if (!m_bts)
+		return cs;
+
+	unencoded_octets = m_llc_queue.octets();
+
+	/* If the DL TBF is active, add number of unencoded chunk octets */
+	if (m_dl_tbf)
+		unencoded_octets = m_dl_tbf->m_llc.chunk_size();
+
+	/* There are many unencoded octets, don't reduce */
+	if (unencoded_octets >= m_bts->bts_data()->cs_downgrade_threshold)
+		return cs;
+
+	/* RF conditions are good, don't reduce */
+	if (m_nack_rate_dl < m_bts->bts_data()->cs_adj_lower_limit)
+		return cs;
+
+	/* The throughput would probably be better if the CS level was reduced */
+	cs -= 1;
+
+	/* CS-2 doesn't gain throughput with small packets, further reduce to CS-1 */
+	if (cs == 2)
+		cs -= 1;
+
+	return cs;
+}
+