[handover] Real handover algorithm
This implements the handover algorithm (and associated parameters)
as described in Chapter 8 of the book "Performance Enhancements in
a Frequency |Hopping GSM Network" by Thomas Toftegard Nielsen and Jeroen
Wigard.
The parameters such as averaging windows are configured in struct
gsm_network. We keep some state to trakc up to 10 neighbors as
they are being reported from the MS.
This has so far only been tested in a network with two BTS that
have each other as neighbor. Networks with morge neighbors might
encounter bugs.
diff --git a/openbsc/src/handover_decision.c b/openbsc/src/handover_decision.c
index 736679a..b37cecd 100644
--- a/openbsc/src/handover_decision.c
+++ b/openbsc/src/handover_decision.c
@@ -32,7 +32,9 @@
#include <openbsc/signal.h>
#include <openbsc/talloc.h>
#include <openbsc/handover.h>
+#include <openbsc/gsm_utils.h>
+/* issue handover to a cell identified by ARFCN and BSIC */
static int handover_to_arfcn_bsic(struct gsm_lchan *lchan,
u_int16_t arfcn, u_int8_t bsic)
{
@@ -50,14 +52,184 @@
return bsc_handover_start(lchan, new_bts);
}
-#define RXLEV_HYST 3
+/* did we get a RXLEV for a given cell in the given report? */
+static int rxlev_for_cell_in_rep(struct gsm_meas_rep *mr,
+ u_int16_t arfcn, u_int8_t bsic)
+{
+ int i;
-/* process an already parsed measurement report */
+ for (i = 0; i < mr->num_cell; i++) {
+ struct gsm_meas_rep_cell *mrc = &mr->cell[i];
+
+ /* search for matching report */
+ if (!(mrc->arfcn == arfcn && mrc->bsic == bsic))
+ continue;
+
+ mrc->flags |= MRC_F_PROCESSED;
+ return mrc->rxlev;
+ }
+ return -ENODEV;
+}
+
+/* obtain averaged rxlev for given neighbor */
+static int neigh_meas_avg(struct neigh_meas_proc *nmp, int window)
+{
+ unsigned int i, idx;
+ int avg = 0;
+
+ idx = calc_initial_idx(ARRAY_SIZE(nmp->rxlev),
+ nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev),
+ window);
+
+ for (i = 0; i < window; i++) {
+ int j = (idx+i) % ARRAY_SIZE(nmp->rxlev);
+
+ avg += nmp->rxlev[j];
+ }
+
+ return avg / window;
+}
+
+/* find empty or evict bad neighbor */
+static struct neigh_meas_proc *find_evict_neigh(struct gsm_lchan *lchan)
+{
+ int j, worst = 999999;
+ struct neigh_meas_proc *nmp_worst;
+
+ /* first try to find an empty/unused slot */
+ for (j = 0; j < ARRAY_SIZE(lchan->neigh_meas); j++) {
+ struct neigh_meas_proc *nmp = &lchan->neigh_meas[j];
+ if (!nmp->arfcn)
+ return nmp;
+ }
+
+ /* no empty slot found. evict worst neighbor from list */
+ for (j = 0; j < ARRAY_SIZE(lchan->neigh_meas); j++) {
+ struct neigh_meas_proc *nmp = &lchan->neigh_meas[j];
+ int avg = neigh_meas_avg(nmp, MAX_WIN_NEIGH_AVG);
+ if (avg < worst) {
+ worst = avg;
+ nmp_worst = nmp;
+ }
+ }
+
+ return nmp_worst;
+}
+
+/* process neighbor cell measurement reports */
+static void process_meas_neigh(struct gsm_meas_rep *mr)
+{
+ int i, j, idx;
+
+ /* for each reported cell, try to update global state */
+ for (j = 0; j < ARRAY_SIZE(mr->lchan->neigh_meas); j++) {
+ struct neigh_meas_proc *nmp = &mr->lchan->neigh_meas[j];
+ unsigned int idx;
+ int rxlev;
+
+ /* skip unused entries */
+ if (!nmp->arfcn)
+ continue;
+
+ rxlev = rxlev_for_cell_in_rep(mr, nmp->arfcn, nmp->bsic);
+ idx = nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev);
+ if (rxlev >= 0) {
+ nmp->rxlev[idx] = rxlev;
+ nmp->last_seen_nr = mr->nr;
+ } else
+ nmp->rxlev[idx] = 0;
+ nmp->rxlev_cnt++;
+ }
+
+ /* iterate over list of reported cells, check if we did not
+ * process all of them */
+ for (i = 0; i < mr->num_cell; i++) {
+ struct gsm_meas_rep_cell *mrc = &mr->cell[i];
+ struct neigh_meas_proc *nmp;
+
+ if (mrc->flags & MRC_F_PROCESSED)
+ continue;
+
+ nmp = find_evict_neigh(mr->lchan);
+
+ nmp->arfcn = mrc->arfcn;
+ nmp->bsic = mrc->bsic;
+
+ idx = nmp->rxlev_cnt % ARRAY_SIZE(nmp->rxlev);
+ nmp->rxlev[idx] = mrc->rxlev;
+ nmp->rxlev_cnt++;
+ nmp->last_seen_nr = mr->nr;
+
+ mrc->flags |= MRC_F_PROCESSED;
+ }
+}
+
+/* attempt to do a handover */
+static int attempt_handover(struct gsm_meas_rep *mr)
+{
+ struct gsm_network *net = mr->lchan->ts->trx->bts->network;
+ struct neigh_meas_proc *best_cell = NULL;
+ unsigned int best_better_db = 0;
+ int i, rc;
+
+ /* find the best cell in this report that is at least RXLEV_HYST
+ * better than the current serving cell */
+
+ for (i = 0; i < ARRAY_SIZE(mr->lchan->neigh_meas); i++) {
+ struct neigh_meas_proc *nmp = &mr->lchan->neigh_meas[i];
+ int avg, better;
+
+ /* skip empty slots */
+ if (nmp->arfcn == 0)
+ continue;
+
+ /* caculate average rxlev for this cell over the window */
+ avg = neigh_meas_avg(nmp, net->handover.win_rxlev_avg_neigh);
+
+ /* check if hysteresis is fulfilled */
+ if (avg < mr->dl.full.rx_lev + net->handover.pwr_hysteresis)
+ continue;
+
+ better = avg - mr->dl.full.rx_lev;
+ if (better > best_better_db) {
+ best_cell = nmp;
+ best_better_db = better;
+ }
+ }
+
+ if (!best_cell)
+ return 0;
+
+ LOGP(DHO, LOGL_INFO, "%s: Cell on ARFCN %u is better: ",
+ gsm_ts_name(mr->lchan->ts), best_cell->arfcn);
+ if (!net->handover.active) {
+ LOGPC(DHO, LOGL_INFO, "Skipping, Handover disabled\n");
+ return 0;
+ }
+
+ rc = handover_to_arfcn_bsic(mr->lchan, best_cell->arfcn, best_cell->bsic);
+ switch (rc) {
+ case 0:
+ LOGPC(DHO, LOGL_INFO, "Starting handover\n");
+ break;
+ case -ENOSPC:
+ LOGPC(DHO, LOGL_INFO, "No channel available\n");
+ break;
+ case -EBUSY:
+ LOGPC(DHO, LOGL_INFO, "Handover already active\n");
+ break;
+ default:
+ LOGPC(DHO, LOGL_ERROR, "Unknown error\n");
+ }
+ return rc;
+}
+
+/* process an already parsed measurement report and decide if we want to
+ * attempt a handover */
static int process_meas_rep(struct gsm_meas_rep *mr)
{
- struct gsm_meas_rep_cell *mr_cell = NULL;
- unsigned int best_better_db;
- int i;
+ struct gsm_network *net = mr->lchan->ts->trx->bts->network;
+ int av_rxlev;
/* we currently only do handover for TCH channels */
switch (mr->lchan->type) {
@@ -68,37 +240,38 @@
return 0;
}
- /* FIXME: implement actual averaging over multiple measurement
- * reports */
+ /* parse actual neighbor cell info */
+ if (mr->num_cell > 0 && mr->num_cell < 7)
+ process_meas_neigh(mr);
- if (mr->num_cell > 6)
- return 0;
+ av_rxlev = get_meas_rep_avg(mr->lchan, MEAS_REP_DL_RXLEV_FULL,
+ net->handover.win_rxlev_avg);
- /* find the best cell in this report that is at least RXLEV_HYST
- * better than the current serving cell */
- for (i = 0; i < mr->num_cell; i++) {
- unsigned int better;
- if (mr->cell[i].rxlev < mr->dl.full.rx_lev + RXLEV_HYST)
- continue;
+ /* Interference HO */
+ if (rxlev2dbm(av_rxlev) > -85 &&
+ meas_rep_n_out_of_m_be(mr->lchan, MEAS_REP_DL_RXQUAL_FULL,
+ 3, 4, 5))
+ return attempt_handover(mr);
- better = mr->cell[i].rxlev - mr->dl.full.rx_lev;
- if (better > best_better_db) {
- mr_cell = &mr->cell[i];
- best_better_db = better;
- }
- }
+ /* Bad Quality */
+ if (meas_rep_n_out_of_m_be(mr->lchan, MEAS_REP_DL_RXQUAL_FULL,
+ 3, 4, 5))
+ return attempt_handover(mr);
- if (!mr_cell)
- return 0;
+ /* Low Level */
+ if (rxlev2dbm(av_rxlev) <= -110)
+ return attempt_handover(mr);
- LOGP(DHO, LOGL_INFO, "Cell on ARFCN %u is better: ", mr_cell->arfcn);
- if (!mr->lchan->ts->trx->bts->network->handover.active) {
- LOGPC(DHO, LOGL_INFO, "Skipping, Handover disabled\n");
- return 0;
- }
+ /* Distance */
+ if (mr->ms_l1.ta > net->handover.max_distance)
+ return attempt_handover(mr);
- LOGPC(DHO, LOGL_INFO, "Starting handover\n");
- return handover_to_arfcn_bsic(mr->lchan, mr_cell->arfcn, mr_cell->bsic);
+ /* Power Budget AKA Better Cell */
+ if ((mr->nr % net->handover.pwr_interval) == 0)
+ return attempt_handover(mr);
+
+ return 0;
+
}
static int ho_dec_sig_cb(unsigned int subsys, unsigned int signal,