Fast fading and effective SINR¶
By default HandoverLab works on the received-power level: path loss, LOS state, shadowing and a
slow time-varying component, with radio-link monitoring on the wideband SINR. Since 2.2.0 the option
radio.fast_fading adds the fast fading of the tapped delay line (TDL) models of TR 38.901 and maps
the SINR at several frequencies to one effective SINR. It is off by default; with it off every
result of 2.1.0 is unchanged bit for bit.
scenario:
base: ns3-urban-3.5ghz
radio:
fast_fading: tdl-c # tdl-a, tdl-b, tdl-c (NLOS), tdl-d, tdl-e (LOS); null: off
delay_spread_ns: 100 # RMS delay spread (TR 38.901 Table 7.7.3-1: 10, 30, 100, 300, 1000 ns)
fading_k_db: null # tdl-d / tdl-e only: another K-factor (TR 38.901 clause 7.7.6)1. The channel¶
| Item | TR 38.901 V19.5.0 | HandoverLab |
|---|---|---|
| Tap delays and powers | Tables 7.7.2-1 to 7.7.2-5 | the five tables, the powers normalised to a unit sum (mean link gain 1) |
| Delay scaling | Eq. 7.7-1: tau_scaled = tau_model x DS | delay_spread_ns |
| Doppler | clause 7.7.2: Jakes spectrum per tap, f_D = |v| / lambda_0 | 32 sinusoids per Rayleigh tap with random arrival angles and phases, f_D from the user's speed and the carrier |
| LOS tap of TDL-D / TDL-E | Rice, K1 = 13.3 dB / 22 dB, Doppler peak at f_S = 0.7 f_D | a specular component at 0.7 f_D next to the Rayleigh part of the first tap |
| Another K-factor | clause 7.7.6, Eq. 7.7.6-1 and 7.7.6-2, delays re-normalised | fading_k_db (the K of Eq. 7.7.6-2: the LOS power over all Rayleigh taps; about 9 dB for the table of TDL-D) |
Every user-cell link fades independently (the TDL models are single-link models; their MIMO extensions of clause 7.7.5 are not used). The channel is evaluated at four points spread over the 24 PRBs of the hypothetical PDCCH (TS 38.133 Tables 8.1.2.1-1/-2) and at eight points spread over the carrier (N_RB of TS 38.101-1/-2 Table 5.3.2-1). The received power of a cell (L1 samples, the link budget of every check) takes the mean gain over the PDCCH band, close to the 20 PRBs of an SSB.
2. From SINR to one effective SINR¶
| Use | Mapping | Why |
|---|---|---|
| Radio-link monitoring (Qout, Qin) | MIESM for QPSK over the PDCCH band: the mean bit-wise mutual information of QPSK (binary input, Gaussian channel) mapped back to the SINR with the same mutual information on a flat channel | TS 38.133 8.1 defines Qout and Qin as the BLER of a hypothetical QPSK PDCCH; the thresholds qout_db / qin_db are flat-channel SINRs, so the faded channel is compared through its flat-channel equivalent |
| Delivered rate | capacity-equivalent SINR over the carrier: 2^(mean log2(1 + SINR)) - 1 | the rate model uses log2(1 + SINR) |
Both mappings return the SINR itself on a flat channel and never more than the mean SINR. Every interfering link has its own fading. Above about 18 dB the mutual information of QPSK is 1 to double precision; the MIESM result is capped there, far above Qout and Qin. No BLER table of another simulator is used; a BLER per MCS (TS 38.214 tables) follows with the slot scheduler.
3. Cost and limits¶
- A single run is about twice as long (32 sinusoids per tap, every step; 10 users on the ns-3 card: about 6 s instead of 3 s for 20 s simulated); many rules on one world take longer (24 rules: about 6 to 8 times), as the batched and compiled loops are not used with fading.
- This version needs the base engine: no beams, layers, NTN or TS 38.133 periods
(
meas_periods: sample,rlm_eval: fixed); such a configuration is refused. - The step stays 10 ms. At 30 m/s and 3.5 GHz the coherence time is about 1 ms, so successive steps see nearly independent fading; the L1 sample of a measurement instant is one such snapshot.
4. Against ns-3 (exploratory)¶
On the ns-3 card (24 A3 settings, four conditions, one seed) against the stored ns-3 results, which were run
without fading: with tdl-c the median handover ratio is 1.01 and the unnecessary-handover ratio 1.18 (0.95 and
1.06 without fading), the rank correlations 0.98; the failure rate stays at about 0.01-0.02 per UE-minute
against 0.43 in ns-3. Fast fading therefore does not close the known gap in failures, which follows the 14 dB gap
in mean serving SINR (see verification).
Against ns-3 runs that use fading themselves (benchmarks/ns3/run_benchmark.py --fading, 96 runs, TR 38.901
fast fading in ns-3), with the criterion fixed before those runs existed (tdl-c, delay spread 100 ns, seed
500; median ratios in [0.8, 1.25], rank correlations at least 0.9): the handover ratio is 1.04 and the
unnecessary-handover ratio 1.24, close to its limit, and the rank correlations are 0.97 and 0.98, so the
criterion passes. As additional rows, tdl-a would miss the unnecessary-handover limit (1.27), tdl-d (1.00 /
1.16) and no fading (0.98 / 1.09) pass. ns-3 itself moves little with fading (mean failure rate 0.40 against
0.43 without, serving SINR 11.5 dB in both), and the failure gap stays as it was (HandoverLab 0.014 per
UE-minute with tdl-c).
5. Checks¶
handoverlab verify --only fading runs the criteria fixed before the model was written: Rayleigh
taps with unit power (KS test), the J0 time correlation, the K-factor of the first TDL-D tap, the
delay spread after scaling, and the two properties of the mappings. tests/test_fading.py also
checks that the option off changes no fingerprint and that a run with fading is reproducible.