Add unit tests for shannon_limit_snr()

tests/link_budgets/test_shannon_limit_snr.py

@@ -0,0 +1,21 @@
+import numpy as np
+import pytest
+
+import sdr
+
+
+def test_standard_limits():
+    assert sdr.shannon_limit_snr(0) == -np.inf
+    assert sdr.shannon_limit_snr(1) == 0
+    assert sdr.shannon_limit_snr(2) == pytest.approx(4.771212547196624)
+
+
+def test_vector():
+    """
+    These numbers were verified visually from published papers.
+    """
+    rate = np.array([1 / 8, 1 / 4, 1 / 3, 1 / 2, 2 / 3, 3 / 4, 7 / 8])  # Code rate, n/k
+    rho = 2 * rate  # bits/2D
+    ebn0 = sdr.shannon_limit_snr(rho)
+    ebn0_truth = np.array([-7.23062536, -3.82775685, -2.3106528, 0.0, 1.8179847, 2.62077655, 3.73571347])
+    assert np.allclose(ebn0, ebn0_truth)