apps/grgsm_trx: add baseband frequency shift feature

An ability to shift the baseband frequency would allow one to
run both base stations and (OsmocomBB-based) mobile stations on
any frequency (e.g. in 2.4 GHz WiFi band)!

This is achieved by adding a given frequency offset to the
result of "ARFCN -> RX/TX frequency" calculation.

Usage example:

  grgsm_trx --freq-offset -500M

Both RX (Downlink) and TX (Uplink) frequencies will be shifted
by 500 MHz back, e.g. tuning request to ARFCN 105 would result
in tuning the radio to 456.0 MHz (instead of 956.0 MHz).

Related: OS#3520 (https://osmocom.org/versions/136)
Change-Id: I42e397e47402a87f4141ef31b25eff4c8c1267e2

apps/grgsm_trx

@@ -25,6 +25,8 @@ import signal
 import getopt
 import sys
 
+from gnuradio import eng_notation
+
 from grgsm.trx import ctrl_if_bb
 from grgsm.trx import radio_if
 from grgsm.trx import fake_pm
@@ -45,6 +47,7 @@ class Application:
 
 	# PHY specific
 	phy_sample_rate = 4 * 1625000 / 6
+	phy_freq_offset_hz = None
 	phy_tx_antenna = "TX/RX"
 	phy_rx_antenna = "RX2"
 	phy_rx_gain = 30
@@ -67,6 +70,10 @@ class Application:
 			self.bind_addr, self.remote_addr,
 			self.base_port)
 
+		# Optional frequency offset
+		if self.phy_freq_offset_hz is not None:
+			self.radio.freq_offset_hz = self.phy_freq_offset_hz
+
 		# Power measurement emulation
 		# Noise: -120 .. -105
 		# BTS: -75 .. -50
@@ -110,6 +117,7 @@ class Application:
 			 "  -G --tx-gain      Set TX gain (default %d)\n" \
 			 "     --rx-antenna   Set RX antenna (default %s)\n" \
 			 "     --tx-antenna   Set TX antenna (default %s)\n" \
+			 "     --freq-offset  Shift baseband freq. (e.g. -500M)\n" \
 			 "     --ppm          Set frequency correction (default %d)\n"
 
 		print(s % (
@@ -128,7 +136,7 @@ class Application:
 				"i:b:p:a:s:g:G:h",
 				["help", "remote-addr=", "bind-addr=", "base-port=",
 				"device-args=", "sample-rate=", "rx-gain=", "tx-gain=",
-				"ppm=", "rx-antenna=", "tx-antenna="])
+				"ppm=", "rx-antenna=", "tx-antenna=", "freq-offset="])
 		except getopt.GetoptError as err:
 			# Print(help and exit)
 			self.print_help()
@@ -167,6 +175,10 @@ class Application:
 				self.phy_tx_antenna = v
 			elif o in ("--ppm"):
 				self.phy_ppm = int(v)
+			elif o in ("--freq-offset"):
+				# Convert /\d+(\.\d+)?(M|k)?/ to Hz
+				offset_hz = eng_notation.str_to_num(v)
+				self.phy_freq_offset_hz = offset_hz
 
 	def sig_handler(self, signum, frame):
 		print("Signal %d received" % signum)

python/trx/radio_if.py

@@ -4,7 +4,7 @@
 # GR-GSM based transceiver
 # Follow graph implementation
 #
-# (C) 2016-2017 by Vadim Yanitskiy <[email protected]>
+# (C) 2016-2018 by Vadim Yanitskiy <[email protected]>
 # (C) 2017      by Piotr Krysik <[email protected]>
 #
 # All Rights Reserved
@@ -29,6 +29,7 @@
 
 from math import pi
 
+from gnuradio import eng_notation
 from gnuradio import digital
 from gnuradio import blocks
 from gnuradio import uhd
@@ -59,6 +60,7 @@ def change_sign(self, msg):
 
 class radio_if(gr.top_block):
 	# PHY specific variables
+	freq_offset_hz = None
 	rx_freq = 935e6
 	tx_freq = 890e6
 	osr = 4
@@ -274,11 +276,21 @@ def calc_phase_inc(self, fc):
 		return self.ppm / 1.0e6 * 2 * pi * fc / self.sample_rate
 
 	def set_rx_freq(self, fc):
+		if self.freq_offset_hz is not None:
+			fc += self.freq_offset_hz
+			print("[#] Shifting RX freq. to %s (offset is %s)"
+				% (eng_notation.num_to_str(fc),
+					eng_notation.num_to_str(self.freq_offset_hz)))
 		self.phy_src.set_center_freq(fc, 0)
 		self.rotator_src.set_phase_inc(self.calc_phase_inc(fc))
 		self.rx_freq = fc
 
 	def set_tx_freq(self, fc):
+		if self.freq_offset_hz is not None:
+			fc += self.freq_offset_hz
+			print("[#] Shifting TX freq. to %s (offset is %s)"
+				% (eng_notation.num_to_str(fc),
+					eng_notation.num_to_str(self.freq_offset_hz)))
 		self.phy_sink.set_center_freq(fc, 0)
 		self.rotator_sink.set_phase_inc(-self.calc_phase_inc(fc))
 		self.tx_freq = fc