Transceiver52M: Set sample rate from within the radio device

The GSM transceiver only operates at a whole number multiple of
the GSM rate and doesn't care about the actual device rate and
if resampling is used. Therefore GSM specific portion of the
transceiver should only need to submit the samples-per-symbol
value to the device interface.

Then, the device should be able to determine the appropriate
sample rate (400 ksps or 270.833 ksps) and if resampling is
appropriate.

Signed-off-by: Thomas Tsou <tom@tsou.cc>

1 files changed, 32 insertions, 7 deletions

diff --git a/Transceiver52M/UHDDevice.cpp b/Transceiver52M/UHDDevice.cpp
index a6fc12d..8ef03bc 100644
--- a/Transceiver52M/UHDDevice.cpp
+++ b/Transceiver52M/UHDDevice.cpp
@@ -35,6 +35,8 @@
 #define NUM_TX_CHANS      2
 #define TX_CHAN_OFFSET    2e6
 #define B100_CLK_RT       52e6 
+#define B100_BASE_RT      GSMRATE
+#define USRP2_BASE_RT     400e3
 #define TX_AMPL           0.3;
 #define SAMPLE_BUF_SZ     (1 << 20)
 
@@ -51,6 +53,8 @@ struct uhd_dev_offset {
 	double offset;
 };
 
+static TIMESTAMP init_rd_ts = 0;
+
 /*
  * Tx / Rx sample offset values. In a perfect world, there is no group delay
  * though analog components, and behaviour through digital filters exactly
@@ -93,7 +97,28 @@ static double get_dev_offset(enum uhd_dev_type type, int sps)
 	return 0.0;	
 }
 
-static TIMESTAMP init_rd_ts = 0;
+/*
+ * Select sample rate based on device type and requested samples-per-symbol.
+ * The base rate is either GSM symbol rate, 270.833 kHz, or the minimum
+ * usable channel spacing of 400 kHz.
+ */ 
+static double select_rate(uhd_dev_type type, int sps)
+{
+	if ((sps != 4) && (sps != 2) && (sps != 1))
+		return -9999.99;
+
+	switch (type) {
+	case USRP2:
+		return USRP2_BASE_RT * sps;
+		break;
+	case B100:
+		return B100_BASE_RT * sps;
+		break;
+	}
+
+	LOG(ALERT) << "Unknown device type " << type;
+	return -9999.99;
+}
 
 /** Timestamp conversion
     @param timestamp a UHD or OpenBTS timestamp
@@ -185,7 +210,7 @@ private:
 */
 class uhd_device : public RadioDevice {
 public:
-	uhd_device(double rate, int sps, bool skip_rx);
+	uhd_device(int sps, bool skip_rx);
 	~uhd_device();
 
 	bool open(const std::string &args);
@@ -314,9 +339,8 @@ void uhd_msg_handler(uhd::msg::type_t type, const std::string &msg)
 	}
 }
 
-uhd_device::uhd_device(double rate, int sps, bool skip_rx)
-	: desired_smpl_rt(rate), actual_smpl_rt(0),
-	  tx_gain(0.0), tx_gain_min(0.0), tx_gain_max(0.0),
+uhd_device::uhd_device(int sps, bool skip_rx)
+	: tx_gain(0.0), tx_gain_min(0.0), tx_gain_max(0.0),
 	  rx_gain(0.0), rx_gain_min(0.0), rx_gain_max(0.0),
 	  tx_freq(0.0), rx_freq(0.0), tx_spp(0), rx_spp(0),
 	  started(false), aligned(false), rx_pkt_cnt(0), drop_cnt(0),
@@ -536,6 +560,7 @@ bool uhd_device::open(const std::string &args)
 	rx_spp = rx_stream->get_max_num_samps();
 
 	// Set rates
+	desired_smpl_rt = select_rate(dev_type, sps);
 	if (set_rates(desired_smpl_rt) < 0)
 		return false;
 
@@ -1077,7 +1102,7 @@ std::string smpl_buf::str_code(ssize_t code)
 	}
 }
 
-RadioDevice *RadioDevice::make(double smpl_rt, int sps, bool skip_rx)
+RadioDevice *RadioDevice::make(int sps, bool skip_rx)
 {
-	return new uhd_device(smpl_rt, sps, skip_rx);
+	return new uhd_device(sps, skip_rx);
 }