Transceiver52M: Add dual channel diversity receiver option

This patch add support for dual channel diversity on the receive
path. This allows two antennas two shared antennas to be used for
each ARFCN handling channel in the receiver. This configuration
may improvde performance in multi-path fading environments,
however, noise andpotential interference levels are increased due
to the higher bandwidth used.

The receive path is oversampled by a factor of four for a rate
of 1.083333 Msps. If the receive paths are tuned within a
maximum channel spacing (currently set at 600 kHz), then both
ARFCN frequencies are processed by each channel of the receiver.
Otherwise, the frequency shifted diversity path is disabled and
standard non-diversity operation takes place.

Diversity processing is handled by selecting the path with the
higheset energy level and discarding the burst on the second
path. Selection occurs on a burst-by-burst basis.

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

1 files changed, 56 insertions, 8 deletions

diff --git a/Transceiver52M/UHDDevice.cpp b/Transceiver52M/UHDDevice.cpp
index 681f939..915cf32 100644
--- a/Transceiver52M/UHDDevice.cpp
+++ b/Transceiver52M/UHDDevice.cpp
@@ -78,17 +78,46 @@ static struct uhd_dev_offset uhd_offsets[NUM_USRP_TYPES * 2] = {
 	{ UMTRX, 4, 7.3846e-5, "UmTRX 4 SPS" },
 };
 
-static double get_dev_offset(enum uhd_dev_type type, int sps)
+/*
+ * Offset handling for special cases. Currently used for UmTRX dual channel
+ * diversity receiver only.
+ */
+static struct uhd_dev_offset special_offsets[] = {
+	{ UMTRX, 1, 8.0875e-5, "UmTRX diversity, 1 SPS" },
+	{ UMTRX, 4, 5.2103e-5, "UmTRX diversity, 4 SPS" },
+};
+
+static double get_dev_offset(enum uhd_dev_type type,
+			     int sps, bool diversity = false)
 {
+	/* Reject USRP1 */
 	if (type == USRP1) {
 		LOG(ERR) << "Invalid device type";
 		return 0.0;
 	}
 
+	/* Special cases (e.g. diversity receiver) */
+	if (diversity) {
+		if (type != UMTRX) {
+			LOG(ALERT) << "Diversity on UmTRX only";
+			return 0.0;
+		}
+
+		switch (sps) {
+		case 1:
+			return special_offsets[0].offset;
+		case 4:
+		default:
+			return special_offsets[1].offset;
+		}
+	}
+
+	/* Normal operation */
 	switch (sps) {
 	case 1:
 		return uhd_offsets[2 * type + 0].offset;
 	case 4:
+	default:
 		return uhd_offsets[2 * type + 1].offset;
 	}
 
@@ -101,8 +130,15 @@ static double get_dev_offset(enum uhd_dev_type type, int sps)
  * 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)
+static double select_rate(uhd_dev_type type, int sps, bool diversity = false)
 {
+	if (diversity && (type == UMTRX)) {
+		return GSMRATE * 4;
+	} else if (diversity) {
+		LOG(ALERT) << "Diversity supported on UmTRX only";
+		return -9999.99;
+	}
+
 	if ((sps != 4) && (sps != 1))
 		return -9999.99;
 
@@ -212,7 +248,7 @@ private:
 */
 class uhd_device : public RadioDevice {
 public:
-	uhd_device(size_t sps, size_t chans);
+	uhd_device(size_t sps, size_t chans, bool diversity);
 	~uhd_device();
 
 	int open(const std::string &args, bool extref);
@@ -305,6 +341,7 @@ private:
 	std::string str_code(uhd::async_metadata_t metadata);
 
 	Thread async_event_thrd;
+	bool diversity;
 };
 
 void *async_event_loop(uhd_device *dev)
@@ -339,7 +376,7 @@ void uhd_msg_handler(uhd::msg::type_t type, const std::string &msg)
 	}
 }
 
-uhd_device::uhd_device(size_t sps, size_t chans)
+uhd_device::uhd_device(size_t sps, size_t chans, bool diversity)
 	: tx_gain_min(0.0), tx_gain_max(0.0),
 	  rx_gain_min(0.0), rx_gain_max(0.0),
 	  tx_spp(0), rx_spp(0),
@@ -348,6 +385,7 @@ uhd_device::uhd_device(size_t sps, size_t chans)
 {
 	this->sps = sps;
 	this->chans = chans;
+	this->diversity = diversity;
 }
 
 uhd_device::~uhd_device()
@@ -600,8 +638,13 @@ int uhd_device::open(const std::string &args, bool extref)
 	rx_spp = rx_stream->get_max_num_samps();
 
 	// Set rates
+	double _rx_rate;
 	double _tx_rate = select_rate(dev_type, sps);
-	double _rx_rate = _tx_rate / sps;
+	if (diversity)
+		_rx_rate = select_rate(dev_type, 1, true);
+	else
+		_rx_rate = _tx_rate / sps;
+
 	if ((_tx_rate > 0.0) && (set_rates(_tx_rate, _rx_rate) < 0))
 		return -1;
 
@@ -611,7 +654,7 @@ int uhd_device::open(const std::string &args, bool extref)
 		rx_buffers[i] = new smpl_buf(buf_len, rx_rate);
 
 	// Set receive chain sample offset 
-	double offset = get_dev_offset(dev_type, sps);
+	double offset = get_dev_offset(dev_type, sps, diversity);
 	if (offset == 0.0) {
 		LOG(ERR) << "Unsupported configuration, no correction applied";
 		ts_offset = 0;
@@ -625,11 +668,16 @@ int uhd_device::open(const std::string &args, bool extref)
 	// Print configuration
 	LOG(INFO) << "\n" << usrp_dev->get_pp_string();
 
+	if (diversity)
+		return DIVERSITY;
+
 	switch (dev_type) {
 	case B100:
 		return RESAMP_64M;
 	case USRP2:
 		return RESAMP_100M;
+	default:
+		break;
 	}
 
 	return NORMAL;
@@ -1173,7 +1221,7 @@ std::string smpl_buf::str_code(ssize_t code)
 	}
 }
 
-RadioDevice *RadioDevice::make(size_t sps, size_t chans)
+RadioDevice *RadioDevice::make(size_t sps, size_t chans, bool diversity)
 {
-	return new uhd_device(sps, chans);
+	return new uhd_device(sps, chans, diversity);
 }