channel_simulator: ability to connect a real USRP device to the simulator

This commit introduces the ability to connect a real hardware device to
the channel simulator.

It is possible to run an eNB with the libchannel_simulator.so as
liboai_device.so and the "usrp" program and have both connected
as if the eNB was directly talking to the USRP device.

For that, a channel can now have a "sample_advance". (The USRP receives
data from the air for subframe N and sends data for subframe N+x in the
future. In this commit x = 2 subframes.)

The following commands can be run to connect an eNB and the "usrp"
program in FDD mode with 25 RBs.

First, run the the channel simulator. In the directory channel_simulator:

    ./channel_simulator

To run usrp, as root, in the directory channel_simulator/usrp:

    ./usrp

To run the eNB, as root:

    CHANNEL_SIMULATOR_RX_FREQUENCY=2 CHANNEL_SIMULATOR_TX_FREQUENCY=1 CHANNEL_SIMULATOR_TX_SAMPLE_ADVANCE=15360 ./lte-softmodem -O [config file]

Then it is possible to connect a COTS UE.

It is also possible to connect openair UE which uses libchannel_simulator.so
as liboai_device.so. For that, run, as root:

    CHANNEL_SIMULATOR_RX_FREQUENCY=1 CHANNEL_SIMULATOR_TX_FREQUENCY=2 CHANNEL_SIMULATOR_RX_SAMPLE_ADVANCE=15360 ./lte-uesoftmodem -C 2680000000 -r 25 --ue-rxgain 140 --nokrnmod

Connecting both the openair UE and a COTS UE, it is possible to send traffic
between the two.

Note: performances need to be improved.

Note 2: the program "channel_simulator" now requires AVX2 instruction set.

channel_simulator/channel_simulator.c

@@ -73,12 +73,19 @@ oom:
   exit(1);
 }
 
-static int find_or_create_channel(channel_simulator *c, uint64_t freq)
+static int find_or_create_channel(channel_simulator *c, uint64_t freq,
+                                  uint32_t sample_advance)
 {
   channel *chan;
   int i;
-  for (i = 0; i < c->channels_count; i++)
-    if ( c->channels[i].frequency == freq) return i;
+  for (i = 0; i < c->channels_count; i++) {
+    if (c->channels[i].frequency != freq) continue;
+    if (c->channels[i].sample_advance != sample_advance) {
+      printf("ERROR: bad sample_advance\n");
+      exit(1);
+    }
+    return i;
+  }
 
   c->channels_count++;
   c->channels = realloc(c->channels, c->channels_count * sizeof(channel));
@@ -86,6 +93,7 @@ static int find_or_create_channel(channel_simulator *c, uint64_t freq)
 
   chan = &c->channels[i];
   chan->frequency = freq;
+  chan->sample_advance = sample_advance;
   chan->data = calloc(1, c->n_samples * 4);
   if (chan->data == NULL) goto oom;
   chan->connection_count = 0;
@@ -98,7 +106,8 @@ oom:
 }
 
 void channel_simulator_add_connection(channel_simulator *c,
-            int socket, uint64_t rx_frequency, uint64_t tx_frequency)
+            int socket, uint64_t rx_frequency, uint64_t tx_frequency,
+            uint32_t rx_sample_advance, uint32_t tx_sample_advance)
 {
   connection *con;
 
@@ -116,8 +125,10 @@ void channel_simulator_add_connection(channel_simulator *c,
   if (con->tx_buffer == NULL) goto oom;
   con->rx_frequency = rx_frequency;
   con->tx_frequency = tx_frequency;
-  con->rx_channel_index = find_or_create_channel(c, rx_frequency);
-  con->tx_channel_index = find_or_create_channel(c, tx_frequency);
+  con->rx_channel_index = find_or_create_channel(c, rx_frequency,
+                                                 rx_sample_advance);
+  con->tx_channel_index = find_or_create_channel(c, tx_frequency,
+                                                 tx_sample_advance);
 
   c->channels[con->rx_channel_index].connection_count++;
   c->channels[con->tx_channel_index].connection_count++;
@@ -152,6 +163,8 @@ void command_set_frequency(channel_simulator *cs, connection *con, int n)
   unsigned char b[8*2];
   uint64_t rx_frequency;
   uint64_t tx_frequency;
+  uint32_t rx_sample_advance;
+  uint32_t tx_sample_advance;
 
   if (n != 8*2) goto err;
   if (fullread(con->socket, b, 8*2) != 8*2) goto err;
@@ -164,10 +177,15 @@ void command_set_frequency(channel_simulator *cs, connection *con, int n)
   cs->channels[con->rx_channel_index].connection_count--;
   cs->channels[con->tx_channel_index].connection_count--;
 
+  rx_sample_advance = cs->channels[con->rx_channel_index].sample_advance;
+  tx_sample_advance = cs->channels[con->tx_channel_index].sample_advance;
+
   con->rx_frequency = rx_frequency;
   con->tx_frequency = tx_frequency;
-  con->rx_channel_index = find_or_create_channel(cs, rx_frequency);
-  con->tx_channel_index = find_or_create_channel(cs, tx_frequency);
+  con->rx_channel_index = find_or_create_channel(cs, rx_frequency,
+                                                 rx_sample_advance);
+  con->tx_channel_index = find_or_create_channel(cs, tx_frequency,
+                                                 tx_sample_advance);
   cs->channels[con->rx_channel_index].connection_count++;
   cs->channels[con->tx_channel_index].connection_count++;
 
@@ -233,6 +251,8 @@ err:
   c->socket = -1;
 }
 
+#include <immintrin.h>
+
 void channel_simulate(channel_simulator *c)
 {
   int i;
@@ -241,14 +261,15 @@ void channel_simulate(channel_simulator *c)
   channel *chan;
   int16_t *to;
   int16_t *from;
-  int32_t mix[c->channels_count][c->n_samples*2];
+  int16_t mix[c->channels_count][c->n_samples*2] __attribute__ ((aligned (32)));
 
-  memset(mix, 0, c->channels_count * c->n_samples*2 * 4);
+  memset(mix, 0, c->channels_count * c->n_samples*2 * 2);
 
   /* clear channels */
   for (i = 0; i < c->channels_count; i++)
     memset(c->channels[i].data, 0, c->n_samples * 4);
 
+#if 0
   /* basic mixing */
   for (i = 0; i < c->connections_count; i++) {
     con = &c->connections[i];
@@ -269,4 +290,24 @@ void channel_simulate(channel_simulator *c)
       to[k] = v;
     }
   }
+#else
+  /* basic mixing */
+  for (i = 0; i < c->connections_count; i++) {
+    con = &c->connections[i];
+    from = (int16_t *)con->tx_buffer;
+
+    for (k = 0; k < c->n_samples * 2; k+=16) {
+      __m256i *a, *b;
+      a = (__m256i *)&mix[con->tx_channel_index][k];
+      b = (__m256i *)&from[k];
+      *a = _mm256_adds_epi16(*a, *b);
+    }
+  }
+
+  for (i = 0; i < c->channels_count; i++) {
+    chan = &c->channels[i];
+    to = (int16_t *)chan->data;
+    memcpy(to, mix[i], c->n_samples * 2 * 2);
+  }
+#endif
 }

channel_simulator/channel_simulator.h

@@ -5,6 +5,7 @@
 
 typedef struct {
   uint64_t frequency;         /* unit: Hz */
+  uint32_t sample_advance;
   uint32_t *data;
   int      connection_count;
 } channel;
@@ -32,7 +33,8 @@ void init_channel_simulator(channel_simulator *c,
     uint32_t samplerate, int n_samples);
 void cleanup_connections(channel_simulator *c);
 void channel_simulator_add_connection(channel_simulator *c,
-    int socket, uint64_t rx_frequency, uint64_t tx_frequency);
+    int socket, uint64_t rx_frequency, uint64_t tx_frequency,
+    uint32_t rx_sample_advance, uint32_t tx_sample_advance);
 
 void connection_send_rx(connection *c, uint64_t timestamp,
     uint32_t *data, int n_samples);

channel_simulator/connection_manager.c

@@ -12,16 +12,20 @@
 typedef struct {
   uint64_t rx_frequency;
   uint64_t tx_frequency;
+  uint32_t rx_sample_advance;
+  uint32_t tx_sample_advance;
   uint32_t samplerate;
 } init_message;
 
 static int get_init_message(init_message *m, int s)
 {
-  unsigned char b[8*2+4];
-  if (fullread(s, b, 8*2+4) != 8*2+4) return -1;
-  m->rx_frequency = gu64(b);
-  m->tx_frequency = gu64(b+8);
-  m->samplerate   = gu32(b+8*2);
+  unsigned char b[8*2+4*3];
+  if (fullread(s, b, 8*2+4*3) != 8*2+4*3) return -1;
+  m->rx_frequency      = gu64(b);
+  m->tx_frequency      = gu64(b+8);
+  m->rx_sample_advance = gu32(b+8*2);
+  m->tx_sample_advance = gu32(b+8*2+4);
+  m->samplerate        = gu32(b+8*2+4*2);
   return 0;
 }
 
@@ -53,10 +57,12 @@ static void *connection_manager_thread(void *_cm)
       printf("ERROR: get_init_message: out of memory\n");
       exit(1);
     }
-    cm->c[cm->new_connections-1].socket       = t;
-    cm->c[cm->new_connections-1].rx_frequency = m.rx_frequency;
-    cm->c[cm->new_connections-1].tx_frequency = m.tx_frequency;
-    cm->c[cm->new_connections-1].samplerate   = m.samplerate;
+    cm->c[cm->new_connections-1].socket            = t;
+    cm->c[cm->new_connections-1].rx_frequency      = m.rx_frequency;
+    cm->c[cm->new_connections-1].tx_frequency      = m.tx_frequency;
+    cm->c[cm->new_connections-1].rx_sample_advance = m.rx_sample_advance;
+    cm->c[cm->new_connections-1].tx_sample_advance = m.tx_sample_advance;
+    cm->c[cm->new_connections-1].samplerate        = m.samplerate;
     lock_signal(&cm->l);
     connection_manager_unlock(cm);
   }

channel_simulator/connection_manager.h

@@ -8,6 +8,8 @@ typedef struct {
   int      socket;
   uint64_t rx_frequency;
   uint64_t tx_frequency;
+  uint32_t tx_sample_advance;
+  uint32_t rx_sample_advance;
   uint32_t samplerate;
 } new_connection;
 

channel_simulator/main.c

@@ -28,7 +28,8 @@ int main(void)
           continue;
         }
         channel_simulator_add_connection(&c,
-            cm.c[i].socket, cm.c[i].rx_frequency, cm.c[i].tx_frequency);
+            cm.c[i].socket, cm.c[i].rx_frequency, cm.c[i].tx_frequency,
+            cm.c[i].rx_sample_advance, cm.c[i].tx_sample_advance);
       }
       connection_manager_clear(&cm);
     }
@@ -36,9 +37,12 @@ int main(void)
 
     for (i = 0 ; i < c.connections_count; i++) {
       connection *con = &c.connections[i];
-      channel    *ch  = &c.channels[con->rx_channel_index];
-      connection_send_rx(con, c.timestamp, ch->data, c.n_samples);
-      connection_receive_tx(&c, con, c.timestamp + c.n_samples, c.n_samples);
+      channel    *ch_rx  = &c.channels[con->rx_channel_index];
+      channel    *ch_tx  = &c.channels[con->tx_channel_index];
+      connection_send_rx(con, c.timestamp + ch_rx->sample_advance,
+                         ch_rx->data, c.n_samples);
+      connection_receive_tx(&c, con, c.timestamp + ch_tx->sample_advance
+                                                 + c.n_samples, c.n_samples);
     }
 
     cleanup_connections(&c);
@@ -46,12 +50,14 @@ int main(void)
     c.timestamp += c.n_samples;
     channel_simulate(&c);
 
+#if 0
     static int processed = 0;
     processed += c.n_samples;
     if (processed >= samplerate/1000) {
       processed -= samplerate/1000;
       usleep(2000);
     }
+#endif
   }
 
   return 0;

channel_simulator/usrp/Makefile

+CC=gcc
+CPP=g++
+CFLAGS=-Wall -g -pthread
+
+PROG=usrp
+OBJS=main.o usrp.o ../utils.o
+
+$(PROG): $(OBJS)
+	$(CPP) $(CFLAGS) -o $@ $^ -luhd -lboost_system
+
+%.o: %.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+%.o: %.cc
+	$(CPP) $(CFLAGS) -c -o $@ $<
+
+clean:
+	rm -f $(PROG) *.o core

channel_simulator/usrp/main.c

+#include "usrp.h"
+#include "../utils.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <netinet/in.h>
+#include <netinet/tcp.h>
+#include <arpa/inet.h>
+#include <unistd.h>
+#include <string.h>
+#include <sys/types.h>
+#include <pthread.h>
+
+#include <inttypes.h>
+
+typedef struct {
+  char *data;
+  uint32_t n_samples;
+  uint64_t timestamp;
+} buffer_t;
+
+void receive_from_channel_simulator(int sock, buffer_t *buf)
+{
+  unsigned char b[8+4];
+  uint32_t n_samples;
+
+  if (fullread(sock, b, 8+4) != 8+4) goto err;
+
+  buf->timestamp = gu64(b);
+  n_samples = gu32(b+8);
+  if (n_samples != buf->n_samples) {
+    free(buf->data);
+    buf->n_samples = n_samples;
+    buf->data = malloc(buf->n_samples * 4); if (buf->data == NULL) goto err;
+  }
+  if (fullread(sock, buf->data, buf->n_samples * 4) != buf->n_samples * 4)
+    goto err;
+
+  return;
+
+err:
+  printf("ERROR: receive_from_channel_simulator failed\n");
+  exit(1);
+}
+
+void send_to_channel_simulator(int sock, char *usrp_data, int n_samples,
+                               uint64_t timestamp)
+{
+  unsigned char b[8+4];
+
+  pu64(b, timestamp);
+  pu32(b+8, n_samples);
+  if (fullwrite(sock, b, 8+4) != 8+4 ||
+      fullwrite(sock, usrp_data, n_samples * 4) != n_samples * 4) {
+    printf("ERROR: send_to_channel_simulator failed\n");
+    exit(1);
+  }
+}
+
+int connect_to_channel_simulator(void)
+{
+  int port = 4024;
+  char *ip = "127.0.0.1";
+  struct sockaddr_in addr;
+  int sock;
+  int v;
+
+  sock = socket(AF_INET, SOCK_STREAM, 0);
+  if (sock == -1) { perror("socket"); exit(1); }
+
+  v = 1;
+  if (setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &v, sizeof(v)) == -1)
+    { perror("channel_simulator: setsockopt(NODELAY)"); exit(1); }
+
+  addr.sin_family = AF_INET;
+  addr.sin_port = htons(port);
+  addr.sin_addr.s_addr = inet_addr(ip);
+
+  while (1) {
+    printf("trying to connect to %s:%d\n", ip, port);
+
+    if (connect(sock, (struct sockaddr *)&addr, sizeof(addr)) == 0) {
+      printf("connection established\n");
+      return sock;
+    }
+
+    perror("channel_simulator");
+    sleep(1);
+  }
+}
+
+void init_connection(int sock, uint64_t rx_frequency, uint64_t tx_frequency,
+                     int samples_per_subframe)
+{
+  unsigned char b[8*2+4*3];
+
+  pu64(b, rx_frequency);
+  pu64(b+8, tx_frequency);
+  pu32(b+8*2, samples_per_subframe * 2);         /* RX sample advance */
+  pu32(b+8*2+4, 0);                              /* TX sample advance */
+  pu32(b+8*2+4*2, samples_per_subframe * 1000);
+
+  if (fullwrite(sock, b, 8*2+4*3) != 8*2+4*3) {
+    printf("ERROR: init_connection failed\n");
+    exit(1);
+  }
+}
+
+void go_realtime(void)
+{
+  struct sched_param sparam;
+  int policy;
+  memset(&sparam, 0, sizeof(sparam));
+  sparam.sched_priority = sched_get_priority_max(SCHED_FIFO)-1;
+  policy = SCHED_FIFO ;
+  if (pthread_setschedparam(pthread_self(), policy, &sparam) != 0) {
+    printf("ERROR: Failed to set pthread priority\n");
+    exit(1);
+  }
+}
+
+int main(void)
+{
+  int sock;
+  buffer_t buf = { data:NULL, n_samples:0, timestamp:0 };
+  uint64_t sim_timestamp;
+  uint64_t usrp_timestamp;
+  int samples_per_subframe = 7680;
+  int tx_sample_advance = 0; //40;
+  char *usrp_data;
+
+  go_realtime();
+
+  usrp_init_connection(2560000000, 2680000000);
+
+  sock = connect_to_channel_simulator();
+  init_connection(sock, 1, 2, samples_per_subframe);
+
+  receive_from_channel_simulator(sock, &buf);
+  sim_timestamp = buf.timestamp - 2 * samples_per_subframe + buf.n_samples;
+
+  usrp_data = calloc(1, buf.n_samples * 4);
+  if (usrp_data == NULL) {
+    printf("ERROR: out of memory\n");
+    exit(1);
+  }
+
+  send_to_channel_simulator(sock, usrp_data, buf.n_samples, sim_timestamp);
+  sim_timestamp += buf.n_samples;
+
+  usrp_start();
+  usrp_timestamp = usrp_read(usrp_data, buf.n_samples);
+
+  while (1) {
+    usrp_write(buf.data, buf.n_samples,
+               usrp_timestamp - buf.n_samples + 2 * samples_per_subframe - tx_sample_advance);
+    usrp_timestamp = usrp_read(usrp_data, buf.n_samples);
+    send_to_channel_simulator(sock, usrp_data, buf.n_samples, sim_timestamp);
+    sim_timestamp += buf.n_samples;
+    receive_from_channel_simulator(sock, &buf);
+  }
+
+  return 0;
+}

channel_simulator/usrp/usrp.cc

+/* g++ bug-3.13.1.0.cc -luhd -lboost_system */
+#include <uhd/usrp/multi_usrp.hpp>
+#include <stdio.h>
+#include <stdlib.h>
+
+static uhd::usrp::multi_usrp::sptr usrp;
+static uhd::rx_streamer::sptr rx_stream;
+static uhd::tx_streamer::sptr tx_stream;
+static uhd::rx_metadata_t rx_md;
+static uhd::tx_metadata_t tx_md;
+
+extern "C" {
+
+#include "usrp.h"
+
+void usrp_init_connection(uint64_t rx_freq, uint64_t tx_freq)
+{
+  std::string args = "type=b200";
+  uhd::device_addrs_t device_adds = uhd::device::find(args);
+  if (device_adds.size() == 0) { printf("no device\n"); exit(1); }
+  if (device_adds[0].get("type") != "b200") { printf("no b200\n"); exit(1); }
+
+  double usrp_master_clock = 30.72e6;
+  args += boost::str(boost::format(",master_clock_rate=%f") % usrp_master_clock);
+  //args += ",num_send_frames=256,num_recv_frames=256, send_frame_size=3840, recv_frame_size=3840";
+  args += ",num_send_frames=256,num_recv_frames=256";
+
+  usrp = uhd::usrp::multi_usrp::make(args);
+
+  usrp->set_clock_source("internal");
+  usrp->set_master_clock_rate(30.72e6);
+
+  usrp->set_rx_rate(7680000, 0);
+  usrp->set_rx_freq(rx_freq, 0);
+  usrp->set_rx_gain(40, 0);
+
+  usrp->set_tx_rate(7680000, 0);
+  usrp->set_tx_freq(tx_freq, 0);
+  usrp->set_tx_gain(90, 0);
+
+  uhd::stream_args_t stream_args_rx("sc16", "sc16");
+  stream_args_rx.args["spp"] = str(boost::format("%d") % 768 );
+  stream_args_rx.channels.push_back(0);
+  rx_stream = usrp->get_rx_stream(stream_args_rx);
+
+  uhd::stream_args_t stream_args_tx("sc16", "sc16");
+  stream_args_tx.channels.push_back(0);
+  tx_stream = usrp->get_tx_stream(stream_args_tx);
+
+  usrp->set_tx_bandwidth(20e6);
+  usrp->set_rx_bandwidth(20e6);
+}
+
+void usrp_start(void)
+{
+  uhd::stream_cmd_t cmd(uhd::stream_cmd_t::STREAM_MODE_START_CONTINUOUS);
+  cmd.stream_now = false; // start at constant delay
+  cmd.time_spec = usrp->get_time_now() + uhd::time_spec_t(0.05);
+  rx_stream->issue_stream_cmd(cmd);
+
+  tx_md.start_of_burst = true;
+  tx_md.end_of_burst = false;
+}
+
+uint64_t usrp_read(char *buf, int samples_count)
+{
+    std::vector<void *> buff_ptrs;
+    buff_ptrs.push_back(buf);
+
+    int recv = rx_stream->recv(buff_ptrs, samples_count, rx_md);
+    return rx_md.time_spec.to_ticks(7680000);
+}
+
+void usrp_write(char *buf, int samples_count, uint64_t timestamp)
+{
+  tx_md.time_spec = uhd::time_spec_t::from_ticks(timestamp, 7680000);
+  tx_md.has_time_spec = true;
+  int sendv = tx_stream->send(buf, samples_count, tx_md, 10 /*1e-3*/);
+  tx_md.start_of_burst = false;
+}
+
+#if 0
+  while (1) {
+    char buf[7680*2*2];
+    std::vector<void *> buff_ptrs;
+    buff_ptrs.push_back(buf);
+
+    int recv = rx_stream->recv(buff_ptrs, 7680, rx_md);
+    //printf("got %d samples ret %d [%lld]\n", recv, rx_md.error_code, rx_md.time_spec.to_ticks(7680000));
+
+    unsigned long long ts = rx_md.time_spec.to_ticks(7680000) + 4 * 7680;
+
+    tx_md.time_spec = uhd::time_spec_t::from_ticks(ts, 7680000);
+    tx_md.has_time_spec = true;
+
+    int sendv = tx_stream->send(buf, 7680, tx_md, 1e-3);
+    //printf("send %d samples\n", sendv);
+
+    tx_md.start_of_burst = false;
+  }
+
+  return 0;
+}
+#endif
+
+} /* extern "C" */

channel_simulator/usrp/usrp.h

+#ifndef _USRP_H_openair
+#define _USRP_H_openair
+
+#include <stdint.h>
+
+void usrp_init_connection(uint64_t rx_freq, uint64_t tx_freq);
+void usrp_start(void);
+uint64_t usrp_read(char *buf, int samples_count);
+void usrp_write(char *buf, int samples_count, uint64_t timestamp);
+
+#endif /* _USRP_H_openair */

targets/ARCH/tcp_bridge/channel_simulator.c

@@ -23,6 +23,8 @@ typedef struct {
   int samples_per_subframe;
   uint64_t rx_frequency;
   uint64_t tx_frequency;
+  uint32_t rx_sample_advance;
+  uint32_t tx_sample_advance;
   lock_t lock;
   buffer_t * volatile rx_head;
   buffer_t *rx_last;
@@ -31,6 +33,7 @@ typedef struct {
   volatile int first_rx_ok;
   uint64_t rx_next_timestamp;
   lock_t tx_lock;
+  lock_t tx_buf_lock;
 } channel_simulator_state_t;
 
 void add_rx_buffer(channel_simulator_state_t *c, buffer_t *b)
@@ -48,22 +51,21 @@ void add_rx_buffer(channel_simulator_state_t *c, buffer_t *b)
 
 void add_tx_buffer(channel_simulator_state_t *c, buffer_t *b)
 {
-  lock(&c->lock);
+  lock(&c->tx_buf_lock);
   if (c->tx_head == NULL) {
     c->tx_head = b;
   } else {
     c->tx_last->next = b;
   }
   c->tx_last = b;
-  lock_signal(&c->lock);
-  unlock(&c->lock);
+  unlock(&c->tx_buf_lock);
 }
 
+/* called with c->lock acquired */
 uint32_t get_rx_sample(channel_simulator_state_t *c, uint64_t timestamp)
 {
   uint32_t *b;
   uint32_t ret;
-  lock(&c->lock);
 again:
   while (c->rx_head == NULL) lock_wait(&c->lock);
   if (c->rx_head->timestamp + c->rx_head->n_samples <= timestamp) {
@@ -82,7 +84,6 @@ again:
   }
   b = c->rx_head->b;
   ret = b[timestamp - c->rx_head->timestamp];
-  unlock(&c->lock);
   return ret;
 }
 
@@ -90,7 +91,6 @@ uint32_t get_tx_sample(channel_simulator_state_t *c, uint64_t timestamp)
 {
   buffer_t *cur;
   uint32_t ret;
-  lock(&c->lock);
 again:
   if (c->tx_head == NULL) { ret = 0; goto done; }
   if (c->tx_head->timestamp + c->tx_head->n_samples <= timestamp) {
@@ -119,7 +119,6 @@ again:
     cur = cur->next;
   }
 done:
-  unlock(&c->lock);
   return ret;
 }
 
@@ -157,9 +156,14 @@ void *rx_thread(void *_c)
       if (out == NULL) goto err;
       out_size = n_samples;
     }
+    lock(&c->tx_buf_lock);
     for (i = 0; i < n_samples; i++)
-      out[i] = get_tx_sample(c, timestamp + n_samples + i);
-    pu64(b, timestamp + n_samples);
+      out[i] = get_tx_sample(c, timestamp - c->rx_sample_advance
+                                          + c->tx_sample_advance
+                                          + n_samples + i);
+    unlock(&c->tx_buf_lock);
+    pu64(b, timestamp - c->rx_sample_advance
+                      + c->tx_sample_advance + n_samples);
     pu32(b+8, n_samples);
     lock(&c->tx_lock);
     if (fullwrite(c->sock, b, 8+4) != 8+4 ||
@@ -179,14 +183,41 @@ err:
 
 void init_connection(channel_simulator_state_t *c)
 {
-  unsigned char b[8*2+4];
+  unsigned char b[8*2+4*3];
+  char *env;
+
+  env = getenv("CHANNEL_SIMULATOR_RX_SAMPLE_ADVANCE");
+  if (env != NULL) {
+    c->rx_sample_advance = atoi(env);
+    printf("channel_simulator: setting rx_sample_advance to %"PRIu32"\n",
+           c->rx_sample_advance);
+  } else {
+    c->rx_sample_advance = 0;
+    printf("channel_simulator: environment variable"
+           " CHANNEL_SIMULATOR_RX_SAMPLE_ADVANCE"
+           " not found, using 0\n");
+  }
+
+  env = getenv("CHANNEL_SIMULATOR_TX_SAMPLE_ADVANCE");
+  if (env != NULL) {
+    c->tx_sample_advance = atoi(env);
+    printf("channel_simulator: setting tx_sample_advance to %"PRIu32"\n",
+           c->tx_sample_advance);
+  } else {
+    c->tx_sample_advance = 0;
+    printf("channel_simulator: environment variable"
+           " CHANNEL_SIMULATOR_TX_SAMPLE_ADVANCE"
+           " not found, using 0\n");
+  }
 
   pu64(b, c->rx_frequency);
   pu64(b+8, c->tx_frequency);
-  pu32(b+8*2, c->samples_per_subframe * 1000);
+  pu32(b+8*2, c->rx_sample_advance);
+  pu32(b+8*2+4, c->tx_sample_advance);
+  pu32(b+8*2+4*2, c->samples_per_subframe * 1000);
 
   lock(&c->tx_lock);
-  if (fullwrite(c->sock, b, 8*2+4) != 8*2+4) {
+  if (fullwrite(c->sock, b, 8*2+4*3) != 8*2+4*3) {
     printf("ERROR: channel_simulator: init_connection failed\n");
     exit(1);
   }
@@ -257,6 +288,10 @@ int channel_simulator_set_freq(openair0_device* device, openair0_config_t *opena
   channel_simulator_state_t *c = device->priv;
   unsigned char b[8+4+8*2];
 
+  if (getenv("CHANNEL_SIMULATOR_RX_FREQUENCY") != NULL ||
+      getenv("CHANNEL_SIMULATOR_TX_FREQUENCY") != NULL)
+    return 0;
+
   pu64(b, 0);
   pu32(b+8, 8*2);
   pu64(b+8+4, openair0_cfg[0].rx_freq[0]);
@@ -316,8 +351,8 @@ int channel_simulator_read(openair0_device *device, openair0_timestamp *timestam
   uint32_t *r = (uint32_t *)(buff[0]);
   uint64_t ts = get_rx_timestamp(c);
   int i;
-  for (i = 0; i < nsamps; i++) r[i] = get_rx_sample(c, ts + i);
   lock(&c->lock);
+  for (i = 0; i < nsamps; i++) r[i] = get_rx_sample(c, ts + i);
   c->rx_next_timestamp += nsamps;
   unlock(&c->lock);
   *timestamp = ts;
@@ -329,6 +364,8 @@ __attribute__((__visibility__("default")))
 int device_init(openair0_device* device, openair0_config_t *openair0_cfg)
 {
   channel_simulator_state_t *channel_simulator = (channel_simulator_state_t*)malloc(sizeof(channel_simulator_state_t));
+  char *env;
+
   memset(channel_simulator, 0, sizeof(channel_simulator_state_t));
 
   /* only 25, 50 or 100 PRBs handled for the moment */
@@ -359,9 +396,27 @@ int device_init(openair0_device* device, openair0_config_t *openair0_cfg)
 
   init_lock(&channel_simulator->lock);
   init_lock(&channel_simulator->tx_lock);
+  init_lock(&channel_simulator->tx_buf_lock);
+
+  env = getenv("CHANNEL_SIMULATOR_RX_FREQUENCY");
+  if (env == NULL) {
+    printf("channel_simulator: CHANNEL_SIMULATOR_RX_FREQUENCY not set, used provided value %g\n",
+           openair0_cfg[0].rx_freq[0]);
+    channel_simulator->rx_frequency = openair0_cfg[0].rx_freq[0];
+  } else {
+    channel_simulator->rx_frequency = atoll(env);
+    printf("channel_simulator: set RX frequency to %"PRIu64"\n", channel_simulator->rx_frequency);
+  }
 
-  channel_simulator->rx_frequency = openair0_cfg[0].rx_freq[0];
-  channel_simulator->tx_frequency = openair0_cfg[0].tx_freq[0];
+  env = getenv("CHANNEL_SIMULATOR_TX_FREQUENCY");
+  if (env == NULL) {
+    printf("channel_simulator: CHANNEL_SIMULATOR_TX_FREQUENCY not set, used provided value %g\n",
+           openair0_cfg[0].tx_freq[0]);
+    channel_simulator->tx_frequency = openair0_cfg[0].tx_freq[0];
+  } else {
+    channel_simulator->tx_frequency = atoll(env);
+    printf("channel_simulator: set TX frequency to %"PRIu64"\n", channel_simulator->tx_frequency);
+  }
 
   /* let's pretend to be a b2x0 */
   device->type = USRP_B200_DEV;