USRP平台实现802.11

This directory contains examples for transmitting and receiving 802.11 style bpsk. The following are brief descriptions of the python scrips in this directory. bbn_80211b.py: This module instantiates the gr-blocks that do the encoding and decoding. For both the transmitter and receiver the user can choose whether to use a raised-cosine baseband pulse, or the 802.11 Barker baseband pulse. Currently, the transmitter does not work very well with the Barker baseband pulse, however given a sufficient signal to noise ratio (and processing power), the receiver can correctly decode 802.11 1 Mbps packets. Also, once in a while, the receiver can correctly decode an 802.11 2 Mbps (qpsk) packet, however qpsk tranmission is not yet supported (although it is possible to transmit and receive at 2 Mbps using bpsk and 2 bits per symbol). bbn_80211b_pkt.py: This module is wraps the classes in bbn_80211b.py. The user should instantiate the classes in this file, and not instantiate the classes in bbn_80211b.py directly. The bbn_80211b_mod_pkts class instantiates the transmit packet queue and provides the send_pkt function. The send_pkt function takes a payload, prepends the 802.11 long preamble and plcp header, appeands the payload crc, and queues the packet for transmission. The bbn_80211b_demod_pkts class instantiates the receive queue and creates a thread to watch for received packets. bbn_80211b_transmit_path.py: This file contains a class that connects an instance of bbn_80211b_mod_pkts to the usrp. bbn_80211b_test.py: This class instantiates the transmitter and receiver and simulates an additive white Gaussian noise channel. Its sends a few packets across the simulated channel, and prints out their contents when they are received. bbn_80211_rx.py: bbn_80211_tx.py: These python scripts respectively receive and send packets over the usrp. There are several different ways of running these scripts. The transmit script uses a raised cosine baseband pulse, and the receive script can be configured to look for either a raised cosine, or a Barker pulse. To send some packets between two usrps at 500 kBps, on one machine start the receiver: ./bbn_80211_rx.py and on the other machine run the transmitter: ./bbn_80211_tx.py If it works, the receiver will output a "PKT" message for each packet it receives. By default, both the transmitter and receiver use 8 samples per symbol, run at 4 megasamples per second, and use 2.4Mhz as the center frequency. The receiver configures the usrp to use 8 bits for each I and Q sample, and the transmitter uses 16 bits for each I and Q sample. To send data at 1 Mpbs, you can configure the receiver to use 4 samples per symbol: ./bbn_80211_rx.py --spb 4 and do the same for the transmitter: ./bbn_80211_tx.py --spb 4 On a fast machine, the receiver program should be able to capture real 802.11 1 Mbps packets and a few 2 Mbps packets. Running in this mode seems to max out the cpu on our 1.6 Ghz Pentium M. Despite the fact that the symbol rate of 802.11 bpsk and qpsk is only 1 megasymbols per second, the energy in the 802.11 signal is spread over 11 Mhz (at baseband) using an 11-chip Barker spreading code. Fortunately, under high signal-to-noise ratio conditions, it is not necessary to sample the entire 11 Mhz bandwidge to recover the symbols. We have succeeded in recovering 802.11 packets using an 8 Mhz sampling rate. To try to catch 802.11 1 Mpbs packets, use the following command: ./bbn_80211b_rx.py -d 8 -f 2412e6 -b The "-d 8" tells the receiver to run at 8 megasamples per second (instead of 4), and "-f 2412e6" means to listen on 802.11 channel 1. (The 802.11 channels are 5 Mhz apart, so channel 2 is at 2427, channel 3 is at 2422 etc..) The "-b" means to look for the Barker code. A message will be printed out for each received packet that passes the crc check. Use the "-p" option to disable crc checking. Note that when crc checking is enabled the crc is sripped off the packet, so disabling crc checking causes the received packet to be 4 bytes longer. bbn_80211_tap.py: This script creates a tap interface that can used to send and receive packets. On Linux a standard ethernet tap (called gr0) is created, and on NetBSD an 802.11 (called tap0) interface is created. Most of the options in the other scripts are available for the tap script. However, the tap script has a "-z" option for transmit only (disable the reciver), and the "-t" option to enable the transmitter (while leaving the receiver enabled). If you use both -r and -t options at the same time the transmitter will be enabled and the reciver will be diabled. To run the tap in 802.11 channel 6 without transmitting, use the following command: ./bbn_80211b_tap.py The defaults are shown in this command ./bbn_80211b_tap.py -d 8 -f 2437e6 -b