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Presented By: Nishant Divecha

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1 Presented By: Nishant Divecha
802.11b Modulation Scheme Presented By: Nishant Divecha

2 Evolution of the b Major Problems at the Physical Layer caused by nature of the chosen media that had to be addressed in b Bandwidth allocation; External interference; Reflection.

3 Frequency hopping spread spectrum (FHSS);
The physical layer of the original standardized three wireless data exchange techniques: Infrared (IR); Frequency hopping spread spectrum (FHSS); Direct sequence spread spectrum (DSSS). Infrared The use of infrared for WLAN has not been accepted by public. There was no successful commercial implementations of IR technology. FHSS First step in the evolution to the DSSS. The idea is to transmit on a given frequency for a very short time and switch to another frequency according to a pre-defined frequency hopping pattern known to both transmitter and receiver. This allows to deal with high energy interference in a narrow band as well mutual interference of two FHSS transmitters positioned close to each other.

4 802. 11 frequency hopping separates the whole 2
frequency hopping separates the whole 2.4Ghz ISM band into channels spaced 1MHz. The transmitter has to change channels at least 2.5 times per second (every 400msec or less). The hopping patterns are described by 3 sets containing 26  hopping sequences each. The sets are defined in such way that the sequences  in each set, when set up on different access points, provide minimum mutual interference. The frequency picked up according to the hopping pattern is then modulated using two-level GFSK (Gaussian Frequency Shift Keying) for 1Mbps and four-level GFSK modulation for 2Mbps data rate.  The FHSS is quite stable to interference, cost effective and simple data transmission technique but it is not widely used for WLANs nowadays. Mostly because of its growing requirements to bandwidth when data transmission rates are increased. Everyone Should be familiar with the DSSS systems, since it was presented last week.

5 Complementary Code Keying
Why CCK? Because it was easy to integrate it with the 1 and 2 Mps (original version). Also it maximizes the trhoughput The CCK modulation is based on the use of the polyphase complementary codes.  The codes posses M-ary orthogonal (close to zero autocorrelation if shift is is not 0) properties. The polyphase complementary codes are not binary, they are complex codes.(it is based on quadrature and in phase architecture using complex symbols) The picture below shows a polyphase code with its real component placed in the vertical plane and the complex component in the horizontal plane. Assuming the data transmission rate is set to 11Mbps, the CCK modulator is fed by bytes of data at the rate of 1.375MBytes/sec. The modulator uses 6 bits of each byte to pick one of 64 unique orthogonal eight chips long polyphase complementary codes (like the one on the picture). The other two bits of the byte are used to rotate the whole code word (0, 90, 180 or 270 degrees). Finally, 11 million times per second, the real and complex parts of the resulted code go to the I(in-phase) and Q(quadrature) channels of the IQ modulator. The resulted symbol rate is 11Mbps, the bandwidth occupied by the channel is 22MHz and consequently the CCK modulation may coexist with original DSSS. CCK allows for multichannel operation in the 2.4 GHz band by using the existing 1 and 2 Mbps DSSS channelization scheme Even faster a operated at 54Mbps through OFDM

6 DSSS Frequency Channel Plan
13 channels in Europe (ETSI) from 2412 MHz to 2472 MHz, spaced by 5 MHz. Only first 11 channels available in USA. • 22 MHz carrier bandwidth. • Only 3-4 non-overlapping channels (USA: 1,6,11 / ETSI: 1,5,9,13). BAKOM recommends channels 1,7,13 for use in Switzerland Channel width ever 22 MHz Channels are overlapping!

7 DSSS Radio Interface 1 Mbps 1 Msymbol/s BPSK spread by 11 chip Barker code, (-4 dB Bandwidth = 11 MHz, main lobe = 22 MHz), IEEE 2 Mbps 1 Msymbol/s QPSK spread by 11 chip Barker code 5.5 Mbps 2 Msymbol/s QPSK like symbols spread by 8 chip Complementary Code Keying (CCK). IEEE b 11 Mbps 4 Msymbol/s QPSK like symbols spread by 8 chip 54 Mbps OFDM with max. 52 sub-carriers, IEEE a / IEEE g

8 The output of the HFA3861A data scrambler is partitioned into bytes and fed to a serial in parallel out mux circuit that gets clocked at the symbol rate of 1.375MHz. Six bits of the mux output are used to select one of 64 complex codes which are fed to a differential modulator circuit. The other 2 bits of the mux output are used to QPSK modulate, i.e., rotate, the 8 chip complex code word.

9 1 Mbps (802.11), 1 bit/symbol ; 2 symbol values DBPSK, 11 analog chips
DQPSK ; 11 analog chips The specification allows a data rate of 1.0 Mbps or 2 Mbps. A version of differential binary phase shift keying modulation is shown above.

10 5.5 Mbps (802.11b) 11 Mbps (802.11b) 4 bit/symbol ; 8 bit/symbol ;
16 symbol values DQPSK ; 8 analog chips 11 Mbps (802.11b) 8 bit/symbol ; 256 symbol values DQPSK ; 8 analog chips FHSS is more robust while DSSS systems provide a higher throughput and transmission over longer distances 1 microsec and phase of the transmitted signal.

11 a set of 64 eight-bit code words used to encode data for 5
a set of 64 eight-bit code words used to encode data for 5.5 and 11Mbps data rates in the 2.4GHz band of b wireless networking The code words have unique mathematical properties that allow them to be correctly distinguished from one another by a receiver even in the presence of substantial noise and multipath interference. CCK applies sophisticated mathematical formulas to the DSSS codes, permitting the codes to represent a greater volume of information per clock cycle. The transmitter can then send multiple bits of information with each DSSS code, enough to make possible the 11Mbps of data rather than the 2Mbps in the original standard.

12 Typical Receiver can operate at 0dB SNR

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