TEMPERATURAS Y ESTRUCTURA DEL SISTEMA DE ESCAPE

The IEEE 802.11 standard specifies two modulation methods, which are direct-sequence spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) [28]. IEEE 802.11 introduces DSSS as a technology to minimize the interference for the 2.4 GHz frequency band, while IEEE 802.11a uses OFDM modulation to control interference in the 5 GHz frequency band. DSSS supports the transmission rate up to 11Mbit/s at the physical layer, while OFDM supports the transmission rate up to 54Mbits/s.

2.1.4.1 Direct Sequence Spread Spectrum (DSSS)

Direct sequence spread spectrum (DSSS) is a spread spectrum modulation technique. In DSSS, the original data signal is multiplied with a pseudorandom noise spreading code. DSSS provides stronger protection against interfering signals, and makes the interfering signal less perceptible. DSSS also provides security of transmission if the code is not known to the public, which makes it very popular in military applications.

When transmitting data using DSSS, the required data signal is multiplied with what is known as a spreading or chip code data stream. Figure 2.6 shows spreading with

DSSS. The resulting data stream has a higher data rate than the data itself. The data is multiplied using the XOR (exclusive OR) function.

Figure 2.6: Spreading with DSSS

Each bit in the spreading sequence is called a chip, and this is much shorter than each information bit. The spreading sequence or chip sequence has the same data rate as the final output from the spreading multiplier. The rate is called the chip rate, and this is often measured in terms of a number ofM chips/second.

The baseband data stream is then modulated onto a carrier. Figure 2.7 shows the process of DSSS generation. In this way the overall signal is spread over a much wider bandwidth than if the data had been simply modulated onto the carrier. The reason is that signals with high data rates occupy wider signal bandwidths than those with low data rates.

To decode the signal and receive the original data, the signal is first demodulated from the carrier to reconstitute the high speed data stream. Demodulating the received signal

Figure 2.7: DSSS Generation

is achieved using the same carrier as the transmitter and reversing the modulation. The result is a signal with approximately the same bandwidth as the original spread spectrum signal. Additional filtering can be applied to generate the original signal. The receiver then uses the same chip sequence to reconstruct the original data. Figure 2.8 shows the process of DSSS decoding.

Figure 2.8: DSSS decoding

It is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference. This way a base station can send data to several mobile devices on a single channel. Similarly several mobile devices can send data to a single base station, provided that in each case an independent spreading code is used.

2.1.4.2 Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing or OFDM is a modulation format that is being used for many of the latest wireless and telecommunications standards, such as IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac and more.

OFDM is a form of multi-carrier modulation. OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at dif- ferent frequencies to the receiver. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result, when signals are transmitted close to one another, they must be spaced so that the receiver can separate them using a filter, and there must be a guard band between them. Figure 2.9 shows the traditional view of receiving signals carrying modulation. On the other hand, the sidebands overlap from each carrier in OFDM as shown in Figure 2.10. The sidebands in OFDM can still be received without the interference because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

Figure 2.10: OFDM Spectrum

The data to be transmitted on an OFDM signal is divided over a large number of radio frequencies. Each radio frequency carries only a small portion of the total amount of data. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.

The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal.

Thus, using OFDM makes the transmitted signal robust against frequency selective interference and fading, such as multipath fading. If frequency selective fading occurs on the radio channel, only a small portion of the data is affected, while in a broadband transmission with all data on a single carrier the complete radio-frequency signal would be affected.

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter- modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.