Which of the following specifies the correct list of non overlapping channels for DSSS used in the 2.4

DIRECT SEQUENCE SPREAD SPECTRUM

Direct sequence spread spectrum (DSSS) works somewhat differently. With DSSS, the data is divided and simultaneously transmitted on as many frequencies as possible within a particular frequency band (the channel). DSSS adds redundant bits of data known as chips to the data to represent binary 0s or 1s. The ratio of chips to data is known as the spreading ratio: the higher the ratio, the more immune to interference the signal is, because if part of the transmission is corrupted, the data can still be recovered from the remaining part of the chipping code. This method provides greater rates of transmission than FHSS, which uses a limited number of frequencies, but fewer channels in a given frequency range. And, DSSS also protects against data loss through the redundant, simultaneous transmission of data. However, because DSSS floods the channel it is using, it is also more vulnerable to interference from EM devices operating in the same range.

Direct Sequence Spread Spectrum (DSSS)

Direct sequence spread spectrum (DSSS) transmission employs a chipping code to “spread” the transmission over a wider frequency band than it would normally occupy. DSSS is a form of CDMA (see Chapter 4). But instead of a pseudorandom binary sequence (PRBS), an 11-bit Barker code is used as the spreading sequence. However, the same principles apply: The redundant bit pattern in the spreading sequence increases the signals resistance to interference and also distributes the power over a 22-MHz-wide channel that looks essentially like low-power, wideband noise. An important characteristic of DSSS modulation is that it is rejected by conventional, narrowband receivers that might be sharing the same ISM band.

For DSSS operation, the 2.4-GHz ISM band is divided into three nonoverlapping or six overlapping frequency bands, as shown in Figure 25.9. Since the DSSS channel occupies a 22-MHz band, only the nonoverlapping channels are guaranteed to be free from contention with each other. For example, two wireless stations operating on channels 1 and 3 would not associate, but carrier sensing may prevent them from transmitting packets simultaneously.

(d) Direct sequence spread spectrum (DSSS)

DSSS technology breaks down the transmitted stream of data into small pieces across a frequency channel. A redundant bit pattern (known as a chipping code) is generated for each bit transmitted. Generally, the longer the chipping code, the more likely it is that the original transmitted data will be properly received. DSSS technology uses more bandwidth than FHSS, but is considered more reliable and resists interference. Because of the chipping code, data can still be recovered without retransmission of the signal, even in the case of damaged data bits.

Since wireless networking uses radio-wave transmission, radio-wave frequency techniques are particularly important to the wireless networks’ performance. There are three technical issues are particularly relevant; frequency allocation, frequency roaming, and frequency interference.

14.8.2 Direct sequence spread spectrum

In direct sequence spread spectrum (DSSS), the transmitter and receiver contain identical psuedo-random sequence generators producing a pseudo-noise (PN) signal. In the transmitter, the input data stream is XORed with the PN signal before transmission. In the receiver the received signal is XORed with the PN stream to recover the original data stream; this is equivalent to correlation with the known PN sequence. There is an obvious analogy between this process and stream ciphering (Section 14.8) but with the crucial difference that in DSSS the PN sequence is at a much greater clock frequency than the data stream. Each bit of the PN sequence is called a chip, and the clock rate of the PN generator is called the chip rate. In practical systems the chip rate is a large integer multiple L of the databit rate. The bandwidth of the transmitted signal is therefore L times greater than that of the data stream.

The DSSS signal gives LPI (a low probability of intercept) because the total signal power is spread over a wide bandwidth and the signal is noise-like, making it hard to detect. In anti-jamming (AJ) applications, the transmitter introduces an unpredictable element into the modulation of the signal, known also to the receiver but kept secret from opponents, as in stream ciphering. This, together with the wide bandwidth of the transmitted signal, makes jamming more difficult than for conventional signals. In CDMA applications, the various transmitters which are sharing the channel use different fixed PN sequences which are chosen so that their cross-correlation is low. After a receiver has correlated the received signal with its particular PN pattern, the interference from the other PN sequences is therefore low. The chosen sequences must also have noise-like autocorrelation functions, to help the receiver to synchronise correctly to the partially unknown timing of the transmitter. Some often-used sequences with these properties are called Gold and Kasami sequences (Proakis, 1989).

Direct sequence spread spectrum requires the overall channel (including, where relevant, equalisation in the receiver) to have approximately unity gain, pure delay characteristics over the whole signal bandwidth. This is achievable for local radio systems and transmission lines, but can be much harder to achieve over a wide bandwidth in long distance radio links.

11.4 Requirements of Direct-Sequence Spread Spectrum

In the DSSS system, the entire bandwidth of the RF carrier is made available to each user. The DSSS system satisfies the following requirements [16]:

The spreading signal has a bandwidth much larger than the minimum bandwidth required to transmit the desired information which, for a digital system, is baseband data.

The spreading of the information is performed by using a spreading signal, called the code signal (see Appendix D). The code signal is independent of the data and is of a much higher chip rate than the data signal.

At the intended receiver, despreading is accomplished by cross-correlation of the received spread signal with a synchronized replica of the same code signal used to spread the data.

DSSS PHY

In the DSSS PHY, data transmission over the media is controlled by the PMD sublayer as directed by the PLCP sublayer. The PMD sublayer takes the binary information bits from the PLCP protocol data unit (PPDU) and converts them into RF signals by using modulation and DSSS techniques (see Figure 21.7). Figure 21.8 shows the PPDU frame, which consists of a PLCP preamble, PLCP header, and MAC protocol data unit (MPDU). The PLCP preamble and PLCP header are always transmitted at 1 Mbps, and the MPDU can be sent at 1 or 2 Mbps.

The start of frame delimiter (SFD) contains information that marks the start of the PPDU frame. The SFD specified is common for all IEEE 802.11 DSSS radios.

The signal field indicates which modulation scheme should be used to receive the incoming MPDU. The binary value in this field is equal to the data rate multiplied by 100 kbps. Two modulation schemes, differential binary phase shift keying (DBPSK) — for 1 Mbps — and differential quadrature phase shift keying (DQPSK) — for 2 Mbps — are available.

The service field is reserved for future use. The length field indicates the number of microseconds necessary to transmit the MPDU. The MAC layer uses this field to determine the end of a PPDU frame.

The CRC field contains the results of a calculated frame check sequence from the sending station. The ITU CRC-16 error detection algorithm is used to protect the signal, service, and length field.

The SYNC field is 128 bits (symbols) in length and contains a string of 1s which are scrambled prior to transmission. The receiver uses this field to acquire the incoming signal and to synchronize the receiver's carrier tracking and timing prior to receiving the SFD. The SFD field contains information to mark the start of the PPDU frame. The SFD specified is common for all IEEE 802.11 DSSS radios.

All information bits transmitted by the DSSS PMD are scrambled using a self-synchronizing 7-bit polynomial. An 11-bit Barker code (1, −1, 1, 1, −1, 1, 1, 1, −1, −1, −1) is used for spreading. In the transmitter, the 11-bit Barker code is applied to a modulo-2 adder together with each of the information bits in the PPDU. The output of the modulo-2 adder results in a signal with a data rate that is 10 times higher than the information rate. The result in the frequency domain is a signal that is spread over a wide bandwidth at a reduced RF power level. Every station in the IEEE 802.11 network uses the same 11-bit sequence. At the receiver, the DSSS signal is convolved with the same 11-bit Barker code and correlated. The minimum requirement for processing gain (Gp) in North America and Japan is 10 dB.

Each DSSS PHY channel occupies 22 MHz of bandwidth and allows for three noninterfering channels spaced 25 MHz apart in the 2.4 GHz frequency band (see Figure 21.9). With this channel arrangement, a user can configure multiple DSSS networks to operate simultaneously in the same area. Table 21.5 lists the DSSS channels used in different parts of the world. Fourteen frequency channels are defined for operation across the 2.4 GHz frequency band. In North America 11 frequencies are used ranging from 2.412 to 2.462 GHz. In Europe 13 frequencies are allowed between 2.412 and 2.472 GHz. In Japan only channels at the 2.483 GHz frequency are permitted.

Table 21.5. DSSS channels for different parts of the world.

The maximum allowable radiated power for DSSS PHY varies from region to region (refer to Table 21.6). The transmit power is directly related to the range that a particular implementation can achieve.

Table 21.6. Maximum allowable transmit power.

3.9.3 Frequency Hopping Spread Spectrum (FHSS)

In DSSS, the PN sequence spreads the spectrum of the signal by the chipping rate, resulting in the instantaneous widening of the spectrum. On the other hand, in FHSS, the PN sequence is used to drive the synthesizer and pseudo-randomly hop the signal bandwidth across a much wider band. In this case, the synthesizer is being sequentially programmed with different frequencies by the PN sequence to modulate the desired signal over a much wider frequency band. One such example is Bluetooth where frequency hopping is utilized to spread the spectrum as shown in Figure 3.34 and Figure 3.35. Unlike DS systems, FH systems can cover a spectrum of several GHz, which is challenging for DS systems still in today's technology. However, the implication of such large bandwidths is phase incoherency. Frequency synthesizers are unable to maintain frequency coherency over such wide bandwidths. This presents a particular challenge for fast FH systems. This in turn forces the designer to resort to noncoherent modulation schemes.

There are two types of FHSS systems, namely slow frequency hopping and fast frequency hopping. In slow frequency hopping, the symbol rate of the data signal is an integer multiple of the hopping rate. In this scenario, one or more data symbols are transmitted each hop. In fast frequency hopping, on the other hand, each data symbol is transmitted over multiple hops and the hopping rate is an integer multiple of the data symbol rate.

6.5 Comparisons of FDMA, TDMA, and DS-CDMA

The DSSS approach is the basis to implementation of the direct sequence code division multiple access (DS-CDMA) technique introduced by Qualcom. The DS-CDMA has been used in commercial applications of mobile communications. The primary advantage of DS-CDMA is its ability to tolerate a fair amount of interfering signals compared to FDMA and TDMA that typically cannot tolerate any such interference(Figure 6.7). As a result of the interference tolerance of CDMA, the problems of frequency band assignment and adjacent cell interference are greatly simplified. Also, flexibility in system design and deployment are significantly improved since interference to others is not a problem. On the other hand, FDMA and TDMA radios must be carefully assigned a frequency or time slot to assure that there is no interference with other similar radios. Therefore, sophisticated filtering and guard band protection is needed with FDMA and TDMA technologies. With DS-CDMA, adjacent microcells share the same frequencies whereas with FDMA/TDMA it is not feasible for adjacent microcells to share the same frequencies because of interference. In both FDMA and TDMA systems, a time-consuming frequency planning task is required whenever a network changes, whereas no such frequency planning is needed for a CDMA network since each cell uses the same frequencies.

Capacity improvements with DS-CDMA also result from voice activity patterns during two-way conversations, (i.e., times when a party is not talking) that cannot be cost-effectively exploited in FDMA or TDMA systems. DS-CDMA radios can, therefore, accommodate more mobile users than FDMA/TDMA radios on the same bandwidth. Further capacity gains for FDMA, TDMA, and CDMA can also result from antenna technology advancement by using directional antennas that allow the microcell area to be divided into sectors. Table 6.1 provides a summary of access technologies used for various wireless systems.

Table 6.1. Access technologies for wireless system.

802.11b DSSS

The spectral correlation of DSSS signals is well understood, because over short intervals (relative to the code-repetition interval), the signal is well modeled by PSK or QAM with a symbol rate equal to the DSSS chip rate, and over long intervals, the signal is well modeled as a PSK or QAM signal with a symbol rate equal to the code-repetition rate and a pulse function equal to the repeated chipping sequence. Over short intervals, then, the SCF for most DSSS BPSK signals is identical to that for a general BPSK signal. For longer intervals, there are many non-conjugate and conjugate CFs for DSSS BPSK. The non-conjugate CFs are harmonics of the code-repetition rate, which is equal to the data rate (bit rate) when the code is repeated for each bit (as it is in 802.11b DSSS BPSK). The conjugate CFs are equal to the non-conjugate CFs plus the doubled carrier frequency. The non-conjugate CFs for DSSS QPSK are identical to those for DSSS BPSK, and the conjugate CFs are not present.

The symbol rate (data rate) for the DSSS signals is 1.0 MHz, and there are 11 chips per symbol, which means the chip rate is 11.0 MHz. Therefore, we should see non-conjugate CFs of k MHz up to about k = 2 × 11 = 22. Results for captured 802.11b DSSS BPSK and QPSK signals are shown in Figure 18.12, where it is evident that the modeling and mathematical predictions are correct (compare the CFs listed in Table 18.4 with the measurements in Figure 18.12).

Because 802.11b DSSS signals have many CFs and associated strong SCFs, many detection strategies are effective. A particularly effective and low-cost strategy is to jointly detect several of the cycle frequencies using CF detection. This can be done by using the SSCA and searching its output for “chains” of CFs separated by 1.0 MHz, or by using a set of FSM-based cycle-frequency searches in narrow bands of cycle frequencies near the nominal values of k MHz.

11.4.1.2 FHSS PHY

While overshadowed by the DSSS PHY, acquaintance with the FHSS option in 802.11 may still be of interest. In FHSS WLAN, transmissions occur on carrier frequencies that hop periodically in pseudo-random order over almost the complete span of the 2.4 GHz ISM band. This span in North America and most European countries is 2.400 to 2.4835 GHz, and in these regions there are 79 hopping carrier frequencies from 2.402 to 2.480 GHz. The dwell on each frequency is a system-determined parameter, but the recommended dwell time is 20 ms, giving a hop rate of 50 hops per second. In order for FHSS network stations to be synchronized, they must all use the same pseudo-random sequence of frequencies, and their synthesizers must be in step, that is, they must all be tuned to the same frequency channel at the same time. Synchronization is achieved in 802.11 by sending the essential parameters—dwell time, frequency sequence number, and present channel number—in a frequency parameter set field that is part of a beacon transmission sent periodically on the channel. A station wishing to join the network can listen to the beacon and synchronize its hop pattern as part of the network association procedure.

The FHSS physical layer uses GFSK (Gaussian frequency shift keying) modulation, and must restrict transmitted bandwidth to 1 MHz at 20 dB down (from peak carrier). This bandwidth holds for both 1 Mbps and 2 Mbps data rates. For 1 Mbps data rate, nominal frequency deviation is ± 160 kHz. The data entering the modulator is filtered by a Gaussian (constant phase delay) filter with 3 dB bandwidth of 500 kHz. Receiver sensitivity must be better than − 80 dBm for a 3% frame error rate. In order to keep the same transmitted bandwidth with a data rate of 2 Mbps, four-level frequency shift-keying is employed. Data bits are grouped into symbols of two bits, so each symbol can have one of four levels. Nominal deviations of the four levels are ± 72 and ± 216 kHz. A 500 kHz Gaussian filter smoothes the four-level 1 megasymbols per second at the input to the FSK modulator. Minimum required receiver sensitivity is − 75 dBm.

Although development of Wi-Fi for significantly increased data rates has based on DSSS, FHSS does have some advantageous features. Many more independent networks can be collocated with virtually no mutual interference using FHSS than with DSSS. As we will see later, only three independent DSSS networks can be collocated. However, 26 different hopping sequences (North America and Europe) in any of three defined sets can be used in the same area with low probability of collision. Also, the degree of throughput reduction by other 2.4 GHz band users, as well as interference caused to the other users is lower with FHSS. FHSS implementation may at one time also have been less expensive. However, the updated versions of 802.11—specifically 802.11a, 802.11b, and 802.11g—have all based their methods of increasing data rates on the broadband channel characteristics of DSSS in 802.11, while being downward compatible with the 1 and 2 Mbps DSSS modes (except for 802.11a which operates on a different frequency band). Bluetooth has some of the characteristics of 802.11 FHSS but has advanced well beyond the capabilities of the earlier standard.