This post covers WIRELESS COMMUNICATIONS AND NETWORKS by William Stallings.

Basic Ideas

• 802.11 Physical Layer- Fundamental System Design

• Three physical media are defined in the original 802.11 standard:

  • Direct sequence spread spectrum (DSSS) operating in the 2.4-GHz ISM band, at data rates of 1 Mbps and 2 Mbps. In the United States, the FCC (Federal Communications Commission) requires no licensing for the use of this band. The number of channels available depends on the bandwidth allocated by the various national regulatory agencies. This ranges from 13 in most European countries to just one available channel in Japan.

  • Up to three non overlapping channels, each with a data rate of 1 Mbps or 2 Mbps, can be used in the DSSS scheme. Each channel has a bandwidth of 5 MHz. The encoding scheme that is used is DBPSK (differential binary phase shift keying) for the 1 Mbps rate and DQPSK for the 2 Mbps rate. A DSSS system makes use of a chipping code, or pseudonoise sequence, to spread the data rate and hence the bandwidth of the signal. For IEEE 802.11, a Barker sequence is used.

  • A Barker sequence is a binary ${-1, +1}$ sequence ${s(t)}$ of length $n$ with the property that its autocorrelation values $R(\tau)$ satisfy $\lvert R(\tau) \lvert \le 1$ for $ \lvert {\tau} \lvert \le (n - 1)$. Further, the Barker property is preserved under the following transformations.

    $s(t) \rightarrow -s(t)$, $s(t) \rightarrow (-1)^ns(t)$ and $s(t) \rightarrow -s(n - 1 - t)$

    as well as under compositions of these transformations. Only the following Barker sequences are known:

    $n=2 ~~~~~~~~ ++$

    $n=3 ~~~~~~~~ ++-$

    $n=4 ~~~~~~~~ +++-$

    $n=5 ~~~~~~~~ +++-+$

    $n=7 ~~~~~~~~ +-++-+-+$

    $n = 11 ~~~~~~~~ +-++-+++—$

    $n = 13 ~~~~~~~~ +-++-+++–+++++–++-+-+$

  • IEEE 802.11b is an extension of the IEEE 802.11 DSSS scheme, providing data rates of 5.5 and 11 Mbps in the ISM band. The chipping rate is 11 MHz, which is the same as the original DSSS scheme, thus providing the same occupied bandwidth.

  • To achieve a higher data rate in the same bandwidth at the same chipping rate, a modulation scheme known as complementary code keying (CCK) is used.

    Question: Using Equation $R(\tau)= \frac{1}{N}\Sigma_{k=1}^{k=N}B_{k}B_{k-\tau}$, find the autocorrelation for the 11-bit Baker sequence as a function of $\tau$.

  • Frequency-hopping spread spectrum (FHSS) operating in the 2.4-GHz ISM band, at data rates of 1 Mbps and 2 Mbps. The number of channels available ranges from 23 in Japan to 70 in the United States

  • FHSS system makes use of a multiple channels, with the signal hopping from one channel to another based on a pseudonoise sequence. In the case of the IEEE 802.11 scheme, I-MHz channels are used.

  • The details of the hopping scheme are adjustable. For example, the minimum hop rate for the United States is 2.5 hops per second.

  • The minimum hop distance in frequency is 6 MHz in North America and most of Europe and 5 MHz in Japan.

  • For modulation, the FHSS scheme uses two-level Gaussian FSK for the 1-Mbps system. The bits zero and one are encoded as deviations from the current carrier frequency.

  • For 2 Mbps, a four-level GFSK scheme is used, in which four different deviations from the center frequency define the four 2-bit combinations.

  • Infrared: at 1 Mbps and 2 Mbps operating at a wavelength between 850 and

    950nm

  • The IEEE 802.11 infrared scheme is omnidirectional rather than point to point.

  • A range of up to 20 m is possible. The modulation scheme for the 1-Mbps data rate is known as 16-PPM (pulse position modulation).

  • In pulse position modulation (PPM), the input value determines the position of a narrow pulse relative to the clocking time.

  • The advantage of PPM is that it reduces the output power required of the infrared source.

  • For 16-PPM, each group of 4 data bits is mapped into one of the 16-PPM symbols; each symbol is a string of 16 bits.

  • Each 16-bit string consists of fifteen Os and one binary 1. For the 2-Mbps data rate, each group of 2 data bits is mapped into one of four 4-bit sequences. Each sequence consists of three Os and one binary 1.

  • The actual transmission uses an intensity modulation scheme, in which the presence of a signal corresponds to a binary 1 and the absence of a signal corresponds to binary O.

• Channel Structure

  • IEEE 802.11a makes use of the frequency band called the which is divided into three parts. The UNNI-1 band (5.15 to 5.25 GHz) is intended for indoor use; the UNNI-2 band (5.25 to 5.35 GHz) can be used either indoor or outdoor, and the UNNI-3 band (5.725 to 5.825 GHz) is for outdoor use.
  • IEEE 80211.a has several advantages over IEEE 802.11b/g:
  • IEEE 802.11a utilizes more available bandwidth than 802.11b/g. Each UNNI band provides four nonoverlapping channels for a total of 12 across the allocated spectrum.
  • IEEE 802.11a provides much higher data rates than 802.11b and the same maximum data rate as 802.11g.
  • IEEE 802.11a uses a different, relatively uncluttered frequency spectrum (5 GHz).

• Coding and Modulation

  • Unlike the 2.4-GHz specifications, IEEE 802.11 does not use a spread spectrum scheme but rather uses orthogonal frequency division multiplexing (OFDM). OFDM, also called multicarrier modulation, uses multiple carrier signals at different frequencies, sending some of the bits on each channel.
  • This is similar to FDM. However, in the case of OFDM, all of the subchannels are dedicated to a single data source.
  • To complement OFDM, the specification supports the use of a variety of modulation and coding alternatives.
  • The system uses up to 48 subcarriers that are modulated using BPSK, QPSK, 16-QAM, or 64-QAM. Subcarrier frequency spacing is 0.3125 MHz.
  • A convolutional code at a rate of 1/2,2/3, or 3/4 provides forward error correction.
  • The combination of modulation technique and coding rate determines the data rate. Below table summarizes key parameters for 802.11a.

• Question: For IEEE 802.11a, show how the modulation technique and coding rate determine the data rate.

  

• Figure illustrates the physical layer frame format. The PLCP Preamble field enables the receiver to acquire an incoming OFDM signal and synchronize the demodulator.

• Next is the Signal field, which consists of 24 bits encoded as a single OFDM symbol. The Preamble and Signal fields are transmitted at 6 Mbps using BPSK.

• The signal field consists of the following subfields:

• Rate: Specifies the data rate at which the data field portion of the frame is transmitted

• r: Reserved for future use

• Length: Number of octets in the MAC PDU

• P: An even parity bit for the 17 bits in the Rate, r, and Length subfields.

• Tail: Consists of 6 zero bits appended to the symbol to bring the convolutional encoder to zero state

• The Data field consists of a variable number of OFDM symbols transmitted at the data rate specified in the Rate subfield. Prior to transmission, all of the bits of the Data field are scrambled.The Data field consists of four subfields:

• Service: Consists of 16 bits, with the first 6 bits set to zeros to synchronize the descrambler in the receiver, and the remaining 9 bits (all zeros) reserved for future use.

• MAC PDU: Handed down from the MAC layer.

• Tail: Produced by replacing the six scrambled bits following the MPDU end with 6 bits of all zeros; used to re-initialize the convolutional encoder.

• Pad: The number of bits required to make the Data field a multiple of the number of bits in an OFDM symbol (48,96,192, or 288).