WLAN - Presentation Notes

Name: Peter Billes

Class: 298c Advanced Networking

Instructor: Seung Bae Im

Date: Nov. 28, 1995

Wireless Local Area Networks (WLAN's)

1 Introduction

Some wireless systems related to mobile communication, which are getting more and more popular are:
  • cellular radio systems
  • cordless telephones
  • airplane telephones
  • paging and messaging
  • specialized satellite-based message services for truck divers on the
  • highway
  • wireless headphones and microphones
  • remote controls
  • palmtop computer, PDA's like Apple's Newton, which are equipped with infrared transmitters and receivers, and are able to exchange data over wireless connections
  • and wireless local area networks (WLAN's) which will be discussed in this presentation.

    The technologies of transmitting data over wireless connections can be divided in two main categories: radio frequency- (including microwave-) and infrared-transmission. Other methods as ultrasound or carrier currents are not considered here, because they can't provide high-speed data rates and thus are not interesting for the use in wireless infrared local area networks (WLAN's).

    2 Radio Frequency WLAN's

    2.1 Characteristics of radio frequency - based connections

  • Radio frequency can penetrate most objects and are well suited for covering larger areas
  • The frequencies which can be used are restricted because most of them are used for other systems eg. radio broadcast, cellular radio, radar systems.
  • Problems: interference (including multipath fading,dispersion and crosstalk), security.

    2.2 Modulation techniques for radio frequency channels

    Two main modulation schemes can be distinguished: narrow band and spread spectrum techniques. The narrow-band techniques like phase shift keying and frequency shift keying are similar to the infrared methods, and are discussed there. We will discuss here two spread spectrum schemes. Spread spectrum systems use a much higher bandwidth than the bandwidth needed for transferring the data, and avoid this way interference and multipath fading.
    2.2.1 Direct Sequence Spread Spectrum (DS)
    This technic modulates an already modulated carrier a second time, with a wideband spreading signal. This signal is called pseudonoise (PN) code, and has a much higher frequency than the information signal. So one bit of information appears several times in outgoing signal. All users are working in the same bandwidth at the same time. This can cause a signal to be swamped by a nearby transmitter (near-far effect).
    2.2.2 Frequency Hopped Spread Spectrum (FH)
    As in DS the subcarrier is modulated a second time. In FH frequency shift keying is used to alter the frequency depending on the PN code mentioned above. This method can be used to prevent frequency-selective fading. DS and FS use more bandwidth to add redundancy to the information signal. Thus the signal quality is improved. But the techniques are not applicable for high speed systems.

    3 Infrared WLAN's

    3.1 Characteristics of infrared-based connections

  • The working range lies between 700nm and 1500nm and provides theoretically a bandwidth of over 300THz.
  • Infrared radiation can't penetrate walls. This makes it easier to build a cell based network. For example in an office building, each room could be a cell and there would be no interference between two cells.
  • Objects in an office environment have Good reflection properties(40%-90%).
  • There is no multipath fading when intensity modulation is used. (Because the receiver is much bigger than the wavelength)
  • Problems: multipath dispersion, security.

    3.2 Infrared Links

    Optical links can be configured in different ways, depending on orientation, and beam angle of the transmitter and receiver. Figure 1 shows the possible configurations. In the cases (a),(c) and (e) the transmitter (T) and receiver (R) are in transmitsline-of-sight. The beam can travel directly from the transmitter to the receiver, without reflection. In the cases (b),(d) and (f) there is no direct path and before reaching the receiver, the signal is reflected by the ceiling or ceiling and walls (diffuse reflection). The directed, line-of-sight configuration is not capable to support one-to-many and many-to-one connections and it is very unpractical to adjust transmitter and receiver before transmitting data. The adjustment of transmitter and receiver could be no major drawback in the following scenario. In a cell are a fixed number of preadjusted docking stations incorporating one transmitter and one receiver. Mobile computers with conventional network cards can be attached to the docking stations. The docking stations duty is to make the transformation from the signals of the network card to infrared signals.

    Source [3]

    The most challenging configuration but also the one which offers the most freedom, is the non-directed non-light-of-sight. The transmitter sends signals in a wide angle to the ceiling and after one or several reflections the signal arrives at the receiver. The biggest problem in this configuration is the multipath dispersion: after several reflections on different paths a short emitted signal is received as a wide signal. This can cause inter-symbol-interference (ISI) at higher data rates or larger cells. In this configuration the data rate depends on the room size. A mathematical model for this effect is described in [1]. The channel bandwidth-room length product for a room with 3m-high (9ft) walls is about 60Mbps.m . That limits the data rate in a 5x5m (15x15ft) large room to 12Mbps (60Mbps.m/5m).

    Larger bit rates can be obtained with other configurations where the transmitter sends the signal to a designated area on the ceiling and the receiver is facing that area (quasi diffuse transmission figure 1(d)) or where a line-of-sight exists between transmitter and receiver. In [2] the authors refers to a potential application of a high-speed wireless LAN figure 2. The Network consists of several base stations forming a backbone which is connected to a server and eventually to a wired network, and an arbitrary number of mobile stations. The base stations are fixed on the ceiling and in line-of-sight to the mobile stations. In large rooms it is possible to use more than one base station to cover the whole area. In rooms without base stations the portable stations should be able to communicate with one another (ad hoc networking) through a non-directed non-line-of-sight link, where the ceiling acts as main reflector. To provide a full duplex connection, different wavelengths or subcarrier could be used. The latter is better suited for ad hoc networking, as easier to implement in a portable station.

    Source [3]

    3.3 Modulation techniques for infrared channels

    The following modulation techniques use an intensity modulated channel. The main difference between an intensity modulated infrared channel and a conventional wired channel is that the i.m.-channel can only deal with positive input, and the amplitude of the input signal is limited not the power. Different modulation techniques are presented, comparing their power- and bandwidth-efficiency at a given bit rate.
    3.3.1 On Off Keying (OOK)
    With OOK, the beam is on when transmitting 1 and off when transmitting 0. The signal must be coded for being able to distinguish between the transmission of zeros and the transmitter off status, and for synchronization. If the average power is P, then the power used to transmit a 1, must be 2P. The bandwidth efficiency is equal to the bit rate (Rb), as one bit can be transmitted with each signal change.
    3.3.2 Two Pulse Position Modulation (2-PPM)
    This modulation is identical to the Manchester signaling offset by a d.c. 1 is represented by the transmission of high-low (here 2,0 and not 1,-1 because of the offset) and o is coded as low-high. The power efficiency is the same as with OOK, but the used bandwidth is doubled. For each bit two signal steps are necessary.
    3.3.3 Binary Phase Shift Keying (BPSK)
    BPSK transmits a signal in cosine form where 1 is represented by a phase jump of 180 degrees and 0 is represented by no phase jump. John R. Barry shows in [3] pp.113-115, that BPSK suffers a power penalty compared to OOK of 1.5dB due to the additional d.c. and has a bandwidth efficiency of 2Rb. This power penalty is typical for intensity modulated channels and not found in conventional wired environments.
    3.3.4 L-level Pulse-Amplitude Modulation (L-PAM)
    L different amplitude values of a cosine signal can be transmitted over a channel. It is possible to code log2(L) bits in each signal step. For example with L=4 amplitude values 2 Bits can be coded in one step. As result the used bandwidth is Rb/log2(L).
    3.3.5 N-subcarrier Binary Phase Shift Keying (N-BPSK)
    When only one subcarrier is used (N=1), then N-BPSK is equal to BPSK. For N>1 this modulation scheme uses BPSK for each subcarrier.
    3.3.6 N-subcarrier Quadriphase-Amplitude Modulation (N-QPSK) or (N-4-QAM)
    For every subcarrier 4 different phase jumps are defined.
    3.3.7 N-subcarrier L-level Quadriphase-Amplitude Modulation (N-L-QAM)
    For every subcarrier L different phase jumps are defined.
    3.3.8 L-level Pulse Position Modulation (L-PPM)
    This technic has a better power efficiency than the techniques mentioned earlier. But the needed bandwidth is larger. This method is widely used in satellite- and fiber-optic-systems. The input bits are first grouped in blocks of the length log2(L). Then every block is transmitted by choosing one of L different signal values.

    Source [3]

    Table 1 shows the power efficiency and bandwidth requirement for the modulations discussed above. When we compare the subcarrier- with the baseband-modulations, we notice that the later have a better power- and bandwidth efficiency. But the subcarrier modulations have some advantages over the baseband schemes. By assigning different subcarrier frequencies to different users, it is possible to achieve asynchronous multiple access. And subcarrier systems are more robust against multipath dispersion, since the boud rate in each sub-band is lower.

    3.4 Cellular architecture in infrared WLAN's

    The topics discussed in the chapter above are related to layer 1 of the ISO/OSI reference model. In this chapter we will address issues of the 2nd layer, eg. media access and topologies.
    3.4.1 Configuration of one cell
    A cell consists of one base station and a number of portable units. The base station is reachable form every portable unit (in line-of-sight or not). Conventional topologies can be used for a cell architecture, eg. bus structures or star configuration. When the portable units are placed in a way they can 'see' each other, then normal CSMA/CD could be easily used. If some portable units can not communicate with others, then it is necessary that the base station acts as a repeater and sends all received signals back to all portable units figure 3(a). To avoid interference of the up and down link, different wavelength or subcarrier could be used figure 3(b). Wavelength duplex is more complex and expensive to be used in portable units than subcarrier duplex.

    Source [3]

    3.4.2 Using more cells
    A single cell could be extended to cover the entire network. In this case an number of base stations cover the network area (eg. a building) and synchronous the same signals. This technic is called unison broadcast. This method can only be used for a small number of portable units and low speeds. Another problem is the multipath dispersion that occurs when a portable unit receives delayed signals from two or more base stations. The main problem in using different cells is that a portable unit could pass a boundary from one cell to another. We have to assure that the data exchange is not interrupted, and the portable unit can work in the new cell. With this aim in view, cells cold be configured in a way, that they overlap. In two adjacent cells different frequencies could be used as subcarrier. In the boundary area, a portable unit receives and sends signals to both cells, so that no dead zone exists.

    4 Comparison of Radio Frequency and Infrared Communication

    Table 2 summarizes some properties of RF and IR channels. RF WLAN's are better suited for use in areas where the distance between transmitter and receiver is big and no high speed communication is necessary. IR can be used in areas where radio frequency noise is present eg. factories or where cell architectures are easy to design like in office buildings where each cell could be a small room. Security is a problem in both RF and IR systems and encryption should be used. IR communication can provide high data speeds because of the high bandwidth that can be used.

    References:

    [1] John R. Barry, J. M. Kahn, E. A. Lee, and D. G. Messerschmitt. 'Simulation of Multipath Impulse Response for Indoor Diffuse Optical Channels'. Proc. of IEEE Workshop on Wireless Local Area Networks. May, 1991. Worcester, Ma. USA. pp.81-89.

    [2] John R. Barry, J. M. Kahn, E. A. Lee, and D. G. Messerschmitt. 'High-Speed Nondirective Optical Communication for Wireless Networks'. IEEE Networkmagazine. November 1991. pp.44-54.

    [3] John R. Barry, 'Wireless Infrared Communications'. Kluwer Academic 1994.