DSSS PHY Specification for the 2.4 GHz Band Designated for Industrial, Science, and Medical (ISM) Applications

Overview

The PHY for the DSSS system is described in this clause. The radio frequency (RF) LAN system is initially aimed for the 2.4 GHz band designated for ISM applications as provided in the United States according to FCC 15.247 and in Europe by ETS 300-328. The DSSS system provides a WLAN with both a 1 and 2 Mbps data payload communication capability. According to the FCC regulations, the DSSS system will provide a processing gain of at least 10 dB. This will be accomplished by chipping the baseband signal at 11 MHz with an 11-chip pseudonoise (PN) code. The DSSS system uses baseband modulations of differential binary phase-shift keying (DBPSK) and differential quadrature phase-shift keying (DQPSK) to provide the 1 and 2 Mbps data rates, respectively.

Scope The PHY services provided to the IEEE 802.11 WLAN MAC by the 2.4 GHz DSSS system are described in this subsection. The DSSS PHY layer consists of two protocol functions:

A physical layer convergence function, which adapts the capabilities of the physical medium dependent (PMD) system to the PHY service. This function will be supported by the PLCP, which defines a method of mapping the IEEE 802.11 MAC MPDUs into a framing format suitable for sending and receiving user data and management information between two or more STAs using the associated PMD system.
A PMD system, whose function defines the characteristics of, and method of transmitting and receiving data through, a WM between two or more STAs each using the DSSS system.

DSSS PHY Functions The 2.4 GHz DSSS PHY architecture is depicted in the reference model in Figure 5-9. The DSSS PHY contains three functional entities: the PMD function, the physical layer convergence function, and the layer management function. Each of these functions is described in the following section. The DSSS PHY service will be provided to the MAC through the PHY service primitives.

PLCP Sublayer To allow the IEEE 802.11 MAC to operate with minimum dependence on the PMD sublayer, a physical layer convergence sublayer is defined. This function simplifies the PHY service interface to the IEEE 802.11 MAC services.

PMD Sublayer The PMD sublayer provides a means to send and receive data between two or more STAs. This clause is concerned with the 2.4 GHz ISM bands using direct sequence modulation.

Physical Layer Management Entity (PLME) The PLME performs management of the local PHY functions in conjunction with the MAC management entity. The models represented by figures and state diagrams are intended to be illustrations of functions provided. It is important to distinguish between a model and a real implementation. The models are optimized for simplicity and clarity of presentation; the actual method of implementation is left to the discretion of the IEEE 802.11 DSSS-PHY-compliant developer. The service of a layer or sublayer is a set of capabilities that it offers to a user in the next higher layer (or sublayer). Abstract services are specified here by describing the service primitives and parameters that characterize each service. This definition is independent of any particular implementation.