User's Manual Chapter 13

Chapter 7: Operational Description
AT&T Wireless Services
7-93
FCC Type Acceptance Application
10991 Rev. 1.0 7/6/99
Interactions between the physical layer and MAC layer entities are
handled through the definition of abstract primitives that define the
services provided by each layer. The primitives are passed between the
physical and Medium Access Channel (MAC) layer entities at Service
Access Points (SAPs).
7.2.1.1 Services Provided by the Physical Layer
The physical layer provides the following services to the Voice-
Medium Access Layer (V-MAC), Digital-Medium Access Layer (D-
MAC), as well as the VTch interface:
• Transmission and reception of voice and data traffic
• Transmission and reception of control information over the
airlink channel between the RU and the Base
• Forward Error Control (FEC) and detection of messages
corrupted during the transmission or reception process
• RU frame and bit-level synchronization to global time references
transmitted by the serving Base Station
7.2.2 PWAN Airlink Overview
The PWAN airlink is based upon an Orthogonal Frequency Division
Multiplex (OFDM) transmission system. OFDM was specifically
designed to counteract the adverse effects of multipath delay spread in
the radio-frequency environment, while at the same time supporting
high data rates. To accomplish this, OFDM converts a single high speed
serial data stream into several low-speed parallel data streams for
transmission across the airlink medium. At the receiving end of the
airlink, the parallel data streams are re-combined into a single high-
speed serial data stream. By converting from high speed serial to low
speed parallel transmission, the amount of RF energy per bit is greatly
increased due to the lengthened data symbol period. In order to support
parallel transmission at a high effective data rate, an OFDM signal is
made of a large group of sinusoidal carriers or “tones”, each of which is
phase and amplitude modulated to carry unique data symbol
information.
OFDM compensates for the time delay spread of a multipath
propagation channel by transmitting each data symbol as an RF burst,
with a brief “guard interval” separating the end of each burst from the
beginning of the next (refer to Figure 7.8—).