Radio Physical Layer RADIO ACCESS LAYERS

WIRELESS ATM SWITCHES 344 w x for multimedia information access using a fast WATM network 19 . The WAND demonstrator operates at up to 25 Mbitrs in the 5-GHz frequency band, and the project is also investigating performance issues related to bit rates exceeding 50 Mbitrs in the 17-GHz frequency band. The overall goal is to design a WATM access network demonstration system that can be commercialized and standardized in the European Telecommunications Ž . w x Standards Institute ETSI . Other European projects include MEDIAN 20 w x and SAMBA 21 .

12.3 RADIO ACCESS LAYERS

A high-speed but low-complexity wireless access technique is critical for providing QoS-based multimedia services to portable terminals. This section outlines wireless-specific protocol layers. These include a radio physical Ž . Ž . layer, a medium access control MAC layer, and a data link control DLC layer. 1

12.3.1 Radio Physical Layer

WATM requires a high-speed radio technology capable of providing reason- ably reliable transmission and reception in the range of 100᎐500 m. WATM systems may operate in various frequency bands depending on the regulatory policies. Currently, they are usually associated with the recently allocated w x 5 GHz band in the US and the HIPERLAN 22 band in Europe. The expected operating frequency range is on the order of 20᎐25 GHz. Typical target bit rates for the radio physical layer of WATM are around 25 Mbitrs, and a modem must be able to support burst operation with relatively short preambles consistent with transmission of short control packets and ATM cells. The radio physical layer can be divided into the radio physical medium Ž . Ž . dependent RPMD and the radio transmission convergence RTC sub-layers w x 25 . The RPMD sublayer specifies the lower-level transmission require- ments, while the RTC sublayer specifies details of formatted data transmis- sion. w x In 12 , the RTC sublayer adopts a dynamic TDMArTDD approach where several virtual circuits are multiplexed in a single radio channel. The TDMArTDD frame structure is shown in Figure 12.3. Ž . The frame consists of 480 slots every 1.5 ms 25.6 Mbitrs . Each slot is formed by 10 bytes, including 8 data bytes and a 2-byte Reed᎐Solomon code w Ž .x Ž . RS 10, 8 for FEC. The frame is divided into an uplink mobile to base and Ž . a downlink base to mobile part. The downlink subframe consists of the 1 w x In 22 the radio access layers consist of a radio physical layer and a radio DLC layer that Ž . contains a MAC sublayer and a logical link control LLC sublayer. RADIO ACCESS LAYERS 345 Fig. 12.3 Dynamic TDMArTDD frame structure. Ž . Ž . modem preamble P , the subframe delineation overhead O , and the Ž . control region C , followed by WATM cells. The preamble is required for TDMA frame delineation and modem synchronization. The frame header in Ž . the downlink subframe delineation overhead O consists of a frame number, Ž . a radio port identifier, and reser®ed bytes. The number of control packets C Ž . delineates the control region. Control packets 8-byte are embedded in the TDMArTDD slots. They are used by the MAC and DLC layers. WATM Ž . cells 56-byte transport multiplexed downlink user information. The base station controls the uplink bandwidth allocation for WATM cells from each mobile, taking into account the number and type of active WIRELESS ATM SWITCHES 346 connections and their bandwidth requirements. Mobiles assemble a subframe similar to that of the base station. The slotted ALOHA region is used by the mobile terminals to send their bandwidth requirements to the base station.

12.3.2 Medium Access Control Layer