Wireless LAN Standards

The IEEE802 standard has been in existence for over 10 years, and a number of U.S.
vendors have been producing wireless LAN products for commercial in-building
applications, that is, private access networks. The idea is to extend wireless LANs into
the public access network space using either the unlicensed Industrial Scientific Medical
(ISM) band at 2.4 GHz or the ISM band at 5 GHz.
Present IEEE 802.11 products use direct-sequence spread spectrum (DSSS). Each bit
transmitted is modulated by an 11-bit barker sequence (a pseudorandom sequence) to
give just over 10 dB of processing gain. The data is either differentially binary phase
shift keyed or differentially quadrature phase shift keyed onto the RF carrier, which
uses 25 MHz channel spacing. The 2.4 GHz ISM band allocated in the United States is
2.402 to 2.480 GHz, that is, 78 MHz giving 3 × 25 MHz channels with some guard band.
Afrequency-hopping physical layer is also specified and an infrared physical layer. RF
products to date have all been DSSS, and this seems likely to remain the case for the
foreseeable future.

The physical layer header determines which RF interface and modulation option are
used and defines the packet size and channel coding used (single block code parity
check). Some synchronization bits at the beginning of the header provide the basis for
locking onto the RF carrier and correlating to the PN-coded channel.
Direct sequence tends to be used because it provides better performance via more
processing gain and coherent demodulation. However, the RF PA needs to be linear,
and the usual IQ balance issues have to be addressed. The present MAC (Medium
Access Control) layer is Ethernet based and doesn’t support quality of service, but this
is presently being redefined (as 802.11e).
IEEE 802a defines the standard for a 54 Mbps LAN product at 5 GHz using OFDM.
This means that a similar multicarrier approach is being adopted by wireless LAN vendors,
digital TV (in Europe and Asia), and, as we will see later, a number of fixed-access
wireless vendors. This is a 300 MHz bandwidth allocation (12 × 25 MHz channels). Net
throughput per channel is 31 Mbps. 802.11g then backward-engineers this standard to
retrofit back into the 2.4 GHz ISM band with the option of using OFDM or DSSS. Net
throughput per channel is 12 Mbps.
Other flavors of IEEE 802 include 802.11d to cover application in other RF bands,
802.11f to address handover protocols, 802.11h to cover power control, 802.11I to cover
authentication and encryption, and 802.11j to cover 802.11/HIPERLAN interworking.
A number of companies are also promoting OFDM-based solutions with optimized
IP protocol backhaul and local area/wide area handover including billing integration.
One example is Flarion (www.flarion.com). There are also competing standards from
Japan (Home RF2 using frequency hopping) and a proposed future HIPERLAN2 (an
ETSI specification).
The addition of authentication and encryption to IEEE802 has made it possible to
deliver a public access network proposition, though issues still need to be resolved as
to how billing is managed as a user moves from a wireless LAN to cellular coverage.
The scenario is that a user arrives at an airport having been on the wide area cellular
network. The user’s laptop (with an integrated wireless LAN transceiver) locks onto
the local wireless LAN base station and downloads/uploads any files waiting to be
received/sent. If the user walked back out of the airport, the laptop would camp back
on to the cellular network.