Wireless Future - Telenor

has to pass via the AP, even for communication
between two MTs associated to the AP. It is
assumed that the majority of traffic for the centralised
mode is between entities where only one
is associated to the AP. The other network topology
is called ad-hoc or direct mode, and is used
for communication between two MTs associated
with the same AP. Here, the user data is transmitted
directly between the two MTs and not via
the AP. The control information, however, is
transmitted via the AP, and the AP still controls
the medium access.
The HiperLAN/2 standard covers the two lowest
layers of the OSI model (physical and data link
control layers) and in addition several core network
specific convergence layers (CL) are
defined to allow HiperLAN/2 to be used as an
access technology for UMTS, Ethernet, IEEE
1394. The protocol stack with the three basic
layers; Physical layer (PHY), Data Link Control
layer (DLC), and the Network Convergence
layer (CL) is shown in Figure 3. Next, descriptions
of the main features associated with the
different layers are attempted. Other overview
articles on HiperLAN/2 can be found in [2], [3]
and [4].
3 Description of the
Different Layers
3.1 Physical Layer
In broad terms the physical layer receives (or
delivers) data (called protocol data unit, PDU)
from the MAC layer, then performs scrambling,
coding (FEC) and interleaving on the data before
multiplexing the data on to a number of orthogonal
sub-carriers. This multiplexing process is
also called Orthogonal Frequency Division Multiplexing
(OFDM) [5], and is a special form of
multicarrier modulation. By splitting the high
rate data stream into several lower rate parallel
data streams the signals are less sensitive to
inter-symbol interference (ISI), which can be
Parameter Value
Channel spacing (and system clock) 20 MHz
FFT length 64
Number of used subcarriers 52
Number of data carriers 48
Number of pilot carriers 4
Telektronikk 1.2001
a problem in multipath environments. Another
important feature of the physical layer is that
seven different modulation and coding modes
can be used on the sub-carriers and are displayed
in Table 2. This makes link adaptation possible
where the most appropriate PHY mode is used
according to the current radio channel and application
requirements. Mode 1 (BPSK) represents
the most robust mode (lowest error rate for a
given C/I) and gives a bit rate of 6 Mbit/s,
whereas mode 7 gives the highest bit rate of
54 Mbit/s but with the disadvantage of being
more sensitive to C/I. More detailed discussions
on the performance of the physical layer can be
found in [6] and [7].
The basic parameters of the physical layer are
shown in Table 3.
Figure 3 Layer model of the
HIPERLAN/2 system
Mode Modulation Coding rate R Nominal bit rate [Mbit/s]
1 BPSK 1/2 6
2 BPSK 3/4 9
3 QPSK 1/2 12
4 QPSK 3/4 18
5 16QAM 9/16 27
6 16QAM 3/4 36
7* 64QAM 3/4 54
Modulation schemes on subcarriers BPSK, QPSK, 16 QAM (64 QAM optional)
DLC
Demodulation Coherent
Radio Link Control
(RLC)
Convergence Layer
(CL)
Error Control
(EC)
Medium Access Control
(MAC)
Physical Layer
(PHY)
Table 2 PHY modes defined
for HIPERLAN/2 (* optional)
Guard Interval length 800 ns (400 ns optional)
Channel coding Convolutional code, constraint length 7 Table 3 Basic PHY layer
parameters
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