Data Link Control Layer DLC
The main task of forwarding is, to handle the
transmission of data packets from the AP to the EPs and from
the EPs to the RMTs. Our investigations led to the so-called
BEACON concept (Figure 1) in time domain
for the multihop DLC, which is a suitable extension of the
H/2 DLC. H/2 conform MTs expect every 2 ms the broadcast of
a so-called BCH. The BCH contains general information like
net ID etc. and a pointer, which marks the beginning of the
FCH within the MAC frame. The FCH contains the information
about the structure of the current MAC frame and is therefore
different for each AP and/or EP. The BEACON concept relies
on the H/2 option for sector antennas, where different sectors
get their own BCHs and FCHs – which are serially transmitted
by the AP. In contrast to a single AP, where the AP transmits
the BCHs/FCHs for all sectors, here the EPs synchronize first
to the BCH of the AP and transmit afterwards serially their
own BCHs/FCHs. This avoids collisions on the air interface,
while each RMT sees a regular BCH from an EP or the AP.
Every second frame the AP/EPs transmit crosswise
empty frames. For this purpose the MAC scheduler at the AP/EP
has to know additionally that it is an AP/EP, i.e. a slightly
modified scheduler is needed for the AP and EPs. The MTs/RMTs
need no modification.
Figure 1: Schematic
DLC-protocol for BEACON concept
Figure 2 shows a general challenge
for forwarding. As forwarding requires at least two orthogonal
resources in time, frequency or code - to transmit data packets
in a first step from AP to EP and in second step from EP to
RMT – the gain in throughput and/or distance by EPs in
a LOS scenario is very small.
Figure 2: Maximum End2End
User Throughput vs. Distance for Forwarding under LOS Conditions
Under the assumptions described above the
achievable throughput decreases very rapidly with the number
of EPs. In Figure 3 a star arrangement of EPs – i.e.
all EPs are located around the AP – is investigated under
different PHY mode selections for the first hop between AP
and EP and the second hop between EP and RMT. As can be seen
already for 7 EPs throughput breaks down completely and no
user data can be transmitted at all. The simulations in Figure
3 have been done for the so-called SF-FSA-concept (Subframe
Fixed Slot Allocation). The BEACON concept performs only slightly
better. In the meantime we have proposed and analyze techniques
to decrease the protocol overhead.
Figure 3: End2End throughput
for multihop networks with SF-FSA DLC-protocol
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