Received Signal Weakness (RSW) Metric
Futurewei Inc.
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Routing
Mobile Ad Hoc Networks [manet]
Mobility
Metric
The Received Signal Weakness (RSW) metric is a simple cost metric that
enables selection of a route with the high end-to-end signal strength.
It is often desirable to identify which of several available routes
offers the best signal strength for data transmission, de-emphasizing
other considerations such as number of hops. However, signal strength
is in certain ways less suitable for use as a routing metric; in
particular, the signal strength of a path with several hops is not
as easy to calculate as cost metrics such as hop count.
Instead of signal strength, we calculate a metric proportional to the
weakness of the signal, in order to obtain a cost metric. The route
having the links with the best signal strength is then chosen in
preference to other routes, by choosing the route presenting the
lowest cost as measured by the Received Signal Weakness (RSW) metric.
The total signal weakness cost for a route is the sum of
the signal weakness measurements at each hop, so that the RSW cost
metric is additive, monotonic, and easy to calculate.
The received signal strength for packets received from a neighbor
is an important factor relevant to the reliability of the link
between the receiving node and its neighbor. Notice that the received
signal strength can vary over time even if the neighboring devices
are not moving.
For a route R as follows composed of links between nodes N_1 ... N_k:
N_1 <--> N_2 <--> N_3 <--> .... <--> N_k
denote the link between
N_{i} and N_{i+1} by L_{i,i+1} and the received signal weakness over
link L_{i,i+1} by RSW_{i,i+1}.
The RSW cost for route R is the
sum of the RSW costs for each link, or in other words
M_rsw(R) = SUM M_rsw(L_{i,i+1}) [i == 1..k-1], where M_rsw is the
metric value for the RSW metric.
The received power as measured (say, in mW) for incoming packets may
have quite a large dynamic range, but the measurements are also quite
variable and so great precision is unlikely to be required. In order
to fit in eight bits, the received power measurement is normalized to
be within the range from 0 to 1, where the minimum measurable power
P_min maps to 1 (the highest cost value) and the maximum measurable
power P_max maps to 0 (the lowest cost value). In other words, the
measured received power P_meas maps to a normalized value
P_norm = (P_max - P_meas) / (P_max - P_min).
It is desirable to increase the cost of low signal strength so
that weak signals are strongly disfavored. For this purpose, P_norm,
which is a positive number no greater than 1, can be exponentiated.
Using RSW_exponent = (1/8) is proposed for this purpose, and
effectively reduces the cost associated with using links that have
good measured values for the received signal strength.
For the purposes of this initial draft, it is proposed to use
a precision that can be carried in an 8 bit metric. That would
allow Max_RSW to attain the value 255, but that value should be
reserved to indicate a route cost of "infinity"; i.e., the route
cost is too large to be represented. For that reason, Max_RSW is
defined to be 254. In addition, we define Min_RSW to be 1, so that
there is some nonzero RSW cost for every link even if the measurement
of the received signal strength is the same as P_min.
These definitions of Max_RSW and Min_RSW determine the scaling
factor for P_norm, namely (Max_RSW - Min_RSW).
Given the scaling factor and shaping function P_norm^RSW_exponent as
above, the RSW metric is defined as
M_rsw = floor((Max_RSW-Min_RSW) * (P_norm^RSW_exponent)) + MinRSW
To be useful with AODVv2 , it is
helpful to define functions Cost() and Loop_Free() for the RSW metric.
The purpose of the Loop_Free() function is to provide assurance that a
selected route is loop-free.
The definition of the Cost() function for RSW is exactly
the same as the RSW metric, M_rsw. In other words, using RSW,
Cost(L) = M_rsw(L) and Cost(R) = M_rsw(R) for a link L and a route R.
For routes R1 and R2, Loop_Free(R1, R2) for RSW is defined
as follows:
LoopFree(R1,R2) := M_rsw(R1) < M_rsw(R2)
or, in other words, LoopFree(R1,R2) returns TRUE if the cost of R1
is less than the cost of R2 (cost as measured by the RSW metric).
This document does not introduce any security mechanisms,
and does not have any impact on existing security mechanisms.
The routing metric defined in the document should
be assigned a value from the "AODVv2 Metric Types"
registry .
RSW for IEEE 802.15.10 Layer-2 Routing
(https://mentor.ieee.org/802.15/dcn/15/15-15-0925-03-0010-received-signal-weakness-rsw-metric-specification.docx)
https://mentor.ieee.org/802.15/dcn/15/15-15-0925-03-0010-received-signal-weakness-rsw-metric-specification.docx
https://mentor.ieee.org/802.15/dcn/15/15-15-0925-03-0010-received-signal-weakness-rsw-metric-specification.docx