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Industry opens eyes to address 75% under-utilisation of cell site backhaul
At CBNL, we have a philosophy of exploiting the statistical properties of mobile backhaul traffic to build efficient and cost-effective networks.
We've made extensive measurements of 2G, 3G and 4G backhaul traffic and our most striking observation is that the backhaul peak-to-mean ratio in data-dominated mobile networks averages about 4:1.
The logical consequence - assuming you don't constrain the RAN by under-provisioning the tail links to nodeB’s (and who would do that?) - is that the mean utilisation of traditional point-to-point (PTP) microwave links is only about 25%.
So we were pleased to see one PTP vendor present data that confirmed this low average utilisation at the CEPT SE19 spectrum engineering working group meeting in Copenhagen in March.
Slide 7 in the vendor’s presentation confirmed that 60% of links in one large mobile operator's network in Europe operate at 30% or lower utilisation.
Why is that?
Well, imagine we have a nodeB with the average peak-to-mean ratio in its traffic of 4:1.
Let's say its peak backhaul demand is 40Mbps to make the numbers easy (about right for a very busy HSPA+ tri-cell).
I'm going to provision exactly 40Mbps of PTP microwave backhaul to this node B.
I know the peak load is 40Mbps so using the peak-to-mean ratio of 4:1 I can derive that the mean load is 40 / 4 = 10Mbps.
Therefore, since there is no other source of traffic on the PTP link I can immediately calculate its utilisation, which is defined as the mean load over the capacity: 10Mbps / 40Mbps = 25%.
How backhaul can improve utilisation and operator profitability
At CBNL, we wouldn’t settle for our radios transmitting idle patterns for 75% of the time, so we implement a point-to-multipoint (PMP) multiple access scheme.
This means there is more than one source of offered load for the radio link, and so we can very easily achieve much higher air-link utilisation than PTP.
This translates directly into lower spectrum lease costs and other opex savings for operators.
With rising data traffic, mobile operators will not be able to operate profitable networks with inefficiencies of this scale spread throughout their network!
It's great to see other thought leaders in the industry facing up to this issue.
Of course, we think point-to-multipoint microwave is a better solution than the peculiar asymmetric channel arrangement that certain PTP vendors are proposing - more details on that another time.