A model is developed to predict traffic levels in transport networks used to backhaul LTE eNodeBs.
Backhaul traffic is made up of a number of different components of which user plane data is the largest,
comprising around 80-90% of overall traffic, slightly less when IPsec encryption is added. The remainder
consists of the transport protocol overhead and traffic forwarding to another base-station during handover.
Network signalling, management and synchronisation were assumed to be negligible.
User plane traffic was depends on the characteristics of cell throughput that can be delivered by the LTE air
interface. Simulations of LTE cell throughput showed very high peaks were possible, corresponding to the
maximum UE (user equipment) capabilities of up to 150Mbps. However, such peaks were only found to
occur under very light network loads of less than one user per cell. During ‘busy times’ with high user traffic
demands, cell throughputs were significantly lower than the quiet time peaks: A heavily loaded 20MHz 2x2
LTE downlink cell limits at around 20Mbps cell throughput. In this scenario, the overall spectral efficiency of
the cell is brought down by the presence of ‘cell edge users’, with poor signal quality and correspondingly low
These results reveal that the cell throughput characteristics for data carrying networks are quite different to
those of voice carrying networks. In a data dominated LTE network, the peak cell throughputs in the
hundreds of Mbps will occur during quiet times. Conversely in voice dominated networks, cell throughput is
related to the number of active calls, hence peaks occur during the ‘busy hours’. Since cell throughput peaks
occur rarely and during quiet times, it is assumed that they do not occur simultaneously on neighbouring
cells. On the other hand, the ‘busy time’ mean traffic will occur on all cells at the same time. The total user
plane traffic for a tri-cell eNodeB (an LTE base station) is modelled as the larger of the peak from one cell, or
the combined busy time mean of the three cells. The same rule is applied to the calculation of traffic from
multiple aggregated eNodeBs.
For the LTE downlink, peak cell throughput is around 4-6x the busy time mean, so for backhaul traffic
aggregates of less than 4-6 cells typical of the ‘last mile’ of the transport network, it is the quiet time peak
that dominates capacity provisioning. For aggregates of 6 or more cells (e.g. two or more tricell eNodeBs), it
is the busy time mean that dominates provisioning of the ‘core’ and ‘aggregation’ regions of the transport
network. From a technical perspective, it may not seem practical to provision the last mile backhaul for a
peak rate that rarely occurs in practice. However, the ability to deliver such rates may be driven by marketing
requirements, as consumers are more likely to select networks or devices which can advertise higher
The results presented in this paper represent mature LTE networks with sufficient device penetration to fully
load all cells during the busy times. It is recognised that it may take several years to reach such a state, and
even then, not all cells may reach full load. The lighter levels of loading likely in the early years of the
network will reduce the ‘busy time mean’ figures applicable to the aggregation and core regions of the
transport network. However, the quiet time peaks if anything will be more prevalent, and so provisioning in
the last mile will have to accommodate them from day one.
The transport provisioning figures given this paper are provided as guidelines to help the industry understand
the sorts of traffic levels and characteristics that LTE will demand. They should not be interpreted as
requirements, and it should be recognised that provisioning may need to be adjusted according to the
particular deployment conditions of individual RAN sites. Results are given for a range of uplink and downlink
scenarios applicable to Release 8 of the LTE specifications. These include 10MHz and 20MHz system
bandwidths, various MIMO configurations, and different UE categories
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