LTE Backhaul Requirements: A Reality Check

12 Dec

LTE mobile broadband technology is now being launched across the world with more than 140service providers committed to implement it within the next two years. 3GPP, the LTEstandards defining body, has detailed the requirements of the new radio and core networkdomain, but not for the packet backhaul network linking the two domains together. Therefore,the industry has been left to extrapolate requirements for the backhaul network based on thebest-case radio interface capabilities. This leads to exaggerated predictions and is insufficientto plan backhaul networks. This paper estimates backhaul capacity more realistically bycombining factors of LTE channel capacity, radio propagation, cell site design and trafficaggregation.

Base stations transmit and receive user traffic over the assigned LTE radio or channel. Trafficvolume is directly proportional to the LTE radio channel size. 3GPP defined channels of 5MHz,10MHz, 20MHz and others in the R8 standard. The standard also defines radio interfacespectral efficiency targets for download (5 bits/Hz) and for upload (2.5 bits/Hz) of this radiochannel.To estimate the available over-the-air LTE radio peak rate, we need to look at how an LTEradio base station maximizes the available radio channel. The main parameter is the receivedsignal quality measured at the handset. The closer a handset is to the base station, the betterthe radio signal quality and the higher the radio peak rate. LTE defines three modulationsettings: QPSK, 16QAM and 64QAM. The best results are achieved using 64QAM, but this isonly available very close to the cell centre and represents the maximum peak rate (100%).16QAM corresponds to 66% and QPSK to 33% of this maximum. This reduced peak rate isavailable throughout most of the cell coverage area, with QPSK right up to the cell edge.All LTE peak rate figures assume the use of the entire LTE cell by a single user handset andinclude the radio layer 1 and layer 2 overhead of 10-25% (e.g., error correction, MAC control).In reality, multiple handsets will share the available radio resources of the LTE cell, resultingin a lower peak rate as well as average throughput per user.Figure 1 below provides an overview of realistic user handset peak data rates. As an example,the calculation for a 10MHz LTE spectrum at medium cell centre distance is as follows:Theoretical peak data rate (50Mbps) x 66% (16QAM) x 85% (minus L1&2 overhead) = 28 Mbps.




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