BaiTip of the Day - December 14th, 2016 - Subframes and Special Subframes


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Today’s tip is meant to help Baicells customers tweak their LTE settings to better fit their customer needs and demographics. This involves changing Subframe and Special Subframe Assignments.

The Baicells Nova R9 eNodeB operates in TDD mode using the Type 2 LTE Frame Structure. The operator can customize uplink and downlink bandwidth ratios using the Subframe Assignment.

As shown in the top section of the figure below, a LTE radio frame is composed of two half frames, each of 5ms duration resulting in total frame duration of about 10ms. Each radio frame will have total 10 Subframes and each Subframe will have 2 time slots. Subframe configuration is based on Uplink Downlink configuration (0 to 6). Currently, the Nova R9 uplink and downlink ratio assignment options are (1 and 2). Subframe Assignment (1) equals a DL:UL ratio of 2:2, while a Subframe Assignment (2) selection is a DL:UL ratio of 3:1. The default is Subframe Assignment (2) in the Nova R9.

DL to UL configuration number determines what goes in all the Subframes is mentioned below in the table. Usually in all the cases, Subframe #0 and Subframe #5 are always used by downlink. Highlighted in Green below is the default configuration for the Nova R9. As evident in the top graphic, special subframes (SSF) are introduced at subframe1 and subframe 6.

Baicells currently offers two Special SubFrame (SSF) options (5 and 7). The Nova R9 default is 7 (highlighted in green below). The Special Subframe carries a DwPTS (Downlink Pilot Time Slot), a GP (Guard Period) and a UpPTS (Uplink Pilot Time Slot). For the 5ms DL to UL switch point periodicity case, SSF (Special Subframes) exist in both the half frames. For the 10ms DL to UL switch point periodicity case, SSF exists only in first half frame.

Each half-frame of 5 ms carries one Special Subframe that is divided into 3 parts as shown in below figure: DwPTS (Downlink Pilot Time Slot), a GP (Guard Period) and a UpPTS (Uplink Pilot Time Slot). Subframe 0 and DwPTS are reserved for downlink; Subframe 2 and UpPTS are reserved for UL. Remaining fields are dynamically assigned between UL and DL.

This Special Subframe replaces Subframe 1. The individual time duration in OFDM symbols of the Special Subframe parts are adjustable.

  1. The Guard Period (GP) implements the DL->UL transition point and the GP has to be large enough to cover the propagation delay of DL interferers. Its length determines the maximum supportable cell size.

  2. DwPTS is considered as a “normal” DL subframe and carries reference signals and control information as well as data for those cases when sufficient duration is configured. It also carries Primary Synchronization Signals (PSS).

  3. UpPTS is primarily intended for Sounding Reference Signals (SRS) transmission from UE. UpPTS is mainly used for Random Access Procedure (RACH) transmission.

The customer UE always needs a Guard Period in order to switch from receiver to transmitter. The Guard Period includes RTD (Round Trip Delay) as shown in figure below. Thus, if an operator wants to achieve more distance involving a longer Round Trip Delay, a longer Guard Period (GP) may need to be used. In this case, the Special Subframe Assignment should be changed to 5.

If distances over 6 miles/10 kilometers are needed in your deployment. The operator can change the Zero Correlation Zone Config from 10 to 12 by going to LTE Settings->Random Access Parameters in the eNodeB WebGui.

These changes should also improve performance for your longer distance clients. Our theoretical limitations at this time with these changes are 10 miles/16 kilometers. Below are simulation results for Subframe 2 and Special Subframe 7 testing in a lab environment. Actual outside results may be impacted by variables such as wind-blown foliage, interference, backhaul capacity and other attenuating circumstances not normally seen in a lab environment.