https://ourtechplanet.com/lte-throughput-optimization-part-2-spectral-efficiency/
Solution type 1:Transition to Higher CQI layer
Consider two LTE layers, for instance L800 and L1800 with same bandwidth. In this case, L800 will
have a higher coverage as it is a lower frequency. So, the user count on L800 will be higher compared
to L1800. However, the lower frequency layer also has higher interference since it has a bigger
coverage radius. So, that will result in a lower CQI and a bad throughput. L1800 throughput will usually
be better even with same bandwidth because it will have better CQI. So, the most important thing is to
ensure that the layer with the better CQI gets most of the traffic. This can be done in many ways and I
have jotted down a few of those.
The easiest way is to give a higher priority to L1800 and that will shift most of the UEs in L1800
coverage away from L800. This will ensure better CQI for users and thus a better throughput. Another
way would be to keep them on same priority and provide a frequency offset to move the users to
L1800. This is more reasonable if L1800 is also getting overloaded then the amount of load to be
shifted can be tuned by varying the offsets.
I prefer load shifting by cell reselection instead of handovers. If the handover thresholds are
changed or frequency priority based handovers are used, then it initiates gap periods. For UE, to move
from one frequency to another frequency in connected mode, it needs to measure the target frequency.
In order to measure the target frequency, the UE goes into a gap mode of 6 ms. This gap mode
repeats itself after every 40 or 80ms. So, if it repeats every 40ms then that means that the UE cannot
be scheduled for 6ms in every 40ms. Moreover, when the UE gets data, it needs to send a HARQ
ACK/NACK after 4ms. So, it means that since the eNB knows that the UE will be in gap mode so the
eNB will not schedule any data for the UE 4ms before the gap mode. That makes it 10ms in each 40ms
that the UE cannot be scheduled which is around 25% of the time. So, inter-frequency handovers
should be minimized as it can cause a 25% degradation in throughput. Cell reselection works in idle
mode so it is a much better way to move users between the layers.
Load Balancing
Internal - General Use
Another way is to enable load balancing between the layers and ensuring that the higher CQI layer
gets more load. Load balancing usually also comes in two modes


Connected Mode: In this case, the eNB calculates the PRBs or user count and tries to maintain
target load values by performing load based handovers between the layers.
Idle Mode: In this case, the eNB sends the frequency in the RRC Release command to the UE.
eNB increases the priority of the target frequency for that UE temporarily and the UE tries to
reselect to that frequency in idle mode.
Once again, I prefer idle mode based load balancing as it does not introduce the inter-frequency
handovers and also gets the work done. But idle mode based load balancing will not have significant
impact in case the layers have different priorities since one layer already has higher priority and idle
mode based load balancing also moves users by increasing the priority. So, if the UEs are not moving
to higher priority layer than that means that the layer has coverage constraints and then the idle mode
based load balancing will also be unable to shift the load.
Vertical Beam-Width
Another important factor is that many times, the low band like L800 has a bigger vertical beamwidth
than the corresponding higher band. This effectively means that at the same tilt value, the L800 will
have a much bigger coverage foot print than the L1800. So, before making any mobility strategy, it is
important to verify the antenna patterns especially the vertical beam-width for all the layers. If the
beam-width of one layer is significantly wider than the other, then ensure to put a tilt offset between the
two to keep an optimum and balanced coverage.
– Scheduler Fairness:
Another important factor is the scheduler type. A scheduler can work in multiple modes
Round Robin: In this mode, the scheduler provides equal resources to all users. This is not an
optimum algorithm as different users have different data requirements.
Max C/I: This mode provides significantly higher resources to users in good coverage conditions. This
mode can starve the cell edge users and they will not get enough data resulting in degradation in user
experience.
Proportion Fair: This scheme maintains a fairness between all users maintaining a healthy resource
sharing between all user types. The basic concept of this mode is to strike a balance between users
and it does that by prioritizing based on CQI and data rates. So, if the CQI is high, it will give resource
to that user first but since it needs to maintain a fair data rate for all users, the cell edge users will also
be scheduled. This scheme is essentially a combination of both round robin and Max C/I as it provides
more resources to users with higher CQI as compared to round robin but it also provides more
resources to cell edge users when compared to Max C/I. Hence, it gets the name Proportional fair.
The user throughput KPI improves with Max C/I scheduler as it provides more resources to
good users resulting in higher user throughput but the cell throughput is improved with
Proportional Fair algorithm as it strikes a balance between all users. So, if the user throughput
KPI is to be improved then the scheduler can be tilted towards Max C/I while Proportional Fair can be
used if cell throughput gain is required. The optimization at this level really needs deep understanding
of the scheduler’s algorithm and it also depends if the specific vendor provides the options to play with
the scheduling weights.
These are the basics to improve the spectral efficiency for a network. In the next part, I will explain the
features that can be used to improve throughput along with the scenarios where they will be applicable.
Internal - General Use
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Throughput improvement in ZTE:“ucDLRetrRBOptSwtch” parameter in ZTE equipment. Throughput is improving by this, but this is also
increasing the BLER.
Parameter Description:
This parameter enables or disables the optimization for the retransmission of RBs in the downlink to
improve downlink spectrum efficiency. If it is set to Open, the RBs that are first retransmitted in the
downlink with a NACK response received are compressed.
Internal - General Use