SON Offload Method 3 Results

Method 3 took just 23 steps (Table 7.2) to deliver its optimised solution (Figure 7.8) and actually because of its ability to jump to the correct tilt and power setting for each change, rather than the trial and error approach adopted by the other algorithms, within its first nine steps (Figure 7.9) Method 3 had delivered more contiguous coverage to the high traffic area of KHS than any of the other algorithms did over all of their entire steps. Whilst Method 3 did not provide as great an increase in the microcell coverage areas (235%) as Method 1, the coverage increases it did provide was precisely aimed at the high traffic area of KHS, which in turn at increased traffic offload onto the microcells. For this reason Method 3 provided the greatest traffic offload (175% microcell traffic increase) onto the microcells of all the four SON methods considered. It also provided contiguous coverage along KHS whilst maintaining the users Ec/Io outage levels below the target level of 2% (Figure7.10). Method 3 led to a reasonable 1.7dB reduction in the macrocell RSCP coverage across the cells of the central 2x2km region of the simulation area.

Existing Macro BTS KHS Lamppost Microcells

Figure 7.8 Microcell best server coverage areas delivered by Method 3.

Figure 7.9 Percentage of the KHS high traffic area covered versus the optimisation steps

taken to achieve these coverage levels for the four SON methods considered.

Figure 7.10 User average Ec/Io and Ec/Io outage delivered by Method 3 for the Central

Area (CC) and KHS area.

Existing Macro BTS KHS Lamppost Microcells 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 1 11 21 31 41 51 61 71 81 91 101 111 121 131 141 151 161 Co ve ra ge P er cen ta ge Optimisation Step

Micro Cell Coverage Area (% KHS Area Covered)

Method 1 Method 1+ Method 2 Method 3 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6% 1.8% -13 -12.8 -12.6 -12.4 -12.2 -12 -11.8 -11.6 -11.4 -11.2 -11 -10.8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Use r O u ta ge Ec/I o [d B] Optimisation Step

Method 3: User Mean Ec/Io & Ec/Io Outage CC User Ec/Io KHS User Ec/Io % CC Outage

7.6 Chapter Summary

This chapter has presented four proposed SON methods developed to maximise traffic offload from a 3G/WCDMA macrocell network onto low power street level 3G/WCDMA microcells deployed on a shared carrier frequency with the macrocell layer. Two out of the four algorithms use microcell based measurements only and the remaining two have been designed around the automatic collection and mapping of geo-located UE measurement reports on to a series of what has been termed here 3D Xmaps.

The first microcell based measurement method (Method 1) used a majority voting approach based upon downlink measurements at each lamppost mounted microcell to target the surrounding macrocells seen to be causing the most interference. Method 1’s algorithm was then able to tilt and down power these surrounding sites without worrying about the consequences of its actions on the performance of the macrocell layer and whilst this led to a greatly increased microcell coverage area (albeit in the wrong areas), it also led to a severe RSCP coverage reduction (4.3dB) on the macrocell layer. Method 1 took 162 steps to reach its final solution and clearly as explained earlier, if each step was reliant on having a day’s worth of network statistics in order to assess the effect of each change, then an algorithm taking this many steps to arrive at a solution would not be practical to deploy in a real network. Method 1 also did not achieve contiguous coverage along KHS and for this and the previous two reasons Method 1 is discounted as being a suitable SON method for microcell offload.

The second microcell based measurement method (Method 1+) again used majority voting to target the surrounding macrocells, but this time the algorithm also monitored the effect each change had on the overall macrocell interference level seen across all of the microcells on KHS. If the macrocell change increased the interference level seen by the microcells then the macrocell that had just been tilted had its tilt reverted back to its previous configuration and then that cell was excluded from any further changes. This approach certainly ensured that macrocells were not downtilted towards KHS and it also avoided the situation seen from Method 1’s results where many of the surrounding macrocells were downtilted to their maximum values, resulting in significant coverage reduction on the macrocell layer. Method 1+ provided a reasonable coverage and traffic increase and did this in just 26 steps making it a practical algorithm based on the use of daily networks statistics between each change. It also provided contiguous microcell coverage along KHS.

The first of the Xmap based methods, Method 2, selected its target macrocells based upon the coverage overlap seen between the micro and macrocells and then cautiously optimised the macrocell network by ensuring each macrocell change did not result in the macrocell’s coverage area being reduced by more than 10% from its original value. Whilst the algorithm aimed to maximise coverage rather than traffic carried, it still failed to provide contiguous

coverage along KHS. Method 2 took 77 steps to achieve its final solution and because of this and its failure to provide contiguous coverage Method 2 is discounted as being a suitable SON method for microcell offload.

The second of the two Xmap based methods, Method 3 was built upon on Method 2’s Xmap approach but including the pseudo post reports in its decision making process. It also reduced the number of steps required to achieve its final solution by predicting the effect each proposed change would have on the macrocell and microcell networks. Method 3 operated by targeting traffic offload onto the microcells rather than purely additional microcell coverage and by doing so delivered the greatest increase in microcell traffic of all the four SON methods considered. Method 3 took just 23 steps to reach its final solution, but had provided most of its gains within the first 9 steps.

In summary two out of the four proposed methods appear to be viable SON based methods for increasing the traffic offload onto the microcell layer. The proposed post-based measurement method, Method 1+ appears from simulation to be a very simple yet effective SON method in order to increase the coverage and the traffic carried by the microcell layer without having a negative impact on the surrounding macrocells’ network performance. It also lends itself to a simple implementation within the Radio Access Network (no need for an MDT server). However, the proposed Xmap based approach of Method 3 appears from simulation to be the most effective method at increasing both microcell coverage and traffic offload onto the microcell layer. The results from both these two approaches suggest that measurement based (microcell or UE) closed loop SON methods for maximising small cell offload do appear to be a realistic possibility and are worthy of further investigation.

Chapter 8