GO ASIA TRAIL 
I published the following article in March this year, but things have changed since then and I switched from ahamo to Rakuten Mobile.
We think it is important to be able to connect to the internet with a smartphone anywhere, not only for trail running expeditions overseas, but also for normal sightseeing purposes to make international travel more comfortable. In the past, the common practice was to purchase a prepaid SIM after arriving at the local airport and connect with a SIM-free device.
However, since ahamo appeared in March 2021, several Japanese services have introduced plans that allow overseas data roaming as is for a monthly fee. However, since ahamo was introduced just as the Corona Disaster was happening, it was difficult to understand the advantages of seamless overseas data roaming, but I was able to experience this on the Formosa Trail when overseas travel was liberalized after the fall of 2022.
The collapse of the docomo quality myth
It seems to have been reported on SNS since early 2023, but I myself felt that this docomo's line quality had dropped around April 2023, I think.
I felt that it was inevitable that communication was unstable in Osaka's downtown areas such as Umeda and Shinsaibashi, but even in the suburbs of Osaka where I live, the quality of the line was deteriorating and so-called "packet jamming" occurred, resulting in a noticeable slow or no communication.
And when I went to Taiwan in April 2023, the performance of international data roaming on ahamo's line was clearly worse than the previous November 2022, and I could not communicate comfortably. At that time, I thought it was inevitable because the first two days were in Kenting, a rural area in the southernmost tip of Taiwan, but the same problem continued to occur after moving to Tainan. I thought this was a bad idea, so I made a new contract for a local prepaid SIM in Tainan and tried to use it, and it was very comfortable.
International roaming means borrowing a local line and going directly to connect to the network in Japan. As a result, it connects to DOCOMO's network in Japan, so I am guessing that this is where the bottleneck was.
DOCOMO's poor line quality is a mistake in DOCOMO's 5G network strategy
We used to hear "docomo is expensive, but the quality of the line is good, so you can use docomo" or "docomo has the widest area, even in rural or mountainous areas," but this will not work at all anymore in 2023.
I am not a specialist in the details of mobile communication technology, so I cannot discuss it in detail, but in a nutshell, I feel that one of the reasons is that they made a major error of judgment in the design and strategy of their 5G network. Although some improvements have been made at the tuning level, unfortunately, I feel that it will be tough to solve the problem fundamentally without also reviewing the design of base stations and backbones. I believe that this is not a story that can be recovered in a few months, and that the problem will probably continue for several years.
So for the time being, it will be tough to use docomo lines (not only docomo and AHAMO, but also MVNO lines).
https://japan.cnet.com/article/35210244/
Currently using Rakuten Mobile + IIJmio (KDDI line)
I remember a long time ago, the phrase "usable in the world but out of range at home" was popular on 2chan to make fun of Vodafone's poor line quality, and a phenomenon similar to this has been occurring since around April 2023. As expected, it became difficult to keep ahamo's line for overseas expeditions several times a year, so we sought to switch the line.
As a result, we now have the following lines
- Main: Rakuten Mobile (980-2980 yen per month (tax excluded))
- Sub: IIJmio (850 yen per month (tax included))
For my usage, Rakuten Mobile costs only ¥1980 per month (tax not included), and the monthly fee for both is almost the same as ahamo, and since the line is redundant, I have peace of mind even if one carrier should fail. Also, Rakuten Mobile offers free overseas data roaming up to 2GB per month.
When I traveled to Korea last month, I used Rakuten Mobile's overseas data roaming to connect to the Internet, and I had no problems with the quality of the connection. I was able to connect to the Internet just fine, and there was no time lag.
I was a little worried about the low data capacity of 2GB, but since I was connected to WiFi at the hotel, I was able to reduce the data capacity to 800MB for 4 days and 3 nights. (In Taiwan, WiFi in many hotels is slow, and in many cases, it is more comfortable to use a mobile connection after all.)
If you think 2GB is not enough at all, you might want to think about signing up for a local prepaid SIM. Even in that case, you can use it as a "bridge line" from the moment you arrive at the airport to the time you sign up for a prepaid SIM.
Rakuten Mobile's overseas data roaming capacity can be added to 1GB for 500 yen.
Rakuten Mobile's line quality is improving.
The quality of Rakuten Mobile's connection has improved considerably compared to when the service was first launched. When the service first started, I was allowed to use it when it was free for one year, but to be honest, it was not practical at that time. However, the quality of Rakuten Mobile's line in recent months has been getting better. Especially in downtown Umeda, Osaka, the connection is honestly better than docomo's.
When I launched the MUJI Passport app at MUJI in the Hankyu Umeda Station building, I had a problem with the MUJI Passport barcode not showing up at all with a docomo line, but with Rakuten Mobile, it showed up right away. Of course, Rakuten Mobile is weak in some buildings, but in that case, I can switch to a spare IIJmio line and there is no problem. I can live my life without any inconvenience.
And what about in the mountains, I have not felt any inconvenience in the mountains I often visit (Minoh - Ikeda, Rokko mountain range, Kyoto). In May 2023, I visited Akiyoshidai in Yamaguchi, Japan, and had no problem even with an optimistic connection. However, it is true that IIJmio (KDDI line) is still reliable in some places, so I think Rakuten Mobile + some kind of backup line (KDDI or SoftBank line) is preferable.
Conclusion
- You should get rid of docomo line for a few years.
- Rakuten Mobile + a fallback line is preferable.
- Rakuten Mobile comes with overseas data roaming up to 2GB per month.
Frankly, I would be very happy if Rakuten Mobile could expand its overseas roaming capacity to 3GB. I wonder how they will do it...it must be tough since the entire Rakuten Group is in the red right now due to the capital investment in Rakuten Mobile. However, the deficit is shrinking and the company is expected to be in the black by 2024. The decline in docomo's line quality may be a tailwind. I really hope Rakuten Mobile does well.