Friday, May 01, 2015

Impact of a small cell on the RF-EMF exposure in a train

Joel's comments:  Given the prevalence of cell phone use on subways, buses, and trains, secondhand exposure to RF radiation can be significant and may pose health risks to passengers. See for example "Secondhand Exposure to Cell Phone Radiation: An Emerging Public Health Problem?"  Thus, it is important we develop strategies to minimize these exposures either through adoption of harm reduction policies or technologies.

The following study estimates the exposure of a passenger to radio frequency (RF) radiation on a train when only one cell phone is in use, both with and without a miniature mobile phone base station installed on the train.  Since the scenario under investigation involves specific mobile phone equipment and only a single user, the generalizability of the study results is rather limited.


Impact of a small cell on the RF-EMF exposure in a train

Aerts S, Plets D, Thielens A, Martens L, Joseph W. Impact of a small cell on the RF-EMF exposure in a train. Int J Environ Res Public Health. 2015 Feb 27;12(3):2639-52. doi: 10.3390/ijerph120302639.


The deployment of a miniature mobile-phone base station or small cell in a train car significantly ithe coverage and the capacity of a mobile network service on the train. However, the impact of the small cell on the passengers' exposure to radio-frequency electromagnetic fields (RF-EMF) is unknown. In this study, we assessed experimentally the RF-EMF exposure of a mobile-phone user who is either connected to the outdoor macrocell network or to an in-train small cell, while traveling on the train, by means of the absorbed-dose concept, which combines the base station downlink exposure with the mobile-phone uplink exposure. For Global System for Mobile Communications (GSM) technology at 1800 MHz, we found that by connecting to a small cell, the brain exposure of the user could realistically be reduced by a factor 35 and the whole-body exposure by a factor 11. 
Open source paper:
... several studies on the exposure of the general public to radio-frequency (RF) electromagnetic fields (EMF) have established that public transport (bus, train, etc.) has become the dominant micro-environment in terms of RF-EMF exposure, with the largest RF-EMF strengths attributed to mobile-phone use [3-6].

... caused by ... (1) the fast movement of the train, forcing the mobile phone to repeatedly connect to a different base station (macrocell) (i.e., a handover), during which the power of the mobile device is set to its maximum [7]; (2) the metal frame of the train that behaves more or less like a Faraday cage, significantly attenuating any signal that penetrates it (hence, any mobile device inside the train is forced to radiate stronger for the transmitted signal to possess enough power to reach the base station); and (3) the large amount of people present in a small environment which is the train car, increasing the chances of mobile-phone use, and thus reinforcing the aforementioned factors.

A combined solution that would effectively eliminate the first two factors is to bring the mobile-phone base station inside the train. This can be done by deploying a small cell in the train car, a miniature Wi-Fi-like base station (with a maximum output power of about 100 mW) to which mobile-phone users can connect directly and continuously, instead of repeatedly connecting to far-off macrocell base stations.
In this study, we considered the following sources of RF-EMF: (1) the subject’s mobile phone (a near-field source); (2) the base station to which the mobile phone was connected (a far-field source), and in case of the small-cell scenario; (3) the macrocell as well, as it is still present and radiating. We did not consider, however, other macrocell base stations, mobile phones of other persons, or the small-cell-to-macrocell-connection antenna on the roof of the train (Figure 1) ...
It’s important to keep in mind that the contribution of the mobile phones of other people in the train to the far-field exposure of a person (in this study downlink and far-field exposure are interchangeable, but with the addition of other users, their uplink signals add to the subject’s far-field exposure) can be quite significant. In fact, in the study by Bolte and Eikelboom [6], this contribution was found to be more than 10 times higher (92 µW/m2) than the base stations’ contribution (7 µW/m2), and Plets et al. [20] calculated that in a train car with 15 (average) users, their uplink signals add to the (average) subject’s total exposure up to 24% of his own uplink exposure. Taking into account other users’ mobile phones, we thus find that the small cell has an increased beneficiary effect on the exposure of passengers who are light users (whose exposure is dominated by the contributions of the small cell base station and/or other passengers’ mobile phones), and a comparable effect on the exposure of heavy users (whose exposure is dominated by their own mobile phone) (with the maximum reduction unaffected), compared to a scenario without other users. Additionally, it is probable that the small cell radiates more power when more users are connected to it, as is the case with macrocell base stations. However, assuming the PSC values above are average values over the train ride, we do not have to keep account for this in our analysis.
We are confident that using two different mobile phones for our measurements does not significantly bias our exposure comparison. In this study, we attempted at a general comparison of the exposure in a macrocell and a small-cell scenario for an average GSM1800mobile-phone user on the train. In practice, there are many different types of mobile phones, and it is impossible to account for all small differences in their effective output power or the absorption of their radiation in the body and the brain.
It should further be noted that all doses were averaged over sizeable volumes: the whole body or the brain’s grey mass...

The influence of a small cell on a mobile-phone user in a train is twofold. On the one hand, its vicinity to the passengers could result in a substantially increased downlink exposure; however, this is highly dependent on its effective output power. On the other hand, for the same reason, and also due to the elimination of handovers, the transmit power of any mobile phone will be considerably lower, significantly reducing the exposure due to one’s mobile device (and those of others). Combining both exposure factors, it is found that, in a realistic one-user scenario for GSM1800, the user’s total exposure of the body can be reduced by a factor 11, and of the brain by a factor 35; while both could be maximally reduced by a factor 60.
However, whether the total human RF-EMF exposure in the train due to mobile communications is reduced by the deployment of a small cell ultimately depends on several factors, including the output power of the small cell, the number of small cells in the train (depending on how many simultaneous users have to be served), as well as the number of users in the train, and how long they use their devices. This will be the subject of future research.
--Joel M. Moskowitz, Ph.D., Director
Center for Family and Community Health
School of Public Health
University of California, Berkeley

Electromagnetic Radiation Safety

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