Wisps use two different bands primarily. The ones who take low cost and an easier life as priorities over any assurance of quality use the interference prone 2.4GHX "wifi" band. No realistic quality of service can be provided on this band as there are so many devices on it including microwave cookers and only three non overlapping channels available.
The others use the 5 GHz spectrum which requires more technical ability and has higher costs to implement but otherwise has a lot of channels available.It is currently lacking in serious interference sources too.
Wisps can co-exist in the same way as mobile providers, it does require co-operation with channelling between the WISPs though which may not always occur.
In countires such as the USA where WISPs have been around for a lot longer than the UK they have a lot of over crowding issues in some areas with multiple providers.
WISP over crowding in the UK is unlikely to occur, especially with the BDUK focus on big telco's and Fibre when it comes down to funding decisions.
Bill Lewis - MD
Kijoma Broadband - (Division of Kijoma Solutions Ltd)
Fixed wireless ISP - ISPA/CISAS/RIPE members