The problem with dynamic spectrum access is not what is happening today, but what happens if it is successful
Getting the most out of valuable radio spectrum was the topic of a gathering of European regulators, industry analysts and advisers at the Dynamic Spectrum Access forum, hosted by Forum Europe in Brussels on 7 March 2012.
Broadly speaking, dynamic spectrum access (DSA) refers to making better use of radio spectrum through re-use of 'idle' bandwidth – spectrum that is not fully utilised in either frequency or time.
The concept of DSA has become particularly relevant in the past few years as the use of wireless data services has expanded. The dramatic increase in use of mobile data, driven mainly by mobile subscribers using smartphones and tablet devices, is well understood. However, wireless data usage is diversifying into a wide range of industry sectors, and machine-to-machine (M2M) applications are a particular growth area. M2M applications are not nearly as time sensitive as smartphone and tablet data traffic and can, potentially, tolerate higher levels of interference than other uses of wireless data because they can schedule data transfer to fit within available resources. Hence, accommodating M2M applications in underutilised spectrum in bands assigned for other primary uses appears to be an ideal way to meet their growing demand.
M2M is not the only application for DSA; others might include rural broadband services (which can often fit into the 'gaps' left by the less intensive use of spectrum by licensed services in rural areas). Another possible application of DSA is a next generation of Wi-Fi technology. Wi-Fi currently operates in licence-exempt spectrum (at 2.4GHz or 5GHz), but with the 2.4GHz band becoming increasingly congested, there is commercial interest in extending into other bands. In particular, bands below 1GHz are attractive, because of their greater range and indoor penetration (into buildings).
Accordingly, although DSA could apply to any underutilised spectrum, in practice the industry is focusing on opening up TV white spaces to secondary users, which is seen as the first step in extending DSA more widely to other bands.
The progress towards DSA was the topic of Forum Europe's recent event. A number of interesting implications emerged from that discussion:
- Currently the TV broadcast spectrum is attracting the most interest of advocates of DSA. However, proof of concept in the broadcast bands raises the prospects of licensed spectrum being opened to secondary use in other bands – such as spectrum used by cellular mobile services. The reaction of mobile operators to having spectrum assigned to them on a 'non-exclusive' basis, and whether DSA will devalue mobile spectrum, or increase its value by extending use to hitherto unused portions, is yet to be seen
- The concept of 'licensed shared access' (i.e. two or more users being assigned a common bandwidth, with the same or similar rights to interference protection) is a key development, which the European Commission is expected to consult upon later in 2012
- Protection of the incumbent uses of that spectrum is often challenging to achieve. Use of geo-location databases is emerging as the primary means of co-ordinating this, but as discussed in Brussels, implementing these databases is challenging – particularly because DTT receivers are not designed to accommodate secondary use in the same band as the primary DTT transmission.
- One key risk is that the DSA use of a particular band takes off to such an extent that it impairs the ability of secondary users to operate viably. There are already indications that Wi-Fi spectrum is so popular in some city centres that single-user throughput reduces to kilobits per second rather than megabits per second. The same could happen in theory in TV white spaces, or in other bands in which Wi-Fi services might opportunistically use under a DSA regime.
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