Unpaired Spectrum lessons
Some readers may remember being exhorted at kindergarten to share toys with our fellow tots. In the face of exploding demand for mobile broadband services and a looming scarcity of allocated spectrum to meet it, a critical emerging question for the mobile sector is how best to use all of the spectrum there is available now and in future in the most techno-economic efficient and effective way.
Two elements of the best solution are emerging that could and should be parts of it, namely: (a) Utilization of all the unpaired spectrum that is available as well as the paired spectrum that has dominated mobile network deployments until now, and (b) Pooling or sharing of spectrum between operators to support the most economic and capable network deployments that require access to large channel bandwidths.
TDD and FDD – Not Necessarily Either/Or
Both modes of operation – paired (FDD) and unpaired (TDD) – have qualities and limitations that in the era of mobile broadband can make them potentially complementary with respect to their ability to meet overall traffic demands efficiently. FDD operation, which has traditionally dominated in cellular networks, avoids the problem of interference between uplink (UL) and downlink (DL) transmissions by locating them in separate frequencies. In contrast the risk of this type of interference is inherent in TDD operation in which DL and UL transmissions share the same frequencies and must be separated in time.
A characteristic of TDD systems is that they require guard periods (or “dead time”) between UL and DL transmissions to account for the time needed for equipment to switch between transmit and receive modes, and to accommodate the transmission time between the mobile and base station. Inevitably this dead time is therefore longer the greater the distance between the MS and the BS with which it is communicating
The data rate that a TDD system can deliver to and from an MS is reduced as a result of this dead time, so that the distance from the BS at which the maximum data rate to an MS falls off significantly is smaller than the distance at which this occurs in an FDD cell.
In contrast to the reduced data rate performance at TDD cell edges compared to FDD cells, the latter are inherently inflexible with respect to the ratio of UL and DL traffic they can handle. In contrast TDD systems can in principle be adjusted to handle varying patterns of UL and DL traffic volumes by changing the proportions of time allocated to the two directions of transmission.
A number of algorithms have been proposed to make these adjustments so as to exploit this capability of TDD systems efficiently. However if there is significant inter-cell interference the use of dynamic DL/UL allocation may be difficult or impossible, since fluctuations in the ratios of UL and DL traffic do not occur in lockstep across cells.
TDD for Mobile Broadband and the Superiority of LTE
Historically the unpaired spectrum allocated in bands such as PCS (1900 MHz) and GSM 1800 MHz has hardly been exploited for 2G or 3G networks. However in the mobile broadband era these blocks of spectrum as well as of course new TDD spectrum, such as the 50 MHz in the ITU Option 1 mid-segment of the 2.6 GHz band, may prove to be valuable. Fortunately one broadband wireless system – TDD or TD-LTE – is rapidly becoming available in multiple frequencies, capable of being deployed in multiple channel widths from 1.4 to 20 MHz, and designed to be interoperable with FDD LTE.
Furthermore the ideal configuration of LTE in dense urban areas is likely to involve a combination of FDD and TDD cells, with the latter serving especially hot spots where traffic densities are the highest, and its property of a more rapid fall off of data rate as the distance from an MS to the BS increases is irrelevant.
Spectrum sharing improves network economics
While LTE can be deployed in relative slivers of spectrum, its techno-economics are substantially greater when it is deployed in wide channel widths, say 15 or 20 MHz. As one example the three established mobile operators in South Africa each have 5 MHz of TDD spectrum in the 3G or UMTS band (i.e. 2010-2025 MHz).
If they pooled this currently unused spectrum more capacity could be made available at lower cost to mobile broadband users than if they each deployed TDD networks separately. Pooling or sharing of spectrum inevitably entails complex issues of commercial negotiations between the parties involved and agreement on the technical and operational means of making capacity available to each of them.
The potential economic and functional benefits of spectrum sharing are sufficiently large, and likely to be urgently needed, to make it worthwhile to pursue initiatives to tackle and decide how best to implement practical and effective solutions and rules to resolve these operational and commercial complexities.
Unpaired spectrum discussion in forum << comments and views
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