The explosion of mobile devices and the demand for multimedia content is causing wireless data consumption to grow exponentially. As a result, available spectrum is getting more and more scarce and prized. Necessity being the mother of all inventions, significant efforts have been directed in finding ways to exploit more efficiently this very precious resource. Here are 8 innovations that are used, or will potentially be used, to sustain the ever increasing demand for mobile connectivity.
Orthogonal Frequency Division Multiplexing consists in using several narrowband subcarriers to form a wideband signal. Being orthogonal to each other, there is no need for guard band between the subcarriers; they may even overlap without interference. OFDM has been widely adopted, and is now at the heart of most modern broadband communication standards, including ADSL, 802.11a/g/n/ac, Wimax, LTE, and DVB-T.
Multiple Input, Multiple Output encodes multiple data streams orthogonally (or quasi-orthogonally), and sends them through an equal number of antennas, while the receiver decodes in a reciprocal way. Instead of trying to minimize the adverse effects of multipath, MIMO exploits spatial diversity to increase throughput, range, and reliability. The linear relationship between the number of antennas and capacity gain makes MIMO a highly valuable technology to optimize spectrum usage. It is no surprise that most modern standards like have adopted MIMO, with the latest provisioning for up to 8 receive and transmit antennas (8×8 MIMO).
3. Frequency Reuse: Smaller Cells
The motivation behind traditional macro-cellular architecture was primarily coverage, not capacity. The need to support a large number of users concentrated in small areas drove the migration from macro to micro, metro, pico, and femtocells. The shorter distance between the two communication points brings many advantages. Of the most important ones, signal-to-noise ratio will be higher, resulting in better, faster data links, and frequencies may be reused more densely. The level of integration that is required in small cells has only been made possible in the past few years, with the coming of powerful embedded processors, and more recently application-specific processors. 59-64
4. Frequency Reuse: Outdoor vs. Indoor
Unlicensed millimeter wave bands are considered by many as the next “2.4 GHz” in terms of ubiquity. The most popular band, at 60 GHz, offers around 5 GHz (depending on region) of unlicensed bandwidth, a tremendous channel capacity if compared to the overall spectrum available for commercial applications. The 60-GHz band sits right on an absorption peak caused by oxygen, and do not propagate through building walls. This latter property may be exploited in creating two types of applications, one indoor and one outdoor, without interfering each other. The WiGig organization has adopted the 60-GHz band for 802.11ad, promising up to 7 Gbits/sec transfer rates. Already in operation are millimeter wave backhaul links with highly directional beams, greatly minimizing potential interference.
5. Wi-Fi Offload
With all smartphones now supporting Wi-Fi, and small cells operating with a radius similar to wireless access points, operators had everything to gain in combining the two to offload traffic from highly valuable licensed bands to unlicensed ISM ones. Although there is no technical difficulties in physically integrating the two technologies, significant efforts are put in adequately distributing traffic based on load, tolerance to latency, mobility, etc.
6. Data Caching
Internet data caching is nothing new. Every web browser on PCs and laptops caches recently downloaded content for quicker access. Today’s tablets and smartphones have sufficient memory resources to also support such functionality. Newer ground is data caching at the access point. Although it does not reduce data traffic between the network and the user equipment, it does contribute to more efficient spectrum use when in cases where wireless backhaul is employed.
7. Dynamic Spectrum Access
Deemed to be the most promising mean to maximize spectrum use, dynamic spectrum access is a process by which radios establish communication parameters such as operating frequency, bandwidth, and transmit power based on spectrum usage in its vicinity, at a given time. Such negotiation and decision may be handled by the radio itself (coined as cognitive radio), but most commonly at the moment, it originates from a centralized server or database. This is the case of White Space Devices as ruled by the FCC. Recent surveys in the United States demonstrated that spectrum activity at any given time and place is very scarce. This scarcity creates great opportunities for radios and networks that have the ability to dynamically allocate spectral resources.
8. Full Duplex Communication
Virtually all communication systems will duplex uplink and downlink in time or in frequency. If it was possible to receive and transmit at the same time and at the same frequency, a factor of two in spectrum efficiency could be gained. Traditional radios must divide receive and transmit in time or frequency, as the strong transmitted signal would otherwise completely saturate the receiver. Research on this topic is ongoing. One potential avenue is to generate a secondary signal that will null the transmit signal from the receiver’s viewpoint.