BACKGROUND

Both Frequency Division Duplex (“FDD”) and Time Division Duplex (“TDD”) modes are commonly used in wireless communications systems. For example, the LTE standard supports both FDD and TDD modes, as another example 802.11 versions (e.g. Wi-Fi) support TDD mode of operation.

In the case of LTE, various numbered bands are defined within what is called “Evolved UMTS Terrestrial Radio Access” (E-UTRA) air interface. Each E-UTRA band not only specifies a particular band number, but it defines whether the band is FDD or TDD, and what bandwidths are supported within the band (e.g. see http://en.wikipedia.org/wiki/LTE_frequency_bands#Frequency_bands_and_channel_bandwidths for a list of E-UTRA bands and their specifications). For example, Band 7 is an FDD band defined as using the frequency ranges of 2,500-2,570 MHz for Uplink (“UL”), 2,620-2,690 for downlink (“DL”), it supports 5, 10, 15, 20 and MHz signal bandwidths within each of the UL and DL bands.

In many cases E-UTRA bands overlap. For example, different bands may be common spectrum that has been allocated in different markets or regions. For example, Band 41 is a TDD band using the frequency ranges of 2,496-2,690 MHz for both UL and DL, which overlaps with both UL and DL ranges in FDD Band 7 (e.g. seeFIGS. 16aand 16b. Currently, Band 41 is used in the U.S. by Sprint, while Band 7 is used by Rogers Wireless in the bordering country of Canada. Thus, in the U.S., 2,500-2,570 MHz is TDD spectrum, while in Canada that same frequency range is UL for FDD spectrum.

Typically, a mobile device, upon attaching to a wireless network, will scan through the band searching for transmissions from one or more base stations, and typically during the attach procedure, the base station will transmit the characteristics of the network, such as the bandwidth used by the network, and details of the protocol in use. For example, if an LTE device scans through 2,620-2,690 MHz in the U.S., it might receive an LTE DL frame transmitted by an eNodeB that identifies the spectrum as Band 41, and if the LTE device supports Band 41 and TDD, it may attempt to connect to the eNodeB in TDD mode in that band. Similarly, if an LTE device scans through 2,620-2,690 MHz in the Canada, it might receive an LTE DL frame transmitted by an eNodeB that identifies the spectrum as Band 7, and if the LTE device supports Band 7 and FDD, it may attempt to connect to the eNodeB in FDD mode in Band 7.

Most early LTE networks deployed worldwide used FDD mode (e.g., Verizon, AT&T), but increasingly TDD mode is being used, both in markets with extensive FDD coverage, such as the U.S. (where Sprint is deploying TDD) and in markets that do not yet have extensive LTE coverage, such as China (where China Mobile is deploying TDD). In many cases, a single operator is deploying both FDD and TDD at different frequencies (e.g. Sprint operates both FDD LTE and TDD LTE in different frequencies in the U.S.), and may offer LTE devices which can operate in both modes, depending on which band is used.

Note that the E-UTRA list of LTE bands is by no means a final list, but rather evolves as new spectrum is allocated to mobile operators and devices to use that spectrum are specified. New bands are specified both in spectrum with no current band that overlaps its frequencies, and in spectrum in bands overlapping frequencies of previous band allocations. For example, Band 44, a TDD band spanning 703-803 MHz, was added as an E-UTRA band several years after older 700 MHz FDD bands were specified, such as Bands 12, 13, 14 and 17.

As can be seen inFIG. 6, the bulk of mobile data used to be voice data (e.g. Q1 2007), which is highly symmetric. But, with the introduction of the iPhone in 2007, and the rapid adoption of Android and then introduction of the iPad in 2009, non-voice mobile data rapidly outpaced the growth of voice data, to the point where, by the middle of 2013, voice data was a small fraction of mobile data traffic. Non-voice data is projected to continue to grow exponentially, increasingly dwarfing voice data.

As can been seen inFIG. 7, non-voice mobile data is largely dominated by media, such as streaming video, audio and Web browsing (much of which includes streaming video). Although some streaming media is UL data (e.g. during a videoconference), the vast majority is DL data, resulting is highly asymmetric DL vs. UL data usage. For example, in the Financial Times May 28, 2013 article, “Asymmetry and the impending (US) spectrum crisis”, it states that “ . . . industry estimates of the ratio of data traffic downlink to data traffic in the uplink ranges from a ratio of about eight to one (8:1)—to considerably more.” The article then points out that the largely FDD deployments in the U.S. are very inefficient in handling such asymmetry since FDD mode allocates the same amount of spectrum to each DL and UL. As another example, Qualcomm estimated DL/UL traffic asymmetry as high as 9:1 for one of the U.S. operators, based on 2009 measurements in live networks (cfr., Qualcomm, “1000x: more spectrum—especially for small cells”, Nov. 2013, http://www.qualcomm.com/media/documents/files/1000x-more-spectrum-especially-for-small-cells.pdf). Thus, even when FDD DL spectrum is heavily utilized (potentially to the point of being overloaded), the UL spectrum may be largely unused.

The Financial Times article points out that TDD is far better suited to such asymmetry since it can be configured to allocate far more timeslots to the DL data than the UL data. For example, in the case when 20 MHz is allocated to FDD (as 10+10 MHz), DL data throughput is limited to a maximum of full-time use of 10 MHz (even when the UL data needs far less than the 10 MHz it has been allocated), whereas when 20 MHz allocated to TDD, DL data throughput can use all 20 MHz the vast majority of the time, allocating the 20 MHz to UL data a small percentage of the time, far better matching the characteristics of data usage today. The article acknowledges that, unfortunately, most existing U.S. mobile spectrum is already committed to FDD mode, but urges the FCC to encourage the use of TDD as it allocates new spectrum.

Although TDD would certainly allow for more efficient use of new spectrum allocations given the increasingly asymmetric nature of mobile data, unfortunately existing FDD networks deployments cannot change operating mode to TDD since the vast majority of users of such LTE FDD networks have devices that only support FDD mode and their devices would cease to be able to connect if the network were switched to TDD mode. Consequently, as LTE data usage becomes increasingly asymmetric, existing LTE FDD networks will see increasing DL congestion, while UL spectrum will be increasingly underutilized (at 8:1 DL:UL ratio, the lower estimate of the May 28, 2013 Financial Times article, that would imply that if the DL channel is fully utilized, only ⅛th, equivalent to 1.25 MHz of 10 Mhz, would be used of the UL channel). This is extremely wasteful and inefficient, particularly given the limited physical existence of practical mobile spectrum (e.g. frequencies that can penetrate walls and propagate well non-line-of-sight, such as ˜450-2600 MHz) and the exponential growth of (increasingly asymmetric) mobile data (e.g. Cisco 2/2013 VNI predicts a 61% CAGR in mobile data growth through 2018, most of which is streaming video and other highly asymmetric data).

