All radio spectrum is typically controlled by each country’s regulatory organization (e.g. Federal Communication Commission in the US). In some cases and some countries portions of the spectrum are set aside for general use such as license-free networks. Part of the spectrum in most countries is controlled for military use, public safety and commercial services.
Only the entities so entitled may use the frequency bands to which they have rights. To design a system adequately, RF system engineers will need to work closely with the customer and carefully follow government codes. The next sections provide information regarding common world-wide frequency bands for LTE, along with a general discussion on LTE channel spacing, and guard band considerations.
This article explain concept of spectrum planning from basic to advanced level. The majority of the information within this section is taken from the 3GPP specifications (e.g., the 3GPP TS.104 specifications for the Base Station and the TS.101 specifications for the User Equipment).
LTE Operating Bands.
The choice of frequency band depends on the system operator’s license for spectrum allocation. This selection primarily impacts the propagation pathloss. The higher frequency bands will typically result in more pathloss and reduced cell radius and coverage area.
The LTE operating bands include new spectrum, as well as the opportunity to re-farm existing legacy spectrum. The following table provides the LTE (E-UTRA) spectrum per 3GPP (TS_36.104 and TS_36.101). This table shows the various FDD and TDD bands. This may vary based on the specific customer and country in which LTE will be deployed.
The system planner should coordinate with Product Management concerning the availability and timing for when the products will support specific frequency bands. The frequency band (e.g. 900 MHz, 2.5 GHz, etc.) impacts the amount of pathloss that will be encountered as the pathloss predicted by the propagation model is a function of the frequency.
Site coverage is inversely related to the frequency band (i.e. the higher frequency band, the smaller the coverage). The following figure shows an example of two different propagation models and highlights the frequency band influence to the pathloss.
In addition to the pathloss, there are other items that are impacted by the spectrum of operation. The following lists some of these items.
- Antenna characteristics are a function of the frequency. The antenna gain at a specific frequency band should be verified through the antenna vendor.
- The supported base station and subscriber device may vary based on the frequency band.
- The supported channel bandwidths vary depending on the given spectrum.
Channel Bandwidths of LTE.
The channel bandwidth is dependent upon the amount of spectrum that the operator has obtained. Not all channel bandwidths are supported in each band. Thus, in addition to the amount of spectrum, the channel bandwidth choice depends on the operator’s spectrum license. The RF system engineer needs to work with the customer to understand which channel bandwidth is appropriate and also ensure that the base stations and subscriber stations to be deployed in the network operate in the desired spectrum and channel bandwidth.
Larger bandwidths increase the cell edge data rates, but slightly decrease the coverage. Assuming a fixed amount of power is available to the subcarriers, if there is a wider channel bandwidth and more subcarriers, then there is less power available per subcarrier with an associated reduction in coverage radius.
Various channel bandwidths are available in the LTE technology allowing for spectrum flexibility. The following channel bandwidths are supported within the LTE specifications:
1.4, 3, 5, 10, 15, and 20 MHz.
The smaller bandwidths are useful in the smaller frequency bands; larger channel bandwidths are more appropriate for the larger frequency bands. The channel edges are defined by the channel bandwidth, using the following equation:
Fc +/- (BWChannel/2)
where
Fc is the center frequency of the carrier.
BWChannel is the channel bandwidth.
The channel bandwidth also impacts the number of resource blocks per slot, as seen in Table 4. The following table provides the channel bandwidths that are supported for each LTE frequency band (per the 3GPP TS 36.101 specification, v8.6.0).
Channel Spacing.
The channel spacing between LTE carriers will depend upon the deployment scenario, the size of the frequency block available, and the channel bandwidths. The nominal spacing between two adjacent LTE carriers is defined as:
Nominal Channel Spacing = [BWChannel(1) + BWChannel(2)]/2.
Where BWChannel(1) and BWChannel(2) are the channel bandwidths of the two respective LTE carriers. The channel spacing can be adjusted to optimize performance in a particular deployment scenario.
The channel center frequency must be an integer multiple of 100 KHz. The downlink and uplink carrier frequencies are uniquely designated by the E-UTRA Absolute Radio frequency Channel Number (EARFCN) in the range of 0 through 65,535.
Downlink:
The downlink carrier frequency is calculated using the following equation:
FDL = FDL_low + 0.1(NDL – NOffs_DL)
Where the FDL_low, NOffs_DL and range of NDL for each operating band are given in the following table. NDL is the downlink EARFCN.
Uplink:
The uplink carrier frequency is calculated using the following equation:
FUL = FUL_low + 0.1(NUL – NOffs_UL)
Where the FUL_low, NOffs_UL and range of NUL for each operating band are given in the
Table 7. NUL is the uplink EARFCN.
Table 7 provides further LTE (E-UTRA) channel number information per the 3GPP specifications (TS_36.104 and TS_36.101).
Not all of the channel frequencies that result from the equations above can be used, as proper spacing needs to be maintained at the band edges and between carriers. As seen in the note in the table above, the channel numbers at the lower and upper end of the operating frequency band edge are excluded from use. Basically half the channel bandwidth is excluded from either end of the operating band. Also, the nominal channel spacing equation needs to be followed to determine the spacing between channels.
For an FDD LTE system, the following table provides the default LTE (E-UTRA) frequency separation from the Tx channel carrier center frequency to the Rx channel carrier center frequency for the given LTE bands.
Guard Band Considerations for LTE.
As implied in the previous section, the typical recommendation for a guard band is to reserve half of the channel bandwidth (e.g. for a 5 MHz channel, this would be 2.5 MHz guard band on each side) as seen in Figure below.
But note that this is a recommendation to begin with; good site engineering practices such as antenna orientation and tilting, site co-location, and optional RF filtering may be able to reduce the guard band further. The guard band may depend to a great extent on how much device-to-device interference can be tolerated between the licensee and any adjacent licensee.
Tighter interference criteria, such as a maximum of 1 dB degradation at 1 meter, typically needs more guard band than the half-channel spacing “rule of thumb” posed above. Researchers and standards bodies are now trying to model the probability of devices coming within close proximity using hot-spot models (rather than assuming uniform device distributions) in order to provide more accurate guard band requirements for device-to-device interference.
It is up to the experienced system planners and installers to ensure the proper site engineering and survey are performed. If the adjacent technology is the same (i.e. both are LTE FDD or LTE TDD), then it is possible through mutual coordination that the amount of guard band can be minimized.
Additional Spectrum Information.
There are concerning various bands, e.g. 2.5 GHz, 2.3 GHz, and 700 MHz. It is important to determine if the base station and subscriber devices will be offered at the given frequencies. Even though there may be some spectrum available in a region, does not imply that the spectrum will be a candidate for LTE deployment.
United States.
2.5 GHz Band.
The Broadband Radio Service (BRS), formerly known as the Multipoint Distribution Service, and the Educational Broadband Service (EBS), formerly known as the Instructional Television Fixed Service, are located in the 2.5 GHz band. These services were originally licensed as interleaved, 6 MHz channels, which were used primarily to provide high-site, high-power, one-way video operations.
Over time, however, these uses evolved to include fixed and mobile, digital, two-way systems capable of providing high-speed, high-capacity broadband service, including two-way Internet access service, via low-power, cellularized communications systems or high power single-site systems. Optimization of these evolving uses, however, was inhibited by the band plan and service rules that governed the BRS/EBS spectrum until 2004.
Until 1996, BRS spectrum was licensed according to Geographic Service Areas with a 35-mile radius. These “incumbent” licenses are grand-fathered and continue to exist today. The designer must work around these sites and provide for suitable interference protections near these existing operations.
In 1996, the FCC conducted an auction and sold licenses for available BRS spectrum according to Basic Trading Areas (BTA) of various sizes (there are 493 BTAs). These BTA licenses were granted subject to the prior rights of the incumbent BRS license holders. EBS spectrum is licensed only as Geographic Service Areas with a 35-mile radius; although in the future, vacant EBS spectrum may be auctioned and licensed as BTAs.
EBS spectrum is licensed exclusively to accredited educational institutions, governmental organizations engaged in the formal education of enrolled students (e.g. school districts), and nonprofit organizations whose purposes are educational. The FCC rules and policies, however, have long permitted EBS licensees to lease up to 95% of their EBS spectrum to nonEBS entities for commercial purposes, subject to compliance with certain requirements.
In 2004 the FCC adopted new, flexible BRS/EBS service rules to facilitate the growth of new and innovative wireless technologies and services, including fixed and mobile wireless broadband services. In addition, the FCC reconfigured the 2.5 GHz band plan to combine previously interleaved BRS/EBS channels, and create upper- and lowerband segments for low-power uses, and a mid-band segment for high-power operations.
By creating contiguous channel blocks, and grouping high- and low-power users into separate portions of the band, the new band plan reduces the likelihood of interference caused by incompatible uses and creates incentives for the development of innovative services, which were inhibited by the prior band plan.
The new BRS/EBS band plan, as seen in Figure below, will allow licensees to use the 2496 to 2690 MHz spectrum in a more economically efficient manner and will support the introduction of next-generation wireless technologies. Under the new regulations, the total spectrum bandwidth licensed by the FCC for EBS and BRS spectrum is 194 MHz.
Approximately 75% of this spectrum is licensed for the Educational Broadband Service and 25% is licensed for the Broadband Radio Service. Individual channels and channel groups of EBS and BRS spectrum will range from 5.5 MHz to 23.5 MHz of spectrum.
The new rules also preserve the operations of existing licensees, including educational institutions currently offering instructional television programming, and continue the FCC’s long-standing policy of allowing up to 95% of EBS spectrum to be leased for commercial purposes.
Licensees must transition to the new band plan by October 19, 2010 (barring certain disputes in the transition planning process), which includes relocating licensees from their current channel assignments to new spectrum designations in the band.
The transition period has three phases:
(1) the Initiation Planning Period, which runs for 30 months from July 19, 2006, during which an Initiation Plan must be filed with the FCC;
(2) the Transition Planning Phase, which is a ninety-day period that commences upon the filing of the Initiation Plan; and
(3) the Transition Completion Phase, which commences after the Transition Planning Period ends, and provides eighteen (18) months within which the transition must be completed to complete the physical transition to the new band plan.
Accordingly, assuming no disputes among affected licensees concerning a given transition plan, the transition period for any given market could last 51 months. Thus, for example, given that the amended rules became effective on July 19, 2006, the transition period for any given market (barring disputes) could last until October 19, 2010. Until a market is transitioned, licensees may continue to operate in accordance with their currently licensed operations.
This band spans from 2495 MHz to 2690 MHz and covers the LTE operating bands 7 and 38 (bands 7 and 38 are only a portion of the entire BRS-EBS band). LTE operating band 7 is designated as FDD, whereas the BRS-EBS band plan implies a TDD scenario. In the United States, the 2.5 GHz band is regulated by the FCC. Thus, the equipment is to comply with at least the following standards:
- Spectral emissions requirements per FCC Part 27.53.
- Maximum EIRP requirements per FCC Part 27.50.
- Spurious emissions requirements per FCC Part 15 Class A.
- EMI requirements per FCC Part 15 Class B.
However, FCC Part 27 is in the process of being amended to accommodate the 2.5 GHz band. Due to this volatility, the specific Part 27 requirements will not be listed here. Please refer to the latest FCC Part 27 Report & Order documentation (04-258) for detailed specifications.
2.3 GHz Band.
This band is also referred to as the Wireless Communications Service (WCS) Spectrum. There are 2 licenses in each of the 52 Major Economic Areas (MEAs) and 2 licenses in each of the 12 Regional Economic Area Grouping (REAGs).
The WCS spectrum is divided into four blocks as illustrated by the following figure. Blocks A and B each consist of two 5 MHz blocks whereas Blocks C and D each have a single 5 MHz block.
- Block A 2305 to 2310 MHz paired with 2350 to 2355 MHz
- Block B 2310 to 2315 MHz paired with 2355 to 2360 MHz
- Block C 2315 to 2320 MHz
- Block D 2345 to 2350 MHz.
Blocks A and B are available in the 52 MEAs whereas Blocks C and D are available in the 12 REAGs.
This band spans from 2.3 GHz to 2.4 GHz, and covers the LTE operating band 40. LTE operating band 40 is designated as TDD, but Block A and B reflect a scenario with paired frequencies.
In the US, the center of this band (i.e. 2320 to 2345 MHz) is occupied by Satellite Digital Audio Radio Services (SDARS) operators XM and SIRIUS and the rules pertaining to the WCS (2.3 GHz wireless) operators include onerous OOBE (Out Of Band Emission) limits on 2.3 GHz wireless operations in order to protect SDARS operations. These OOBE limits will require unusual efforts to control OOBE including the possible use of extreme filters, power limits and/or bandwidth limits.
700 MHz Band.
The following figure provides the revised 700 MHz Band Plan for Commercial Services.
As seen in this figure, this band spans from 698 MHz to 806 MHz. This covers the LTE operating bands 12, 13, 14, and 17. So when these bands are used, the system planner must take care to minimize interference with the existing 700 MHz band.
AWS Band Plan.
The following figure provides the Advanced Wireless Services (AWS) band plan.
This band uses the frequency range from 1710 to 1755 MHz for the uplink and the range from 2110 to 2155 MHz for the downlink. It covers the LTE operating band 4 and parts of bands 3, 9 and 10.
The FCC intended the AWS band to harmonize as much as possible with European bands. The upper AWS band lines up with Europe’s UMTS 2100 base transmit band and the lower AWS band aligns with Europe’s GSM 1800 mobile transmit band.
Outside of the United States.
2.5 GHz Band.
Outside the United States, the 2.5GHz band is regulated by ETSI, and thus all
equipment designed to operate in this band must comply with EN 302 326-2 (spectral
emissions), EN55022:2006 Class B (radiated & conducted emissions), along with EN
301 489-4 (EMC).
2.3 GHz Band.
This band spans from 2.3 GHz to 2.4 GHz, and covers the LTE Operating Band 40. It is assumed that the 2.5 GHz emissions approach will apply to 2.3 GHz, with optional filtering needed for locations that may result in unacceptable adjacent-channel interference (e.g. 2.29 GHz-2.30 GHz is used for deep space research services in Australia).
The 2.3 GHz band is regulated by ETSI, and thus all equipment designed to operate in this band must comply with EN 302 326-2 (spectral emissions), EN55022:2006 Class B (radiated & conducted emissions), along with EN 301 489-4 (EMC). Furthermore, the 2.3 GHz equipment must also comply with the spectral emissions requirements per Australian Communications and Media Authority Radiocommunications Act 1992 Sample Spectrum License.
References.
- 3GPP – http://www.3gpp.com/
- Federal Communications Commission – www.fcc.gov.
- National ITFS Association (NIA) – www.itfs.org.
- ETSI – www.etsi.org.
- “Spectrum Analysis for Future LTE Deployments”, Motorola white paper – http://business.motorola.com/experiencelte/resources.html.