Voice and Data — “4G” LTE

Originally, of course, voice was the focus of cellular phone technology. When data was added, it at first piggy-backed on voice. Over the years several improvements increased the data rate possible in this fashion (e.g. 1xRTT → EVDO_A → eHRPD). But ultimately, the need for high speed data connections hastened a change from a circuit-switched to a packet-switched system.

“Long term evolution”, LTE is a first step in this direction, with a separate packet-switched high speed data channel. The existing circuit-switched channels for voice (CDMA or GSM), were retained — for now.

Unfortunately, this means that antennas and bi-directional amplifiers are needed to support the high speed data channels as well as the voice channels. There are two main approaches to having an LTE data connection as well as a voice connection in areas of marginal reception:

(1) Add a second, parallel system of antennas and bi-directional amplifier for the LTE frequency band;

Suitable bi-directional amplifiers have recently become available. For example:

  Wilson 4G LTE 700 (801865)(*) (for band 13 — currently used by Verizon Wireless)

  Wilson AWS 70 (802365)(*) (for band 17 — used by T-Mobile USA and some Canadian providers)

High gain antennas are also available for the upper 700 MHz band (see below). For example, sector antennas JA750S18-65 (18 dBi, 65°), JA750S14-65 (14 dBi, 65°), and JA750S12-90 (12 dBi, 90°) from JACC Limited:

The latest sector antennas may not always show up right away on the JACC web site — contact them directly for more information.

Further, check Nanhai Microwave Communications Equipment Co. for high gain sector antennas. Again, the web site may not be easy to navigate - may need to look under VHF TV/UHF TV/RFID Antennas 760-780 MHz.

See also 700 MHz Yagi Antennas from ZDA (10, 14 and 16 dBi).

See also 700 MHz Grid Parabolic Antennas from ZDA (10, 12, 15, and 16 dBi).

See also High-Gain All Band LTE/3G/GSM Antenna from Poynting (12 dBi)

See also TESSCO.

In the above, on the left is the 824-894 MHz 18 dBi (65 degree horizontal, 7.5 degree vertical beam width 92" high) J0800S18-65 from JACC for CDMA, and on the right is the 746-787 MHz 16 dBi (90 degree horizontal, 7.2 degree vertical beam width 113" high) PB760-16-90V from Nanhai Microwave for LTE.

(2) Alternatively, use antennas and bi-directional amplifiers that cover both the voice and the LTE data band.

Suitable bi-directional amplifiers have recently become available. For example:

  Wilson Tri-band 4G-V (805165)(*) for band 13 (upper 700 MHz band) — as well as 800 and 1900 MHz bands for voice (Verizon Wireless).

  Wilson Tri-band 4G-C (802770)(*) for AWS (1700/2100 MHz) band — as well as 800 and 1900 MHz bands for voice (T-Mobile USA and providers in Canada).

  Wilson Tri-Band 4G-A (805370)(*) for band 12 (lower 700 MHz band) — as well as 800 and 1900 MHz bands for voice (AT&T Mobility). (Expected Fall 2013).

See also Wilson Tri-band 4G-V, 4G-A, 4G-C Boosters(*)

  Wilson AG Pro Quint Selectable (803470)(*) for (i) Verizon Wireless LTE upper 700 MHz, (ii) AT&T Mobility LTE lower 700 MHz, (iii) T-Mobile USA LTE (AWS 1700/2100 MHz) — and the voice bands (iv) cellular (800 MHz) and (v) PCS (1900 MHz).

  SureCall Force55 CM5000-80 five band amplifier that supports all 2G, 3G and 4G bands used in the USA and Canada (Cellular / PCS / AT&T LTE / Verizon LTE / T-Mobile AWS) — except Sprint/WiMax (suitable for large installations).

  SureCall Fusion5s Fusion5s supports Voice/3G/4G LTE 5-Band for all major carriers (suitable for more moderate installations).

Multiband amplifiers are considerably more expensive than single band, but allow one to save on cabling and antennas.

Many low gain flat panel and dome antennas for interior use (such as Wilson 301135(*) and Wilson 301123)(*) cover both the 800-900 MHz cellular voice channel and the 700-800 MHz channel (band 12 and 13) used by some providers for LTE data. Finding a high gain outside antenna that covers both may, on the other hand, be more of a challenge (although in some cases an antenna designed for the cellular 800-900 MHz band may still have useful gain, at least in the upper part of the 700-800 MHz range — check with the manufacturer).

Finding the frequency band used by one's provider

Different providers use different frequency bands for LTE data transmission. Fortunately many presently use only one band, and so it may not be too hard to determine what that is (contrary to the situation with the frequency bands used for CDMA and GSM voice). For example:

• Verizon Wireless (so far) uses band 13 (upper SMH, 776-787 MHz uplink and 746-757 MHz downlink, FDD)

• U.S. Cellular uses band 12 (lower SMH, 698-716 MHz uplink and 728-746 MHz downlink, FDD)

• AT&T Mobility uses band 17 (lower SMH, 704-716 MHz uplink and 734-746 MHz downlink, FDD)

• T-Mobile (and some Canadian providers) use band 4 (AWS, 1710-1755 MHz uplink and 2110-2155 MHz downlink)

See Frequency Bands and Channel Bandwidths for a more detailed listing.

Presently there appears to be little inter-operability in the LTE world. Different carriers use different frequency bands for high speed data, and equipment manufacturers at times design chips to cover specific bands used by their clients.

In planning a cellular repeater, keep in mind that the cellular world is growing and evolving rapidly. Verizon Wireless, for example, currently uses only a 10 MHz band in the upper 700 MHz range for LTE (band 13), but has recently acquired a 20 MHz channel in the AWS range (band 4) (which is not accessible to their current offerings of smart phones). Also, the FCC is encouraging TV stations to give up spectrum in the 600 MHz range, which may later be used for yet more data channels.

LTE base station antenna identification

Each base station antenna has a unique identifier. For CDMA this takes the form SID:NID:BID, where SID is the System ID, NID is the Net ID, and BID is the Base station ID. For GSM it is MCC:MNC LAC:CID, where is LAC is the Location Area Code and CID is the cell ID, MCC the mobile country code, and MNC, the mobile network code.

Since LTE data channels are entirely separate from the voice channels, their base stations also have their own unique identifier, MCC:MNC TAC:CI (and PCI), where TAC is the Tracking Area Code, CI is the Cell Identity (and PCI is the physical cell ID). Presently the only app that exposes the unique ID for the LTE data channel (as well as detailed LTE signal strength, SNR and CQI indications) is CellTracker — see the last two lines of text in the screen shot:

As with GSM and CDMA, base station IDs are not assigned arbitrarily, but tend to follow certain numbering rules, which may be useful in locating nearby base stations.

A Threshold Phenomenan

It is important to have enough gain when adding support for LTE data channels to an existing cellular repeater setup. The reason is that if the amplified high speed LTE data channel is “weaker” than the existing amplified CDMA or GSM voice channel, then the end user equipment may very well decide that it can do better by using the legacy data communication methods (s.a. 1xRTT, EVDO_A or eHRPD). Then, disappointingly, no “4G” data connection will be made.

Different approaches to handling voice and data

Different providers may deal with the voice and high speed data channels in different ways:

SVLTE (Simultaneous Voice and LTE)

 SVLTE apparently requires two radios to be on at the same time, which has the advantage that voice usage will not impact the data rate — but this increases power consumptions and cost.

CSFB (Circuit Switched Fallback)

 CSFB works with a single radio that can be switched in frequency, and may suffer from somewhat longer call setup time as a result.

While two receivers operating simultaneously do not present a big problem, multiple transmitters do (because of interference from intermodulation products). So some schemes have receivers operating on both the voice and the data channels, but only allow transmission on one of these at a time.

The future — VoLTE?

Voice channels take up spectrum space. This spectrum can ultimately be reclaimed (and used for more data channels) if voice can piggy-back on data, using something akin to “voice over IP” (VoIP), namely “Voice Over LTE” (VoLTE). “Session Initiation Protocol” (SIP) is likely to play a role in the implementation of VoLTE. At that point we might be back again to needing to amplify just one frequency band (namely the LTE band). Although, currently the trend is toward more, not fewer channels, particularly if carrier aggregation and LTE roaming is to be implemented.

There are already some experiments in using LTE alone, without a “companion” CDMA or GSM channel — Google's Project Fi may be one such.

See also “Evolution and Deployment of VoLTE” .

Very Long Term Evolution?

See e.g. “LTE Advanced”.
(*) Wilson Electronics has become weBoost, which may carry some of the above mentioned products. See also RepeaterStore and SureCall.
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Berthold K.P. Horn, bkph@ai.mit.edu