Sector Antenna Number Identification for CDMA and GSM
The “unique IDs” are SID:NID:BID for CDMA , and LAC:CID for GSM.
When attempting to identify base station antennas mounted on the same tower (or the same level of a tower with multiple sets of antennas), it is useful to note that the BIDs (for CDMA) or CIDs (for GSM) are typically related numerically — in groups of three, each covering a sector of 120°. The ‘sector numbers’ are encoded in the BID or CID and go in clockwise order, typically starting with 1, from the antenna aimed closest to North or NorthEast:
1 — antenna pointing approximately North;
2 — antenna pointing roughly 120° clockwise from North (approximately SE);
3 — antenna pointing roughly 240° clockwise from North (approximately SW).
Omni-directional antennas — which are rare — may be assigned sector number 0. Note also that in some rare cases, there might instead be six sectors of 60°. Unfortunately different carriers use different numbering schemes for related sector antennas. Here are some of the more commonly used schemes:
GSM 1: For GSM, the CIDs are treated as decimal numbers. One common scheme is to have the least significant decimal digit of the CID be 1, 2, or 3 for three related sector antennas. For example CIDs: 44441, 44442, and 44443. In some cases, where one might otherwise run out of possible CIDs, the digits 4, 5, and 6 — and perhaps even 7, 8, and 9 — may be used for the same purpose. Vodafone-UK uses this code.
GSM 5: Another scheme employed with some GSM systems uses the 10,000 decimal digit (fifth digit from the right) for this purpose. For example CIDs: 14444, 24444, and 34444. O2-UK uses this code. A slightly different version of this scheme can be used when towers have six sectors instead of three. In this case the second set of three sectors may be numbered 4, 5, and 0 (i.e. what would be a 6 wraps around to 0). Chunghwa Telecom in Taiwan uses this code.
CDMA 1: For CDMA, the BIDs are treated as hexadecimal numbers. One common scheme for CDMA uses the least significant hexadecimal digit as the sector number. For example, 0x4441, 0x4442, and 0x4443. U.S. Cellular uses this code — as does China Telecom in some areas.
CDMA 3: Another scheme used with CDMA systems is to instead employ the third hexadecimal digit from the right (so BIDs for related base stations are separated numerically by 256 decimal). For example, 0x4144, 0x4244, and 0x4344. Verizon Wireless uses this code, as does Sprint/Nextel — and China Telecom in some areas.
In any case, if the ‘sector number’ of the BID/CID of the base station equals
1 — then the antenna probably is roughly South or SouthWest of the cell phone;
2 — then the antenna probably is roughly West to NorthWest or North;
3 — then the antenna probably is roughly East.
This is only approximate, of course, since (i) each sector covers 120°, (ii) antennas may be aimed to best match local topography and road layout; (iii) one is not always connected to the sector antenna aimed in a direction closest to one's location.
Sector Antenna Number Identification for LTE
The unique IDs in the case of LTE are TAC:CI. While not essential for figuring out where the LTE base station antennas are that one might be able to connect to using a cellular repeater, it can be interesting to look more carefully at TAC, CI and PCI.
PCI is a number between 0 and 168 * 3 = 504. PCI may be reassigned to avoid interference between LTE base stations “within ear shot” of one another, and so is not directly useful for unique identification (although, if one adds 1 to the remainder when PCI is divided by 3, one obtains the sector number — see below).
CI is a 28 bit number, the last 8 bits of which are the sector number — while the top 20 bits are the so-called eNodeB number. The last two digits of CI, when expressed as a hexadecimal number, specify a particular antenna in a group of related antennas. That number is typically 0x01, 0x02, or 0x03. These numbers are assigned in a similar fashion to the sector numbers for antennas used for CDMA/GSM voice.
Service providers may use particular parts of the TAC for regional grouping purposes. For Verizon Wireless e.g. the low order 8 bits (last two hex digits) of TAC repesent a “super region” (collection of related nearby TACs) that may cover a small state, or a large city.
Further, part of the TAC may be encoded in the CI as well. For example, the top 20 bits of the 28-bit CI (i.e. the eNodeB number), when divided by 1000 (decimal), may yield the "super region" part (low order 8 bits) of the TAC described above.
By the way, the partial redundancy between TAC, CI and PCI makes it easier to reject bogus LTE base station IDs that arise due to inaccurate decoding, or, more commonly, due to asynchronous updating of the components of the antenna ID in the Android Telephony module.
Relationships between CDMA/GSM voice and LTE data base station antenna numbering
Antennas for LTE (data) often are on the same tower as those for CDMA or GSM (voice) (in some cases, where nearby frequencies are used for voice and data, the same antenna may even be used for both). The unique IDs for these antennas may be related. This does not mean that one has to be connected to corresponding antennas used for voice and data. One often is connected for CDMA/GSM to one base station antenna and for LTE to another antenna, which may be on the same tower or elsewhere.
An example of the relationship between the unique identifiers is the scheme used by Verizon Wireless. To find the CDMA BID, take the LTE CI mod 256,000 (that is, divide by 256 and by 1000 and take the remainder). Express the result as a hexadecimal number — with digits numbered 0 through 5, with 0 being the lowest order digit (i.e. 0x543210). The hexadecimal digits numbered 1 and 5 should be 0s. Rearrange the remaining four digits in the order 0x4032. The result is the BID for the corresponding CDMA base station — typically on the same tower and aimed in the same direction. (This computation is perhaps more easily expressed as a program than in words).
Knowing this relationship can make it easy to map out LTE base station antennas once CDMA base station antennas have been located.
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Berthold K.P. Horn, bkph@ai.mit.edu
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