Capacity LTE

[s.sunilpal123]

Hello friends...

Kindly help me by explaining whether the capacity in LTE is hard or soft...

Also some vendor document which explicitly states so would be very helpful as I have to convince some people.

Thanks in advance....rep n thx assured!

[Resistor]

The capacity in LTE can be viewed as "soft" in the same way in which the capacity of GSM with frequency hopping can be viewed as "soft" when frequency reuse is aggressive. What does this mean? Answer: this means that the capacity of one cell will depend on the loading of the cells around it. If the surrounding cells are not loaded then the capacity of the cell in question will reach the theoretical maximum determined by the number of the available subcarriers (this is determined by the available spectrum and/or the isntalled hardware) and the number of the symbols (this is more or less fixed in FDD, but will vary in TDD implementations depending on the configuration and the proportion of traffic in the UL/DL).

Bottom line: it looks like that LTE will be interference-limited and thus the capacity of a single cell will depend on the general level of system interference from the neighbouring cells. The capacity of the network will depend in the quality of the cell plan (i.e. how much the neighbouring cells interfere each other)

[Resistor]

Additional notes:

1) The notion of "soft" and "hard" limit to the cell's loading (and not capacity) was introduced by Qualcomm in 1996-1997 when they were promoting their, then revolutionary, physical link technology based on CDMA. Few people would remember this now, but in those days CDMA was viewed just as a variant of frequency hopping, in which the hopping was very-very fast and signal generation was done using a different approach (i.e. this is why it was called "direct-spread" CDMA). The term "soft" referred not so much to the commercial standardised IS-95 system, but rather to a hypothetical CDMA system, in which the spreading codes could be viewed as unlimited in number. Then, given that there was no limit on the spreading codes, the only limit on the number of the users served by a single cell came from the ability to recover the CDMA signal from the mix of useful signal and noise. And as soon as "recovering the signal" was brought into the argument it was possible to claim that adding more users simply made system interference stronger and the recovering of the signal less likely, but not impossible. Thus, as more and more spreading codes would be used to serve more and more users, the recovering of the signal for any given user would become less and less likely, but not exactly impossible. Hence it was claimed that there was no particular "hard" limit, which could be shown to divide the "Possible" and "Impossible" to recover the signal for a user, and thus there was no particular "hard" limit on the number of connected users and the cell load. Hence, it was claimed that there would be always a possibility to, perhaps, connect one user more, making it a bit less likely to sustain each of the existing connections, but not entirely unlikely. Hence, the capacity, associated with the number of users, could be viewed as not defined by a fixed integer number. This of course was wrong. Why? Because in reality the channelization codes are not unlimited in number in a practical system and therefore there is a hard limit to the loading and to the number of the connected users, which hard limit is determined by the number of the channelization codes used by the system (for UMTS in the downlink for 12.2kbps speech this limit is 128 when RAB 30kbps is used). However, even when the number of channelisation codes is extremely high, so high as to be viewed as practically unlimited in number (this can be achieved if we give up the requirement for the channelisation codes to be orthogonal), then even in this case there will be a hard limit to the capacity of a CDMA radio link, known as "pole capacity". This limit is a number less or equal to the processing gain and thus it has a fixed upper limit (it becomes equal to the processing gain in the absence of thermal noise and extra-cell interference). The only justification for viewing CDMA systems (and thus for viewing UMTS systems, which have CDMA radio link) as having "soft" cell capacity is that in the DL the practical limit on the number of connected users is more often determined by the lack of sufficient BTS power, than by the number of the available channelization codes. And in the uplink the number of connected users is more oftern determined by a pole capacity much less than the processing gain. Thus, in practice neither of the hard limits to the cell capacity -- the number of channelization codes in the DL and the processing gain in the UL -- applies directly, because the capacity is limited mainly by the level of the extra-cell interference.

2) As the reader might have already concluded from note (1), the distinction between "soft" and "hard" cell load-limit for the systems is artificial. This can be shown by demonstrating that even old analog systems with FDMA could be viewed as having a "soft" loading limit. Here is how it goes: in FDMA user channels are separated by frequency and/or distance. Usually it is assumed that cells, which are close to each other, must not use the same frequency channels. This of course is only an assumption. There is nothing to stop us assign every available frequency channel to every cell. For AMPS, TACS and NMT this could be several hundreds of channels for each cell. Of course, if two users on two separate, but close to each other, cells happen to use the same frequency then their links would suffer interference and could even become untenable, however before that the system will try to perform a intra-cell handover to a different not-interfered frequency on the same cell. At the expense of the minor nuisance of the hard handover, the calls will continue. As more and more users are added to the cells it will become progressively more difficult to add a new user, because all users will suffer from frequent intra-cell handovers. Then, just like in CDMA, where there is never a hard limit between "impossible" and "possible" to recover the user's signal from the noise, in an old analog FDMA system there is no hard limit between "impossible" and "possible" to tolerate a bit more frequent intra-cell handovers. There will always be a possibility to add just one user more, because the number of assigned frequencies to each cell in this hypothetical case is so great that it will never be reached -- then, similarly to CDMA (where long before the number of channelisation codes on a cell in the DL is exhausted, it becomes impossible to connect a new user because of the very low probability for recovering the signal from the noise) in FDMA, long before the number of the assigned frequencies is reached, it will become impossible to connect a new user because of the very low probability for finding a clear frequency through intra-cell handovers. This shows that both systems (CDMA and FDMA) can be operated in a mode in which any hard limits determined by finite resources will never be reached, and the capacity will be determined by the level of the extra-cell interference. In both cases, if a cell is surrounded by other heavily loaded cells its capacity will go down. If the surrounding cells are not loaded then the capacity of the cell in question will go up, possibly even hitting the hard limit in both cases -- CDMA and FDMA.

These two notes demonstrate that it is not so much the type of the system, that determines whether it has "soft" or "hard" limit to its cell load, but it is the mode of its operation. Hence we may answer the above question about "soft" or "hard" capacity limit for LTE cells in this way: LTE cells will be operated in such a mode, that their capacity will be limited not by any hard limits of the installed hardware or another similar finite resource, but by the presence of extra-cell interference. Cells will be so close to each other, that long before their other resources are exhausted the interference at the input of the eNB or UE receiver will become so high that the available reserve power of the UE or eNB transmitter will not be sufficient to improve the SINR above that which makes the radio connection sustainable. Thus, long before any hard limit is hit, it will become nearly impossible (but not exactly impossible) to add another new user to the cell (hence the term "soft" loading limit). Of course, in modern systems this whole reasoning needs to be re-worked and re-stated in the terms of "channel throughput" instead of in the terms of "number of users connected", as we have done above.