Welcome to the World of CDMA
The CDMA Revolution
The great attraction of CDMA technology from the beginning
has been the promise of extraordinary capacity increase over narrowband
mulitiple access wireless technologies. Simple models suggest that the
capacity improvement may be more than 20 times that of the existing narrowband
cellular starndrards, such as AMPS in North America, NMT in Scandinavia,
TACS in the United Kingdom. Historically, the capacity was calculated
using simple arguments. Reality, of course, is much more complicated than
the idealized models. Real cell coverage areas are highly irregular, not
the neat hexagons found in textbook models. Offered load is not spatially
uniform, changes dramatically with time-of-day, and is often subject to
other uncontrollable influences.
An idealized multiple access mobile radio system consists
of a family of base stations, or "cells," geographically distributed
over the service area, and mobile stations. We use the term "mobile"
generically to mean any subscriber station, whether it moves or not. The
majority of new cellular sales are now in fact hand held portable units,
and the market outlook is for that trend to continue for the forseeable
future. However non-traditional uses, such as wireless data modems in
laptops, are also expected to grow dramatically in the near future, and
this applications are likely to be largely static.
Traditional Multiple Access CommunicationTraditionally radio communication systems have separated users by either frequency channels, time slots, or both. These concepts date from the earliest days of radio. Even spark transmitters used resonant circuits to narrow the spectrum of their radiation. Scheuled net operation was probably the first manifestation of time slotting. Modern cellular systems began with the use of channelized analog FM. More recently several hybrid FDM-TDM digital systems have been developed, ostensibly to enhance service quality and capacity. In all these systems, each user is assigned a particular time-frequency slot.
In large systems the assignments to the time-frequency slots cannot be unique. Slots must be reused in multiple cells in order to cover large service areas. Satisfactory performance in these systems depends critically on control of the mutual interference arising from the reuse. The reuse concept is familiar even in television broadcasting, where channels are not used in adjacent cities.
North American cellular allocates approximatley AMPS 416 channels to each operator (30 kHz spacing, with a total allocation of 12.5 MHz in each direction - see the Frequency Plan). An idealized system geometry shown in the figure. The same frequency obviously cannot be reused in any adjacent pair of cells because a user on the boundary between those cells would receive both signals with equal amplitude, leading to an unacceptably high interference level. A plane can be tiled with hexagonal cells, labelled in accordance with the seven-way pattern shown in the figure. Thus, if a unique set of channels is assigned to each of the seven cells, then the pattern can be repeated without violating the adjacency requirement. Although this idealized pattern is not strictly applicable in all real systems, the seven-way resuse pattern is aproximately correct. The capacity of systems built in this way is determined by the bandwidth per channel and the seven-way reuse pattern. In an AMPS system, therefore, the maximum capacity per cell is approximately 416/7 = 59. For three-way sectored cells, the same K=7 reuse applies over all three sectors, that is, only about 19 channels are available in each sector. In an ideal geometry the reuse pattern looks like this, representing channel sets by distinct colors ...
In this connection, it should be noted that achievement of the K=7 reuse, rather than an even larger number, depends on the fact that the effective propagation decay law is faster than free space. That is, in a vacuum electromagnetic radiation decays in intensity like R-2. However measurements have consistently shown that the effective propagation law exponent is typically between -3.5 and -5 in the ground mobile environment. Interestingly, it is easy to show that if the propation law were that of free space, a large cellular system would not be viable at all. The larger-than-free-space propagation exponent means that only the first tier of neighbor cells is significant in the idealized model.
The "Magic" of CDMACDMA offers an answer to the capacity problem. The key to its high capacity is the use of noise-like carrier waves, as was first suggested decades ago by Claude Shannon. Instead of partitioning either spectrum or time into disjoint "slots" each user is assigned a different instance of the noise carrier. While those waveforms are not rigorously orthogonal, they are nearly so. Practical application of this principle has always used digitally generated pseudo-noise, rather than true thermal noise. The basic benefits are preserved, and the transmitters and receivers are simplified because large portions can be implemented using high density digital devices.
The major benefit of noise-like carriers is that the system sensitivity to interference is fundamentally altered. Traditional time or frequency slotted systems must be designed with a reuse ratio that satisfies the worst-case interference scenario, but only a small fraction of the users actually experience that worst-case. Use of noise-like carriers, with all users occupying the same spectrum, makes the effective noise the sum of all other-user signals. The receiver correlates its input with the desired noise carrier, enhancing the signal to noise ratio at the detector. The enhancement overcomes the summed noise enough to provide an adequate SNR at the detector. Because the interference is summed, the system is no longer sensitive to worst-case interference, but rather to average interference. Frequency reuse is universal, that is, multiple users utilize each CDMA carrier frequency... The reuse pattern is now
Capacity is determined by the balance between the required SNR for each user, and the spread spectrum processing gain. The figure of merit of a well-designed digital receiver is the dimensionless signal-to-noise ratio (SNR)
Energy per bit is related to signal power and data rate:
The interference in this equation is the sum of the signals
from all users other than the one of interest.
Near-Far ProblemCDMA (and spread spectrum in general) was always dismissed as unworkable in the mobile radio environment because of what was called the "near-far problem." It was always assumed that all the stations transmitted constant power. In the mobile radio environment some users may be located near the base station, others may be located far away. The propagation path loss difference between those extreme users can be many tens of dB. Suppose, for example that only two users are present, and that both are transmitting with enough power that the thermal noise is negligible. Then the SNR, in dB, is
If there is, say, 30 dB difference between the largest
and smallest path losses, then there is a 60 dB difference between the
SNR of the closest user and the farthest user, because these are the received
powers. To accomodate the farthest users, the spreading bandwidth would
have to be perhaps 40 dB, or 10,000 times the data rate. If the data rate
were 10,000 b/s, then W=100MHz. The spectral efficiency is abysmal, far
worse than even the most inefficient FDMA or TDMA system. Conversely,
if a more reasonable bandwidth is chosen, then remote users receive no
Power ControlThe key to the high capacity of Qualcomm's CDMA is extremely simple: If, rather than using constant power, the transmiters can be controlled in such a way that the received powers from all users are roughly equal, then the benefits of spreading are realized. If the received power is controlled, then the subscribers can occupy the same spectrum, and the hoped-for benefits of interference averaging accrue.
Assuming perfect power control, the noise plus interference is now
or about N=32. The target SNR of 6 dB is a nominal estimate.
Once power control is available, the system designer and operator have
the freedom to trade quality of service for capacity by adjusting the
SNR target. Note that capacity and SNR are reciprocal: a three dB improvement
in SNR incurs a factor of two loss in capacity, and vice-versa.
Embedded Cell Capacity
The discussion leading to equation (9) assumes only a single cell, with no interference from neighboring cells. One might ask what has been gained here. The capacity of an isolated AMPS cell likewise is very high. In fact, there is nothing to stop you from using all the channels if there are no neighbors; reuse is not needed. The capacity of that fully populated AMPS cell would be about 42 channels (1.25 MHz/ 30 kHz channel spacing). This is not greatly different than the number that we just calculated for CDMA.
So what is the point of using CDMA?This is where the big difference is. To find what happens with the neighbor cell interference, we have to add that interference into equation (3) above. The math of this can be found in several of the references [e.g. Gilhousen, et al.]. It turns out that the fraction of the reverse link interference that comes from the neighbor cell is about 60% of the own-cell interference. And, importantly, this answer is not terribly sensitive to the parameters of the model, provided we assume that the mobiles are power-controlled in a sensible way.
For doing the calculation, we just introduce the effective frequency reuse factor F, defined as
From equation (11), it is evident why we call F the effective frequency reuse factor. It plays the same role in the CDMA capacity equation (11) that the narrowband frequency reuse factor K does in AMPS capacity. An omnidirectional CDMA cell, even with other-cell interference, has a higher capacity than an AMPS cell by a factor of K/F = 7/1.6 or about 4.4. The improvement in a sectored cell is even more dramatic because the CDMA capacity is largely unaffected by the sectorization. That is, (11) applies to one sector, with only small modifications because of the interference leakage between sectors, while AMPS does not gain from the sectorization. In this case the gain is K/F = 21/1.6 or about13 times.
Voice CodingBecause the interference is averaged, anything that can be done to reduce the average transmitted power enhances capacity. An obvious target for such power optimization is the speech coding. Human speech is an intermitten information soure. Measurements at Bell Laboratories many years ago suggested that the activity factor in natural human conversation is in the range of 35-40%. If that activity factor can be translated into power gating, then a further increase in capacity of perhaps two times is possible. This is in fact done. It is accomplished by features of both the air interface standard IS-95A and the voice coder service option.The pole capacity becomes
where v is the voice activity factor, approximately 0.5.
The gain over AMPS capacity, by the same assumptions as above, is now
about 26 times - perhaps optimistic given the crude nature of the model,
but suggestive of the substantial improvements possible by converting
System capacity, as you might expect, is affected by
propagation phenomena. Users of analog cellular phones are familiar with
the fading that is so annoying, especially in handheld portables when
standing nearly still. Fading in a moving vehicle is more rapid, being
caused by motion of the vehicle through more or less stationary interference
patterns, with the wavelength being about one foot. CDMA is much more
robust than the analog technologies in the presence of multipath, but
it does affect capacity.
When does multipath cause fading, and when does it not?
When the multipath components are "resolved"
by the CDMA waveform, that is, when their delays are separated by at least
the decorralation time of the spreading, then they can be separated by
the despreading correlator in the receiver. They do not interfere because
each component correlates at a different delay. When the multipath components
are separated by less than the decorrelation time, then they cannot be
separated in the receiver, and they do interfere with one another,
leading to what is sometimes called flat fading.
Coverage versus CapacityThere is some "bad news" arising from the CDMA capacity equation. Namely, the fact that the power that the mobiles are required to transmit goes to infinity as the capacity "pole" is approached. As the required power increases, mobiles at the fringe of coverage will begin to run out of transmitter power. That is, they will be asked to transmit more than their capability allows. The practical consequence of this is that the system load should really be controlled so that the planned service area never experiences coverage failures because of this phenomenon.
There are some interesting mathematical models of this, that we talk about in our Coverage-Capacity pages. It is not really so much a problem as it is a system design consideration. You cannot simultaneously achieve maximum capacity and maximum coverage. It is a tradeoff.
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Copyright © 1996-1999 Arthur H. M. Ross, Ph.D., Limited