During this discussion I will use the generic term "CDMA"
to refer to the IS-95B standard.
Technically speaking, CDMA is only a means to transmit bits of information,
while IS-95B is a transmission protocol that employs CDMA. You may
also hear the term "TDMA" used to refer generically to
the IS-136 standard. Once again, TDMA is only a method of transmitting
bits, while IS-136 is a protocol that happens to employ TDMA.
quite a bit of time reading the messages that flow through the various
PCS newsgroups and forums on the Internet, and if one thing is abundantly
clear, it is that people don't seem to know the true differences
between CDMA and TDMA.
And who could blame them? There is so much hype surrounding these
two competing technologies that it is difficult for a regular PCS
subscriber to know who is telling the truth.
am NOT an RF engineer, nor do I work for any of the cellular or
It is however my hobby to keep up with the latest developments in
mobile communication (as this web site amply demonstrates).
I would like to clear the air by interjecting my own spin on this
debate. I hope that by the time you finish reading this editorial
you will have a better understanding of the true strengths and weaknesses
of both technologies.
begin by learning what these two acronyms stand for.
TDMA stands for "Time Division Multiple Access", while
CDMA stands for "Code Division Multiple Access".
Three of the four words in each acronym are identical, since each
technology essentially achieves the same goal, but by using different
methods. Each strives to better utilize the radio spectrum by allowing
multiple users to share the same physical channel.
You heard that right.
More than one person can carry on a conversation on the same frequency
without causing interference.
This is the magic of digital technology.
the two competing technologies differ is in the manner in which
users share the common resource.
TDMA does it by chopping up the channel into sequential time slices.
Each user of the channel takes turns transmitting and receiving
in a round-robin fashion.
In reality, only one person is actually using the channel at any
given moment, but he or she only uses it for short bursts.
He then gives up the channel momentarily to allow the other users
to have their turn.
This is very similar to how a computer with just one processor can
seem to run multiple applications simultaneously.
on the hand really does let everyone transmit at the same time.
Conventional wisdom would lead you to believe that this is simply
Using conventional modulation techniques, it most certainly is impossible.
What makes CDMA work is a special type of digital modulation called
This form of modulation takes the user's stream of bits and splatters
them across a very wide channel in a pseudo-random fashion.
The "pseudo" part is very important here, since the receiver
must be able to undo the randomization in order to collect the bits
together in a coherent order.
you are still having trouble understanding the differences though,
perhaps this analogy will help you. This my own version of an excellent
analogy provided by Qualcomm:
a room full of people, all trying to carry on one-on-one conversations.
In TDMA each couple takes turns talking. They keep their turns short
by saying only one sentence at a time.
As there is never more than one person speaking in the room at any
given moment, no one has to worry about being heard over the background
In CDMA each couple talks at the same time, but they all use a different
Because none of the listeners understand any language other than
that of the individual to whom they are listening, the background
din doesn't cause any real problem.
this point many people confuse two distinctly different issues involved
in the transmission of digital audio.
The first is the WAY in which the stream of bits is delivered from
one end to the other.
This part of the "air interface" is what makes one technology
different from another.
The second is the compression algorithm used to squeeze the audio
into as small a stream of bits as possible.
latter component is known at the "Voice Coder", or Vocoder
Another term commonly used is CODEC, which is a similar word to
modem. It combines the terms "COder" and "DECoder".
Although each technology has chosen their own unique CODECs, there
is no rule saying that one transmission method needs to use a specific
People often lump a technology's transmission method with its CODEC
as though they were single entities.
We will discuss CODECs in greater detail later on in this article.
encoding schemes differ slightly in their approach to the problem.
Because of this, certain types of human voice work better with some
CODECs than they do with others.
The point to remember is that all PCS CODECs are compromises of
some sort. Since human voices have such a fantastic range of pitch
and tonal depth, one cannot expect any single compromise to handle
each one equally well.
This inability to cope with all types of voice at the same level
does lead some people to choose one technology over another.
of the PCS technologies try to minimize battery consumption during
calls by keeping the transmission of unnecessary data to a minimum.
The phone decides whether or not you are presently speaking, or
if the sound it hears is just background noise. If the phone determines
that there is no intelligent data to transmit it blanks the audio
and it reduces the transmitter duty cycle (in the case of TDMA)
or the number of transmitted bits (in the case of CDMA).
When the audio is blanked your caller would suddenly find themselves
listening to "dead air", and this may cause them to think
the call has dropped.
avoid this psychological problem many service providers insert what
is known as "Comfort Noise" during the blanked periods.
Comfort Noise is synthesized white noise that tries to mimic the
volume and structure of the real background noise.
This fake background noise assures the caller that the connection
is alive and well.
in newer CODECs such as EVRC (used exclusively on CDMA systems)
the background noise is generally suppressed even while the user
This piece of magic makes it sound as though the cell phone user
is not in a noisy environment at all.
Under these conditions, Comfort Noise is neither necessary, nor
desirable. You can read my article on EVRC by clicking here.
that we have a rudimentary understanding of the two technologies,
let's try and examine what advantages they provide.
We'll begin with CDMA, since this newer technology has created the
greatest "buzz" in the mobile communications industry.
of the terms you'll hear in conjunction with CDMA is "Soft
A handoff occurs in any cellular system when your call switches
from one cell site to another as you travel.
In all other technologies this handoff occurs when the network informs
your phone of the new channel to which it must switch.
The phone then stops receiving and transmitting on the old channel,
and it commences transmitting and receiving on the new channel.
It goes without saying that this is known as a "Hard Handoff".
CDMA however, every site are on the SAME frequency.
In order to begin listening to a new site the phone only needs to
change the pseudo-random sequence it uses to decode the desired
data from the jumble of bits sent for everyone else.
While a call is in progress the network chooses two or more alternate
sites that it feels are handoff candidates. It simultaneously broadcasts
a copy of your call on each of these sites.
Your phone can then pick and choose between the different sources
for your call, and move between them whenever it feels like it.
It can even combine the data received from two or more different
sites to ease the transition from one to the other.
arrangement therefore puts the phone in almost complete control
of the handoff process.
Such an arrangement should ensure that there is always a new site
primed and ready to take over the call at a moment's notice.
In theory, this should put an end to dropped calls and audio interruptions
during the handoff process.
In practice it works quite well, but dropped calls are still a fact
of life in a mobile environment.
However, CDMA rarely drops a call due to a failed handoff.
problem facing CDMA systems is channel pollution.
This occurs when signals from too many base stations are present
at the subscriber's phone, but none are dominant.
When this situation occurs the audio quality degrades rapidly, even
when the signal seem otherwise very strong.
Pollution occurs frequently in densely populated urban environments
where service providers must build many sites in close proximity.
Channel pollution can also result from massive multipath problems
caused by many tall buildings.
Taming pollution is a tuning and system design issue. It is up to
the service provider to reduce this phenomenon as much as possible.
defense of CDMA however, I should point out that the new EVRC CODEC
is far more robust than either of the earlier CODECs.
Because of its increased robustness it provides much more consistent
audio in the face of high frame error rates.
EVRC is an 8 kilobit CODEC that provides audio quality that is almost
as good to the older 13 kilobit CODEC.
Since CDMA consumes only as much of the "ether" as a user
talks, switching everyone to an 8 kilobit CODEC was an inevitable
confuse EVRC with the old (and unlamented) 8 kilobit CODEC implemented
in the early days of CDMA deployment.
That CODEC was simply awful, and very few good things could be said
EVRC is a far more advanced compression algorithm that cleans up
many of the stability problems inherent in the two older CODECs.
The sound reproduction is slightly muddier than the 13 kilobit CODEC,
but the improvement in stability makes up for this.
often cite capacity as one CDMA's biggest assets.
Virtually no one disagrees that CDMA has a very high "spectral
It can accommodate more users per MHz of bandwidth than any other
What experts do not agree upon is by how much.
Unlike other technologies, in which the capacity is fixed and easily
computed, CDMA has what is known as "Soft Capacity".
You can always add just one more caller to a CDMA channel, but once
you get past a certain point you begin to pollute the channel such
that it becomes difficult to retrieve an error-free data stream
for any of the participants.
ultimate capacity of a system is therefore dependent upon where
you draw the line.
How much degradation is a carrier willing to subject their subscribers
to before they admit that they have run out of useable capacity?
Even if someone does set a standard error rate at which these calculations
are made, it does not mean that you personally will find the service
particularly acceptable at that error rate.
move away from CDMA now and have a look at TDMA.
Before we can go any further though, I should note that there are
actually three different flavors of TDMA in the PCS market.
Each of these technologies implements TDMA in a slightly different
way.v The most complex implementation is, without a doubt, GSM.
It overlays the basic TDMA principles with many innovations that
reduce the potential problems inherent in the system.
reduce the effects of co-channel interference, multipath, and fading,
the GSM network can use something known as Frequency Hopping.
This means that your call literally jumps from one channel to another
at fairly short intervals.
By doing this the likelihood of a given RF problem is randomized,
and the effects are far less noticeable to the end user.
Frequency Hopping is always available, but not mandated. This means
that your GSM provider may or may not use it.
is a proprietary Motorola technology that no other company seems
to participate in.
Only Motorola makes iDEN phones, and only Motorola makes iDEN infrastructure
Perhaps the company guards its technology on purpose.
iDEN was initially deployed as an alternative to standard packet
radio systems commonly used by public safety and business users.
However, it also provided phone interconnect services that are extinguishable
from phone services offered by the other PCS systems, as well as
packet data services for web browsing and hooking up your laptop
to the Internet.
there is the old IS-136 technology, but this is now an officially
All of the North American providers who used it (Rogers, Cingular,
and AT&T) are abandoning it in favor of GSM.
The same is happening in other parts of the world where IS-136 was
I therefore will not spend much time talking about this variation
Each of these TDMA technologies uses a different CODEC.
GSM sports a CODEC called EFR (short for Enhanced Full Rate). This
CODEC is arguable the best sounding one available in the PCS world.
IS-136 used to sound horrible, but in the fall of 1997 they replaced
their old CODEC with a new one.
This new CODEC sounds much better than the old, but it doesn't quite
match the GSM and CDMA entries.
systems still rely on the switch to determine when to perform a
Unlike the old analog system however, the switch does not do this
in a vacuum.
The TDMA handset constantly monitors the signals coming from other
sites, and it reports this information to the switch without the
caller being aware of it.
The switch then uses this information to make better handoff choices
at more appropriate times.
the most annoying aspect of TDMA system to some people is the obviousness
of handoffs. Some people don't tend to hear them, and I can only
envy those individuals.
Those of us who are sensitive to the slight interruptions caused
by handoffs will probably find GSM the most frustrating.
It's handoffs are by far the most messy. When handoffs occur infrequently
(such as when we are stationary or in areas with few sites), they
really don't present a problem at all.
However, when they occur very frequently (while travelling in an
area with a huge number of sites) they can become annoying.
capacity in a TDMA system is fixed and indisputable. Each channel
carries a finite number of "slots", and you can never
accommodate a new caller once each of those slots is filled.
Spectral efficiency varies from one technology to another, but computing
a precise number is still a contentious issue.
For example, GSM provides 8 slots in a channel 200 kHz wide, while
iDEN provides 3 slots in a channel only 25 kHz wide. GSM therefore
consumes 25 kHz per user, while IS-136 consumes only 8.333 kHz per
When Direct Connect is used on iDEN, 6 users can be stuffed into
a single channel, thus only 4.166 kHz is consumer per user. There
is also a new 6:1 interconnect CODEC coming for iDEN which will
allow 6 phone users per channel.
would be sorely tempted to proclaim that iDEN has 3 to 6 times the
capacity of GSM.
In a one-cell system this is certainly true, but once we start deploying
multiple cells and channel reuse the situation becomes more complex.
Due to GSM's better error management and frequency hopping the interference
of a co-channel site is greatly reduced.
This allows frequencies to be reused more frequently without a degradation
in the overall quality of the service.
is measured in "calls per cell per MHz".
An GSM system using N=4 reuse (this means you have 4 different sets
of frequencies to spread out around town) the figure is 5.0 We get
an efficiency value of 6.6 for N=3.
Unfortunately I could not find any figures for iDEN systems, but
based on similar figures released for the IS-136 system we can expect
efficiency values of 6.0 to 10.0.
this figure for CDMA requires that certain assumptions are made.
Formulas have been devised, and using very optimistic assumptions
CDMA can provide a whopping 45 users per cell per MHz.
However, when using more pessimistic (and perhaps more realistic)
assumptions, the value is 12.
That still gives CDMA an almost 2:1 advantage over GSM, but questionable
advantage over a well-implement iDEN system.
let's deal with another issue involving CDMA and TDMA.
In-building coverage is something that many people talk about, but
few people properly understand.
Although CDMA has a slight edge in this department, due to a marginally
greater tolerance for weak signals, all the technologies fair about
This is because the few dB advantage CDMA has is often "used
up" when the provider detunes the sites to take advantage of
this process gain.
while a CDMA phone might be able to produce a reasonable call with
a signal level of -106 dBm, whereas a GSM phone might need -99 dBm
to provide the same level of service, does this mean that CDMA networks
will always have a 7 dB advantage? If all things were equal, then
yes, but they aren't equal.
I mentioned earlier, channel pollution is a big issue with CDMA
networks and to keep channel pollution to a minimum in urban environments
a CDMA provider needs to keep site overlap to a minimum.
Subsequently, a CDMA network engineer will use that 7 dB advantage
to his advantage by de-tuning the network accordingly.
This means that CDMA users will frequently see markedly lower signal
levels indoors than a GSM user will, but in the end it all works
out about the same.
most important aspect to in-building coverage is the proximity of
the nearest site.
When a site is located just outside of a building it can penetrate
just about any building material.
When a site is much further away however, the signals have a much
harder time of getting past the walls of a structure.
it comes to distance, remember that signals are subject to the "distance
This means that signals decrease by the square of the distance.
A site at 0.25 kilometers away will have 4 times the signal strength
of a site at 0.50 kilometers away, and 16 times that of a site 1.0
Distance squared however is the rate of signal reduction in free
Recent studies have shown that terrestrial communications are usually
subject to rates as high as "Distance cubed", or even
"Distance to the 4th". If the latter is true, then a site
1.0 kilometers away will actually be 256 times weaker than a site
0.25 kilometers away.
penetration is therefore less a technology issue than it is an implementation
Service providers who have sites close to the buildings you commonly
visit will inevitably look better than those who don't.
Never use someone else's in-building experiences unless you expect
to go in the same buildings as they do.
You cannot make useful generalizations about in-building coverage
based upon one person's experience.
does have one peculiarity concerning in-building penetration that
does not affect TDMA.
When the number of users on a channel goes up, the general level
of signal pollution goes up in tandem.
To compensate for this the CDMA system directs each phone to transmit
with slightly more power.
However, if a phone is already at its limit (such as might be the
case inside a building) it cannot do anything to "keep up with
This condition is known as "the shrinking coverage phenomenon"
or "site breathing".
During slow periods of the day you might find coverage inside a
specific building quite good. During rush hour however, you might
find it exceedingly poor (or non-existent).
the end, can we really proclaim a winner in the CDMA vs TDMA war?
For the time being I think not. Perhaps in the future, when newer
technologies built around wider bandwidth CDMA technologies come
into existence, the issue will warrant another look.
By that time, even GSM will have moved to CDMA as its air interface
of choice, but don't let that fool you into believing that they
think the current TDMA air interface is inadequate for its purpose.
Future standards are being built around high speed data.
you are presently in the market for a new phone my advise to you
is to ignore the hype surrounding the technologies and look at service
Judge each with an eye to price, phone choice, coverage, and reputation.
Technology should play a very small roll in your choice (with the
exception of IS-136, whose future is extremely limited at this time).
If you follow this advice, you'll probably be much happier with
the phone and service you inevitably wind up with.