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Comparison between CDMA and TDMA Air Interface for

Cellular Systems
Wong Ngee Hui
School of Electrical & Electronic Engineering
Nanyang Technological University
Nanyang Avenue, Singapore 639798

Abstract division multiple access (CDMA), highlighting the

advantages of each individual system.
Second Generation (2G) mobile cellular
communication systems, which use digital This paper is organized as follows: section 2
technology, are very successful worldwide. This presents an overview of cellular systems. Section 3
paper discusses the basic important properties of compares the multiple access technologies between
different multiple access technologies used for TDMA and CDMA. Section 4 briefly describes
cellular systems. Main focus is on two popular third generation cellular systems. Conclusions are
systems: time division multiple access (TDMA) provided in section 4.
and code division multiple access (CDMA),
highlighting the advantages of each individual 2. OVERVIEW OF CELLULAR SYSTEMS
system. Factors that affect capacity on the two
systems are considered to compare their spectrum
The evolution of mobile cellular communications
efficiency. A brief overview on 3G systems is also
can be found in [1], [4]. The ability to provide
included.
wireless communications to entire population was
developed in the 1960s when Bell Laboratories
1. INTRODUCTION
developed the cellular concept: concept of dividing
a large coverage zone (market) into small cells. The
A mobile cellular communication system provides
same spectrum can be reused by different cells,
a wireless connection to the public switched
which are separated by a minimum reuse distance,
telephone network (PSTN) for any user located
to increase spectrum usage. First generation of
within the radio range of the system [1]. Due to the
cellular systems was implemented in the late sixties
continual advancement in digital and RF circuit
till the late eighties with analog systems using
fabrication, cellular phones are getting more
frequency division multiple access (FDMA).
popular and affordable.
Popular systems examples include Advanced
Mobile Phone System (AMPS) in the US,
In cellular communications, the choice of multiple
European Total Access Cellular System (ETACS)
access is one of the most important issues. There
in Europe and Nippon Telephone (NTT) system in
are three important basic properties of a multiple
Japan.
access. The properties are flexibility, quality and
capacity [2]. Operational flexibility includes easy
Today cellular network are generally called second
frequency/cell planning. Abilities to operate in
generation or 2G technologies that conform to the
different environment provide different services for
2G standards. 2G standards use digital modulation
different cell types and users to support co-
formats and TDMA/frequency division duplex
existence with other systems. Quality will include
(FDD) and CDMA/FDD multiple access
good speech quality. This can be determined by a
techniques. The most popular 2G standards include
subjective test, usually based on a subjective mean
three TDMA standards and one CDMA standard:
opinion score to determine the required carrier-to-
a) Global System Mobile (GSM), which is widely
interference ratio (C/I) [3]. Quality will also
used in Europe, Asia, Australia, South America and
include good coverage to provide the service to
some parts of US; b) Interim Standard 136 (IS-
users on demand and reliable communication in
136), which is also known as North American
terms of low number of dropped calls. High
Digital Cellular (NADC) and popular in America;
capacity radio access is vital for cellular radio
c) Pacific Digital Cellular (PDC), which is a
because it ensures maximum efficiency for the
Japanese TDMA; d) Interim Standard 95 Code
limited and expensive spectrum. This paper
Division Multiple Access (CDMA) or cdmaOne,
discusses the basic important properties of different
which is widely developed in countries such as
multiple access technologies used for cellular
North America, Japan, China, South America and
systems. Main focus is on two popular systems:
Australia.
time division multiple access (TDMA) and code
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Data transmission in 2G is generally limited by the bandwidth allocation between upstream and
user’s data throughput rate and is not sufficient to downstream traffic [6]. In TDMA/FDD operation,
support modern Internet applications. Thus, 2.5G similar frame to TDMA/TDD but different carrier
systems, which are compatible to 2G and able to frequencies for the forward and reverse link.
support the popular web browsing format language, TDMA/FDD requires less transmission bandwidth
Wireless Applications Protocol (WAP) is per radio and also less precise synchronization
introduced. transmissions to minimize interferences [6].

2. COMPARISON BETWEEN CDMA AND 2.2 Advantages of TDMA Systems

TDMA
The non-continuous transmission characteristic of
Multiple access techniques can be grouped into TDMA has posed several advantages over other
wideband and narrowband systems [1] are used to multiple access techniques [1]. Since the subscriber
allow many mobile users to share simultaneously transmitter can be turned off when not in used, this
the finite amount of radio spectrum. This must be results in low battery consumption. The handoff
done without severe degradation in performance of process is also made much simpler, since the
the system, so as to provide high quality mobile is able to detect for other base stations
communication. As present and future standards during idle time slots. Duplexers are not needed as
are based on TDMA and CDMA standards, it is TDMA uses different time slots for transmission
worthwhile to do a comparison between the two and reception. For TDMA/FDD only an addition
popular multiple access systems. switch is needed to switch between transmitter and
receiver respect to CDMA. TDMA has the
2.1 Main Features of TDMA Systems advantage of much stringent power control
requirement. The timeslot structure also supports
In TDMA systems, the radio spectrum is divided mobile assisted or mobile controlled handoff
into time slots and in which time slot only one user (MAHO). Frequency hopping (FH) is one of the
is allowed to either transmit or receive as shown in most valuable features of TDMA systems [4].
Fig. 1 [1]. There are two capacity increasing effects from FH
[6]. The first is frequency diversity and the other is
interference averaging. This is impossible for
CDMA systems. According to [4], the 13 kbps
speech coder in GSM is of better speech quality
than 8.6kbps for IS-95 system.

2.3 Main Features of CDMA Systems

CDMA is a type of Spread Spectrum Multiple

Access. In CDMA, narrowband message signal is
multiplied by a very large bandwidth signal called
the spreading signal. Spreading signal is a Pseudo-
code sequence that has a chip rate, which is an
order of magnitude greater than the data rate of the
Fig. 1. TDMA Scheme message [1]. All users use the same carrier
frequency and may transmit simultaneously. Each
Transmission for any user is non-continuous as user has its own pseudo random codeword, which
TDMA transmit data in a buffer-and-burst method. is approximately orthogonal to all other code
Each channel can be view as a particular time slot words, and all undesired code words will appear as
that re-occurs every frame, where N slots comprise noise. Fig. 3 [1] illustrates this.
a frame. Fig. 2 [1] shows a typical TDMA frame.
Power control in CDMA is essential. Otherwise
some users will jam the others. Power control
ensures that each mobile within the base station
coverage provides the same signal level to the base
station. This is done by rapidly sampling the radio
signal strength indicator (RSSI) levels of each
mobile and then sending a power change command
Fig. 2. TDMA frame structure over the forward link. A tradeoff must be made if a
mobile signal arrives at the cell site with a signal
In TDMA/time division duplex (TDD) half of time that is too weak, the weak signal will probably be
slots are for forward link while others for the dropped. If the power from a mobile user is large,
reverse link. TDMA/TDD facilitates flexible
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the performance of this mobile unit will be electromagnetic load in the air. With CMT there is
acceptable, but it will add undesired interference to also a reduction of the carrier-to-interference ratio
with all other user in the cell. This is so-called C/I by approximately k dB (depending on the
near-far effect. number of base station transmitting and receive by
the mobile station) due to the diminution of the
transmission power in the k base stations by a
factor k. In CDMA, only a correlator, which is
simpler than an equalizer, is needed [11]; however
this is at the expense of slower transmission rate.

3. SPECTRUM EFFICIENCY

It is essential to utilize the limited frequency

spectrum effectively [3]. Spectrum efficiency for
most mobile radio system is simply the channel
efficiency, i.e., the maximum number of channels
in a given frequency band. However, for cellular
systems, the reuse of frequencies must also be
considered. Spectrum efficiency is directly related
Fig. 3. CDMA Scheme to system capacity. A simple calculation is shown
to compare system capacity of TDMA and CDMA
system.
2.4 Advantages of CDMA Systems
3.1 Computation of Radio Capacity
It is bandwidth limited for TDMA systems, while it
is interference limited for CDMA, which make The cellular radio capacity, m, of TDMA can be
CDMA uniquely suitable for interference determined by the relationship [5],
dominated mobile radio environment [5]. Unlike
TDMA, CDMA has a soft capacity limit,
increasing the number of user in a CDMA system
raise the floor noise floor in a linear-manner. Thus, where Bt is the total allocated spectrum for the
there is no absolute limit in the number of user. system, Bc is the channel bandwidth, and K is the
However, the performance will reduce as capacity number of cells in a frequency reuse pattern and
increases [1]. Multi-path fading may also be can be obtained by,
substantially decreases as the signal is spread over
a large spectrum. The inherent frequency diversity K = q2/3 – (2)
will reduce the effects of small-scale fading, if the where q is the co-channel interference reduction
spread spectrum bandwidth is greater than the factor (CIRF).
coherence bandwidth. Thus, addition equalization
hardware is not required for CDMA system [1], In mobile radio environment, we may assume a
[6]. Soft handoff which the MSC may chose the
fourth power rule, i.e. γ = 4,
best version of the signal at any time without
switching frequencies, is quite easily achieved with
CDMA. Frequency and timeslot assignment and
management entail a certain complexity in TDMA
systems, which are not found in CDMA systems
[6]. Another main advantage of CDMA is that
coherent multiple transmission and reception
(CMT) can be realized [7]. In CMT, the base Where M is the total number of equivalent channels
stations surrounding a mobile station and (C/I) is the minimum received carrier-to-
simultaneously transmit to the mobile station. interference ratio per channel or per time slot.
Coherent multiple reception (CMR) in the
downlink is obtained by coherently combine all The C/I ratio of CDMA and TDMA system is
signals arriving at the mobile station. Similarly, in related to Eb/No through (4)
the uplink, signal transmitted by the mobile station
is simultaneously received by several base stations
and are coherently combined to obtain the message.
where Rb is the transmission data rate, Bc is the
CMT and CMR are used to improve the
transmission bandwidth, Eb is the energy per bit and
exploitation of the transmitted power and therefore
No is the interference power per hertz.
reduce the necessary transmitted power and the
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For TDMA capacity calculation, Rb = Bc since Reversed Link:

TDMA use designated time slots, hence C/I is
always positive as baseband Eb/No is always greater Using (6),
than one. However, for CDMA the transmission
bandwidth, which is also the spread-bandwidth is M = 23.22 for C/I = 0.045 (-13.4dB)
Bc>>Rb, and hence C/I is always negative. This is M = 36.71 for C/I = 0.028 (-15.4dB)
because all the channels share the same bandwidth
resulting in the remaining M-1 users appearing as The radio capacity then becomes,
interference to each user.
m= 17.46 channels/cell for C/I = 0.045 (-
Power control can increase the capacity of CDMA 13.4dB)
systems [1], [5]. With forward link power control, m= 27.60 channels/cell for C/I = 0.028 (-
the C/I of a mobile unit at a distance R at the cell 15.4dB)
boundary can be derived [5] as
Hence from the above calculations, with power
C/I=1/1.656M – (5) control, the mobile capacity is forward link limited.
Therefore the capacity of 10.08 channels/cell is
With reverse link power control, interference chosen. Unlike TDMA systems, which are
power for any received signal from the mobile is bandwidth limited, the capacity of CDMA system
(M-1) times the desired signal power as the is interference limited. Therefore, in CDMA
remaining (M-1) mobiles act as interferes. system, the capacity increases as link performance
for each user decrease as shown.
C/I=1/(M-1) – (6)
A way to reduce interference in CDMA system is
Allotted total spread bandwidth is normally Bt = Bc to use directional antennas that receive signals from
= 1.25 MHz. This is ten percent of total spectral only a fraction of current users, thus reducing of
allocation, which is 12.5 MHz for cellular interference. Another way is taking advantage of
telephone service. A bit rate of 8kb/s is assumed as the intermittent nature of speech, which has a duty
an acceptable toll quality vocoded speech. Typical cycle of 3/8 [1], to operate in discontinuous
values of Eb / No = 6.5dB and 8.5dB for CDMA [4]. transmission mode (DTX). In DTX mode, the
The radio capacities are computed for both TDMA transmitter is turned off during the periods of
and CDMA Systems as follows. silence in speech. Thus, the average capacity of a
CDMA system can be increased by a factor
3.2 Capacity for CDMA with Power Control inversely proportional to the duty factor, which is
approximately 2.6.
The C/I required for Eb / No = 8.5 and 6.5dB can be
computed using equation (4) with Bt = 1.25MHz Hence, taking sectorization factor of 3 (i.e. 3
sectors of 120o) and voice activation of 2.6, the
C/I = 0.045 (-13.4dB) for Eb / No = 8.5dB effective radio capacity of CDMA is:
C/I = 0.028 (-15.4dB) for Eb / No = 6.5 dB
m = 10.08 x 2.6 x 3 = 78.62 channels/cell
For CDMA system q = D / R = 2 and hence k = q2 /
3 = 1.33 [5]. 3.3 Capacity for TDMA (GSM)

Forward Link: Assuming system parameters Bt = 1.25MHz, Bc =

200kHz and Rb = 8kb/s. GSM employs 8 time slots
Using (5), in 200kHz channel.

M = 13.41 for C/I = 0.045 (-13.4dB) M = Bt / Bc=1.25M/(200k/8) = 50 channels

M = 21.56 for C/I = 0.028 (-15.4dB) K =3
m = M / K= 50/3 = 16.7 channels/cell
The radio capacity using (3), then becomes,
3.4 Capacity for TDMA (IS-136)
m= 10.08 channels/cell for C/I = 0.045 (-
13.4dB) Interim Standard 136 uses three time slotted users
m= 22.70 channels/cell for C/I = 0.028 (- for each 30KHz channel.
15.4dB)
M = Bt / Bc=1.25M/(30k/3) = 125 channels
K =4
m = M / K= 50/4 = 31.5 channels/cell
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radio channel. The eventual 3G evolutions for

From the above calculations, it can be seen that the CDMA systems from IS-95 lead to cdma2000
capacity of CDMA about 4 times greater than GSM based on the fundamental of IS-95 and IS-95B
system and about 2.2 times greater than IS-136. technologies. The eventual 3G evolutions for GSM,
Thus, CDMA has higher spectrum efficiency. This IS-136 systems lead to Wideband CDMA (W-
can be further explained by the fact that while CDMA). W-CDMA is based on the fundamentals
TDMA reuse frequencies depending on the of GSM, as well as the merged version of GSM and
isolation between cells provided by the path loss in IS-136 through EDGE. This is discussed in
terrestrial radio propagation, CDMA can reuse the [1,8,9,10]
entire spectrum for all cells, this results in an
increase of capacity by a large percentage over the 5. CONCLUSION
normal frequency reuse factor [1]. However
conditions such as non-ideal orthogonal of CDMA Systems based on TDMA and CDMA have been
Pseudo-Code sequence and techniques such as discussed. Both systems have their own
Adaptive Channel Allocation (ACA) in addition of advantages and disadvantages. Although, matured
frequency hopping (FH) that can increase TDMA TDMA technologies have made it seem the
capacity is not included in the calculations. obvious choice. Merits of CDMA such as higher
capacity through voice activation and antenna
4.THIRD GENERATION (3G) CELLULAR sectorization, the ability to support CMT and CMR,
SYSTEMS soft handoff etc cannot be discarded and provide an
edge for CDMA systems. Research and
The worldwide success of 2nd generation cellular implementation of 3G systems is ongoing. With
systems is much greater than what most people many enhancement from the present 2G systems,
expected. However, these 2G systems are limited in 3G will no doubt be as if not more successful.
data rate. Japan is the first to rollout with a detailed
standardization of wideband code-division multiple ACKNOWLEDGEMENT
access (W-CDMA) system. This process
accelerates the standardization in Europe and US. The author would like to thank Dr. Peter Han Joo Chong for the
valuable discussions to enable this comparison to be made.
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