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1 INTRODUCTION

The Code Division Multiple Access (CDMA)

technology has been known for a long time. Namely,

in the Soviet Union (USSR), the first work dedicated

to this muliplexing technique and solutions for

wireless communication applications was published

in 1935 by Russian scientist Dmitry Ageev. During

experiments conducted at that time, it was shown that

by applying linear methods, three types of signal

separation appear: frequency, time, and

compensation. Accordingly, for the first time, the

CDMA technology was used practically in 1957,

when a young Rusian military radio engineer, Leonid

Kupriyanovich, made in Moscow his first

experimental model of a portable automatic mobile

phone system, called the LK-1, with a base station. In

effect, the LK-1 mobile phone, as a precursor to

today’s mobile phones, had a weight of

approximatelly 3 kg, the range of its operation

coverage was up to about 20-30 km,and between 20

and 30 hours of battery life. The base station that

serves mobile phones, as described by the author,

could communicate simultaneously with several

mobile users. However, in 1958, radio engineer

Kuprijanovic made a new experiment with a "pocket"

model of a mobile phone, which weighed only 0.5 kg.

In order to serve a larger number of customers,

Kuprijanovic proposed a device he called a

"correlator". Using the achievements of coding

multiple acces in the development of mobile phones,

in 1958, the intensive research and development have

also begun in the USSR vehicular mobile phones

known as "Altai", for national use in road vehicles

and cars, based on the Soviet standard MRT-1327.

This phone system weighed 11 kg (24 lb), which

initially was placed in the trunk of the vehicles of

high-ranking officials and used a standard handset in

the passenger compartment. The main developers of

the Altai system were Voronezh Science Research

Institute of Communications (VNIIS) and State

Specialized Project Institute (GSPI). In 1963 this

Analyses of Code Division Multiple Access (CDMA)

Schemes for Global Mobile Satellite Communications

(GMSC)

D.S. Ilcev

Durban University of Technology, Durban, South Africa

ABSTRACT: This paper describes in particular Code Division Multiple Access (CDMA) applicable in Global

Mobile Satellite Communications (GMSC). In satellite communication systems, as a rule, especially in GMSC

networks many users are active at the same time. The problem of simultaneous communications between many

single or multipoint mobile satellite users, however, can be solved by using Multiple Access Technique (MAT)

system. Since the resources of the systems such as the transmitting power and the bandwidth are limited, it is

advisable to use the channels with complete charge and to create a different MAT to the channel. This generates

a problem of summation and separation of signals in the transmission and reception parts, respectively.

Deciding this problem consists in the development of orthogonal channels of transmission in order to divide

signals from various users unambiguously on the reception part.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 14

Number 4

December 2020

DOI: 10.12716/1001.14.04.03

806

service started in Moscow, and in 1970 Altai service

was used in 30 USSR cities.

The CDMA technique is a method of channel

access used by various radio communication

technologies, therefore, the CDMA scheme is an

example of multiple access, where several

transmitters can simultaneously send information

over a single communication channel. This allows

several users to share the frequency band (see

bandwidth). To enable this without unnecessary

interference between users, CDMA uses spread

spectrum technology and a special coding scheme

(where each transmitter is assigned a code). This

technique allows several users to share the frequency

band by spoecific separation. To enable this without

unnecessary interference between users, CDMA uses

spread spectrum technology and a special coding

scheme, where each transmitter is assigned a code.

The CDMA sceme is used as an access method in

many mobile phone standards. The IS-95, also called

"cdmaOne", and its 3G evolutionary CDMA2000 are

often referred to as "CDMA", but UMTS, the 3G

standard used by mobile operators, also uses

"broadband CDMA" or V-CDMA, as well as TD-

CDMA. and TD-SCDMA, as its radio technologies.

The succesor for 4G CDMA2000 was Ultra Mobile

Broadband (UMB), however, in November 2008, The

US little know company named Qualcomm

Incorporation announced that it was completing

technology development, favoring Long-Term

Evolution (LTE).

Therefore, the CDMA technique is based around a

special form of transmission known as Direct

Sequence Spread Spectrum (DSSS) scheme, which

history can be directly linked back to the 1940s when

this form of transmission was first envisaged. As

electronics technology improved, it started to be used

for covert military witeless communication in view of

the facts that the transmissions look like noise, it is

difficult to decipher without the knowledge of the

right codes, and furthermore it is difficult to jam.

With the revolution in cellular telecommunications

that occurred in the 1980s a Qualcomm Incorporation

working on DSSS transmissions started to look at this

as the basis for a CDMA as cellular

telecommunications multiple access scheme.

The concept of CDMA had to proved in the field

and accordingly the Qualcomm company was joined

by US network operators Nynex and Ameritech to

develop the first experimental CDMA system. Later

the mutual team was expanded as Motorola and

AT&T (now Lucent) joined to bring their resources to

speed development. As a result this it was possible to

start writing a specification for CDMA project in

1990. With the support of the Cellular

Telecommunications Industry Association (CTIA)

and the Telecommunications Industry Association

(TIA) a standards group was set up. Therefore, an

oficially the CDMA technique was introduced in 1995

and soon become the fastest growing wireless

technology.

Figure 1. Code Division Multiple Access (CDMA)

Technique and CDMA Carrier Spectra

Afterwards, this standard group of CTIA and TIA

then published the standard for the first CDMA

system in the form of IS-95, resulting in the formal

publication of IS-95-A in 1995. The first CDMA

system was launched in September 1995 by

Hutchison Telephone Co. Ltd. in Hong Kong and SK

Telecom in Korea soon followed along with networks

in the USA. This was only one cellular

telecommunications system, although it was the first.

Its development lead on to the DMA2000 series of

standards. The use of CDMA did not stop with

CDMA2000 as it became necessary to evolve the

cellular Global System for Mobile (GSM) standard so

that it could carry data and provide significant

improvements in terms of spectrum use efficiency.

Accordingly CDMA scheme, in the form of

Wideband CDMA (WCDMA) was adopted for this

standard.

The US Loral Space & Communications company,

with Qualcomm Incorporation in 1998 developed the

concept of Globalstar GMSC system that uses CDMA

and Frequency Division Multiple Access (FDMA)

methods for mobile satellite communications. The

Globalstar CDMA technique is a modified version of

the IS-95, which was originally developed by

Qualcomm for Cellular 3G communication services

world wide. Pioneered by Qualcomm the CDMA

signal provides excellent data and voice capacity

through the Globalstar mobile satellite phone

network of 48 satellites. Globalstar chose CDMA

system for use in it satellite communication network

when Globalstar launched service in 2000. The

CDMA technique converts speech signal into digital

format and then transmits it from the Globalstar

Satellite phone up to the satellite systems and down

to the ground station. Every call over the satellite

network has its own unique code which distinguishes

it from the other calls sharing the airwaves at the

same time. The CDMA signal is without interference,

cross talk or static.

2 PRINCIPLES OF CDMA SCHEME

In the CDMA system all fixed and mobile users

occupy the total transponder bandwidth all the time.

The users can be separated because each channel is

multiplied by a unique spreading code. The

composite signal is then modulated onto a carrier.

The information is recovered by multiplying the

demodulated signal with the same spreading code.

For instance, the receiving Mobile Earth Station

(MES) terminal is accordingly able to recover the

transmitted message by a specific user from schore.

No frequency or time coordination is needed before

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accessing the transponder, so new users can easily be

included and the CDMA signals are resistant to

interfering signals from other users. This property

can be utilised in systems with Very Small Aperture

Terminals (VSAT) stations for fixed and mobile

applications, where interference may be received

from adjacent satellites. The main disadvantages are

cost and complexity of the receivers. The simplest

form of satellite VSAT networks is the point-to-point

or point-to-multipoint configuration conecting groind

consumers with MES terminals via Ground Earth

Stations (GES) and Geostationary Earth Orbit (GEO)

and Non-GEO satellites. Usually, for GMSC networks

can be used Medium Earth Orbit (MEO) or Little

Earth Orbit (LEO) satellite constellations.

The CDMA solution is based on the use of the

modulation technique also known as Spread

Spectrum Multiple Access (SSMA), which means that

it spreads the information contained in a particular

signal of interest over a much greater bandwidth than

the original signal. In this MA scheme, the resources

of both frequency bandwidth and time are shared by

all users employing orthogonal codes, which scheme

is shown in Figure 1. (Left). The CDMA scheme is

achieved by a PN (Pseudo-Noise) sequence generated

by irreducible polynomials, which is the most

popular CDMA method. In this way, an SSMA

method using low-rate error-correcting codes,

including orthogonal codes with Hadamard or

waveform transformation has also been proposed.

Therefore, the SDMA system is a scheme where all

concerned Earth stations can use the same frequency

at the same time within a separate space available for

each link. The satellite transponder is a resource that

can be characterized by its available power and

bandwidth, which configuration of CDMA carriers

spectra is illustrated in Figure 1 (Right). The efficient

use of this common resource is a very important

problem in satellite communication. The channels are

designed to the users either fixed (pre-assigned) or

on-demand (demand assignments). Thus, the MAT

configuration permits more than two Earth stations to

use the same satellite network for interchanging

information. Several transponders in the satellite

payload share the frequency bands in use and each

transponder will act independently of the others to

filter out its own allocated frequency and further

process that signal for transmission. This feature

allows any MES terminal located in the

corresponding coverage area to receive carriers

originating from several MES terminals and vice

versa and carriers transmitted by one MES can be

received by any GES terminal. This enables a

transmitting Earth station to group several signals

into a single, multi-destination carrier. Access to a

transponder may be limited to a single carrier or

many carriers may exist simultaneously. The

baseband information to be transmitted is impressed

on the carrier by the single process of multi-channel

modulation.

Figure 2. Satellite CDMA Network Architecture

Concerning the specific encoding process, each

user is actually assigned a signature sequence, with

its own characteristic code, chosen from a set of codes

assigned individually to the various users of the

system. This code is mixed, as a supplementary

modulation, with the useful information signal. On

the reception side, from all the signals that are

received, a given mobile user is able to select and

recognize, by its own code, the signal, which is

intended for it, and then to extract useful information.

The other received signal can be intended for other

users but they can also originate from unwanted

emissions, which gives CDMA a certain anti-jamming

capability. For this operation, where it is necessary to

identify one CDMA transmission signal among

several others sharing the same band at the same

time, correlation techniques are generally employed.

From a commercial and military perspective, this

MAT is still new and has significant advantages.

Interference from adjacent satellite systems including

jammers is better solved than with other systems.

This scheme is simple to operate as it requires no

synchronization of the transmitter and is more suited

for a military system. Small mobile antennas can be

useful in these applications, without the interference

caused by wide antenna bandwidths.

Using multibeam satellites, frequency reuse with

CDMA is very effective and allows good flexibility in

the management of traffic, and the orbit/spectrum

resources. The Power Flux Density (PFD) of the

CDMA signal received in the service area is

automatically limited, with no need for any other

dispersal processes. It also provides a low probability

of intercept of the users and some kind of privacy,

due to individual characteristic codes. The main

disadvantage of CDMA by satellite is that the

bandwidth required for the space segment of the

spread carrier is very large, compared to that of a

single unspread carrier, so the throughput is

somewhat lower than with other systems.

Therefore, in the CDMA scheme, the signals from

various users operate simultaneously, at the same

nominal frequency but are spread in the given

allocated bandwidth by a special encoding process.

Depending on the multiplexing techniques employed

the bandwidth may extend to the entire capacity of

the transponder but is often restricted to its own part,

so CDMA can possibly be combined in the hybrid

scheme with FDMA and/or TDMA. The SSMA

technique can be classified into two methods: Direct

Sequence (DS) and Frequency Hopping (FH). A

combined system of DC and FH is called a hybrid

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CDMA system and the processing gain can be

improved without increases in chip rate.

At present, the CDMA system advantages are

practically effective in new satellite systems, such as

Globalstar, also developed by Qualcomm, which is

devoted to MSS handheld terminals, and Skybridge,

involved in FSS. This type of MA is therefore

attractive for handheld and portable MSS equipment

with a wide antenna pattern. Antennas with large

beam widths can otherwise create or be subject to

interference with adjacent satellites. In any case, this

MA technique is very attractive for commercial,

military, and even TT&C communications because

some Russian satellites use CDMA for command and

telemetry purposes. The Synchronous-CDMA (S-

CDMA) scheme proves efficiently to eliminate

interference arising from other users sharing the same

carrier and the same spot beam. Interference from

other spot beams that overlap the coverage of the

intended spot is still considerable. This process to

ensure orthogonality between all links requires

signaling to adjust transmission in time and

frequency domains for every user independently.

3 CODE DIVISION MULTIPLE ACCESS (CDMA)

NETWORK CONCEPT

As outlined earlier, the CDMA technique is the

earliest implemented in the wireless systems and still

one of the most commonly employed forms of

multiple access techniques for communications via

satellite systems. In the case of CDMA different

Earth stations are able to access the total available

bandwidth of satellite transponder by virtue of their

different carrier frequencies and time, thus avoiding

interference among multiple signals. Therefore, as a

third fundamental MAT system, the CDMA is a

combination of both frequency and time separation.

In fact, it is the most complex technique to

implement, requiring several levels of

synchronization at both the transmission and

reception levels. The CDMA scheme is practical for

digital formatted data only and offers the highest

power and spectral efficiency operation of the three

fundamental techniques.

The functional display of the CDMA process is

similar in presentation to those for two fundamental

FDMA and TDMA systems. Thus, each uplink station

is assigned a time slot and a frequency band in a

coded sequence to transmit its station packets. A

forward uplink link is the transmission direction

from a fixed location, such as a GES terminal or a

base station in wirelless systems to a fixed or mobile

(MES) VSAT stations via LEO/MEO/GEO satellites,

which scenario for CDMA system is shown in Figure

If the link includes a communications relay

satellite, the forward link will consist of both an

uplink, GES terminal or a base station to satellites,

and a downlink, from satellites to MES or cellular

terminals. A return uplink is transmision direction

from a fixedor mobile location, such as fixed or

mobile (MES) VSAT of the GES terminal or a base

station in wireless systems. If the link includes a

communications relay satellite, the return link will

consist of both an uplink, from fixed or mobile (MES)

VSAT stations to LEO/MEO/GEO satellites, and a

downlink, from satellites to the fixed or mobile (MES)

or cellular terminals.

The forward downlink receive station must know

the code of frequency and time locations in order to

detect the complete data sequence. The receive station

(GES terminal) with knowledge of the code can

recoup the signal from the noise-like signal that

appears to a receiver that does not know the code. As

stated, the CDMA scheme is often referred to as the

SSMA technique because of the signal spreading

characteristics of the process, which is achieved by a

PN (Pseudo-Noise) sequence generated by irreducible

polynomials, which is the most popular CDMA

method. In this way, an SSMA method using low-rate

error-correcting codes, including orthogonal codes

with Hadamard or waveform transformation has also

been proposed.

3.1 Direct Sequence (DS) CDMA

In telecommunications, direct-sequence spread

spectrum is a spread-spectrum modulation technique

primarily used to reduce overall signal interference.

The direct-sequence modulation makes the

transmitted signal wider in bandwidth than the

information bandwidth. The data is divided and

simultaneously transmitted on as many frequencies

as possible within a particular frequency band (the

channel). It adds redundant bits of data known as

chips to the data to represent binary 0s or 1s. The

ratio of chips to data is known as the spreading ratio:

the higher the ratio, the more immune to interference

the signal is, because if part of the transmission is

corrupted, the data can still be recovered from the

remaining part of the chipping code. This dominant

DS-CDMA technique is also called Pseudo-Noise

(PN) modulation, where the modulated signal is

multiplied by a PN code generator, which generates a

pseudo-random binary sequence of length (N) at a

chip rate (Rc), much larger than the information bit

rate (Rb). The chip rate sequence is introduced by the

following relation:

Rc = N · Rb (1)

This sequence is combined with the information

signal cut into small chip rates (Rc), thus, speeding

the combined signal in a much larger bandwidth

(W~Rc). Namely, the resulting signal has a wider

frequency bandwidth than the original modulated

signal. In such a way, the transmitting signal can be

expressed in the following way:

s(t) = m(t) p(t) cos (2πfct) = m(t) p(t) cos ωct (2)

where values m(t) = binary message to be transmitted

and p(t) = spreading NP binary sequence.

Consequently, at the GES receiver, the satellite signal

is coherently demodulated by multiplying the

received signal by a replica of the carrier. Neglecting

thermal noise, the receiving signal at the input of the

detector of Low-Pass Filter (LPF) is given by the

following relation:

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r(t) = m(t) p(t) cos ωct (2 cos ωct) = m(t) p(t) + m(t) p(t)

cos 2ωct (3)

The detector LLF eliminates the HF components

and retains only the LW components, such as u(t) =

m(t) p(t). This component is then multiplied by the

local code [p(t)] in phase with the received code,

where the product p(t)2 = 1. At the output of the

multiplier this gives:

x(t) = m(t) p(t) p(t) = m(t) p(t)2 = m(t) [V] (4)

The signal is then integrated over one-bit period to

filter the noise. The transmitted message is recovered

at the integrator output, so in fact, only the same PN

code can achieve the despreading of the received

signal bandwidth.

In this process, the interference or jamming

spectrum is spread by the PN codes, while other

user’s signals, spared by different PN codes, are not

despread. Interference or jamming power density in

the bandwidth of the received signal decreases from

their original power.

Otherwise, the most widely accepted measure of

interference rejection is the processing gain (Gp),

which is given by the ratio Rc/Rb and value of Gp =

20 – 60 dB. The input and output signal-to-noise

ratios are related as follows:

(S/N)Output = Gp (S/N)Input (5)

In the forward link, the LES (Hub Station)

transmits the spread-spectrum signals, which are

spread with synchronized PN sequence to different

MSC users. Since orthogonal codes can be used, the

mutual interference in the network is negligible and

the channel capacity is close to that of FDMA.

Conversely, in the return link, the signals transmitted

from different MES users are not synchronized and

they are not orthogonal. The first case is referred to as

synchronous and the second case as asynchronous

SSMA.

However, the nonorthogonality causes

interference due to the transmission of other MES in

the satellite network and as the number of

simultaneously accessing users increases, the

communication quality gradually degrades in a

process called Graceful Degradation.

3.2 Frequency Hopping (FH) CDMA

The FH-CDMA system works similarly to the DS

system since a correlation process of de-hopping is

also performed at the receiver. The difference is that

here the pseudo-random sequence is used to control a

frequency synthesizer, which results in the

transmission of each information bit rate in the form

of (N) multiple pulses at different frequencies in an

extended bandwidth. The transmitted and received

signals have the following forms:

s(t) = m(t) cos ωc(t) t (6)

r(t) = m(t) cos ωc(t) t · 2 cos ωc(t) t = m(t) + m(t) cos

2ωc(t) t (7)

Thus, at the receiver, the carrier is multiplied by

an unmodulated carrier generated under the same

conditions as at the transmitter. The second term in

the receiver is eliminated by the LPF of the

demodulator. The relation of processing gain for FH

is:

Gp = W/∆f (8)

where W = frequency bandwidth and ∆f = bandwidth

of the original modulated signal. At this point,

coherent demodulation is difficult to implement in

FH receivers because it is a problem to maintain

phase relation between the frequency steps. Due to

the relatively slow operation of the frequency

synthesizer, DS schemes permit higher code rates

than FH radio systems.

4 CONCLUSION

The performances and capacities of MSC for CDMA,

FDMA, and TDMA/FDMA have been analyzed many

years ago for an L/C-band network with global

coverage. For the particular MSS under discussion

and for the particular antenna configurations, both

CDMA and FDMA offer similar performance, FDMA

yielding slightly higher channel capacities at the

design point and CDMA being slightly better at

higher EIRP levels. As the MSS grows and the

antenna beam size decreases, CDMA appears to be a

very efficient system, because it is not limited by L-

band bandwidth constraints. However, CDMA is

wasteful in feeder link bandwidth, and the choice of a

multiple access system must take all parameters into

consideration, such as oscillator stability, interference

rejection, system complexity, etc. as well as system

cost before deciding on a particular multiple access

systems.

The communication satellites for MSC provide

multiple-beam antennas and employ frequency reuse

of the allocated L-band frequency spectrum. It

appears that despite the fact that FDMA and

FDMA/TDMA are orthogonal systems, they

nevertheless suffer from bandwidth limitations and

sensitivity to interbeam interference in L-band.

The CDMA scheme is better at absorbing Doppler

and multipath effects, and it permits higher rate

coding, but it suffers from self-jamming and from

bandwidth constraints in the feeder link. In general,

all three multiple access systems show similar

performance. However, at the chosen design point for

aggregate EIRP, a number of beams, and allocated

bandwidth, FDMA provides still the highest system

channel capacity.

The narrowness of the frequency spectrum

allocated to MSC means that it has to be explored to

the full. Methods available for effective spectrum

utilization include efficient signal design and

subdivision of the total coverage area into narrow

illumination zones. Modern satellites for MSC have

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also onboard processors, which connect an uplink

band to a downlink beam. Processors use A/D

conversion and digital filtering. The A/D converters

quantize the signal and produce quantization noise.

Recently is developed SDMA as an advanced

solution where all concerned MES terminals can

share the same frequency at the same time within a

separate space available for each link. On the other

hand, the RDMA scheme is suitable for a large

number of users in MSC, where all MES terminals

share asynchronously the same transponder by

randomly transmitting short burst or packet

divisions. In addition is developed several mobile

Aloha methods, which successfully increase the

system throughout.

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