Communications and Network, 2013, 5, 374-379
http://dx.doi.org/10.4236/cn.2013.53B2068 Published Online September 2013 (http://www.scirp.org/journal/cn)
Digital Radio Broadcasting for Community Radios Using
Pseudo-random Codes.
Fabrício de Araújo Carvalho, Fernando Walter
Departamento de Telecomunicações, Instituto Tecnológico de Aeronáutica, São José dos Campos, Brazil
Received July, 2013
ABSTRACT
This paper aims to present a digital radio broadcasting system that explores the advantages of pseudo-random codes. In
this context, a transmitter and its dual receiver are able to reuse frequency spectrum bands without interfering on other
existing communication systems. It is proposed a communication system that allows radio broadcasting with the fol-
lowing characteristics: lower transmission power, new communication channels and digital signal processing techinques
to add positioning services in two dimensions.
Keywords: Digital Radio Broadcasting; Software Defined Radio; Spectral Efficiency; Trilateration Positioning
1. Introduction
Since 2005, the Brazilian Ministry of Communications
has opened a public call to assess the existing digital ra-
dio systems. Thus, a possibility arose for the develop-
ment of a national digital communication system to assist
or complement the existing AM/FM modulations already
known.
The current AM/FM analog radios are generally con-
sidered as a means of entertainment, but are also impor-
tant in many areas of public life. In this sense, the lower
power community radio is an important tool for integra-
tion and social development within neighborhoods, as it
allows the union of their listeners around their common
needs. The awareness raised about current problems mo-
tivates social agents to find the most appropriate solu-
tions according to the common interests. Neighborhood
radio broadcasting promotes good socio-environmental
development and consequently improvement of living
standards in the region. Yet, the shortage of available
channels has been seen as an impediment for the disper-
sion of these radios, causing the proliferation of clandes-
tine ones.
The FM receiver has a feature known as capture effect.
This occurs when there are more signals transmitted on
the same carrier frequency, in this case the FM receiver
will respond to the higher power signal and ignore the
others.
Frequency bands in communication systems are allo-
cated according to the type of application and the trans-
mission media used. The frequency spectrum used by all
these means of communication has become a scarce
natural resource. By observing the useful frequency
spectrum (Figure 1), we can see a shortage of available
bands in comparison to the current demand.
Many countries have already modified their conven-
tional radios or are still using them in parallel with the
new digital radio systems. These systems resulted from
experiments carried out by universities and research in-
stitutions. Some of the best-known digital radio systems
have been tested by the Brazilian government (IBOC - In
Band On Channel; DRM - Digital Radio Mondiale).
Tests with the American and European systems will de-
fine the best project in accordance with Brazil´s reality.
For this reason, it is ultimately important for Brazil to
define its own pattern.
Initially, the proposed modulation was exclusively ap-
plied to community digital radio broadcasting to provide
new communication channels with or without minimal
interference as possible on the other ones.
The modulation proposed has the following character-
istics: it provides more communication channels in addi-
tion to the already existing ones; lower power required
Figure 1. Frequency Allocation: 54.0 - 117.975 MHz (Source:
National Telecommunications and Information Admini-
stration, http://www.ntia.doc.gov/osmhome/allochrt. html).
C
opyright © 2013 SciRes. CN

F. de A. CARVALHO, F. WALTER 375
for transmission; high bit rate; positioning service in two
dimensions due to the use of pseudo-random codes,
PRNs.
2. Digital Radio
The PRN codes are used in uncountable communication
systems to allow access through multiple channels to the
medium using the same carrier frequency. This technique
for frequency sharing is known as CDMA.
PRN codes are also used in satellite positioning and
navigation to determine position, speed and moving time
through a process known as trilateration. Position deter-
mination is much easier with these codes because phase
ambiguity is reduced by presenting long periods. One of
the best-known positioning systems with this method is
the GPS system.
Trilateration process of signal is to determine the posi-
tion of the mobile body. It is based on measuring the
propagation time of the Radio Broadcasting station signal
TxN up to the receiver. In this case, it is necessary that
the stations are synchronized. This synchronization can
be achieved when each station is capable of reading a
pilot signal of its neighboring stations.
Figure 2. Digital radio broadcasting.
1

2

3

Figure 3. Digital radio broadcasting.
PRN Codes
The PRN codes used in the radio station are called Gold
codes. These are almost orthogonal, thus there is little
similarity among them.
They are formed by a sequence of binary "0s" and "1s".
In the pseudo-random sequences, the bits are called
"chips". The technical term “chip” is used to distinguish
the code bit from the information bit. We can see in
Equation 1 the PRN code representation ci(t) of the i-th
transmitter.

1
0
,0
Nchip
iil il
lchip
tlT
ctcc or
T


 




1
(1)
where cil corresponds to the chip value ("0" and "1") for a
given l; and l is a counter from 0 to N-1; N is the number
of chips of the code, and Tchip is the chip duration.
The sequence ci(t) is periodic (Equation 2), of Tp pe-
riod (N x Tchip)

1
00
Nchip p
iil
kl chip
tlT kT
ct cT







  (2)
The sequence can be converted into Ci(t), of "1s" and
"-1s", if the multiplication operation is used for the signal
modulation (Equation 3):


cos .
ii
Ct ct


 
(3)
The sequence is generated using two maximum length
registers called G1 and G2 (Figure 4). For a sequence
with a length of 63 chips (= 26 - 1), registrers with 6 cells
or elements are used. Both records are initialized within
1s (module 2 operation). Each record has its values
shifted according to a time reference (10.23 MHz), which
will determine the chip rate and hence the code period.
Figure 4. Gold code generator.

F. de A. CARVALHO, F. WALTER
376
Table 1 shows polynomial G1(t) and G2(t) used to
match each other, the value in each element of the regis-
ters G1 and G2 through a module 2 operation (

).
The PRN sequence determines which station will be
received. The distinction made by the receiver between
the information from the transmitters is made through a
correlation process. The correlation occurs between the
code contained in the signal transmitted by the broad-
casting station and its reply, present in the target recep-
tor.
Correlation makes possible to measure the degree of
similarity between these signals by amplitude of correla-
tion. The amplitude of correlation between distinct and
orthogonal PRNs is approximately equal to zero, Rij (τ)
(cross-correlation) for every τ delay (Equation 4).

  

qualquerpdttCtC
NT
R
chip
NT
ji
ij
/;0
1
0



(4)
In this equation, Ci(t) and Cj(t) are the PRN codes for
the i-th and j-th transmitter, respectively. For the auto-
correlation (same codes), Rii(τ), the amplitude is different
from zero for delay values τ below one chip; 0 ≤ | τ | ≤
Tchip (Figure 5):
As we can observe, the autocorrelation peaks have a
width of two chips and repeat every TC period of the
code (= N x Tchip). The amplitude increases linearly from
a previous chip to a maximum in the alignment, decreas-
ing to zero, one chip after the maximum. For orthogonal
codes, the amplitude Rii (τ) will be approximately zero
for τ delays larger than the chip (Equation 5).
  
0
2
1
1;
0;
chip
NT
iii i
chip
chip
c
chip
RCtC
NT
AparaT
T
para T










tdt
(5)
Figure 6 shows the chip sequence provided by the
PRN code generator (Figure 4).
The autocorrelation for this code is shown in Figure 7.
We can observe the maximum amplitude-normalized
values [63, 15, -1, -17]. The sequence has a 21 chips de-
lay only to emphasize the maximum amplitude.
Figure 8 shows the frequency spectrum of the 10.23
MHz PRN sequence sampled at a rate of 60 MHz.
Tabel 1. PRN code generator polynomials.
Reg. Polinomyal
G1(t) 61
1xx 
G2(t) 6521
1xxxx 
Both transmitter and receiver are designed and en-
coded with the Matlab ® software for the concept tests.
These signals were generated in baseband and intermedi-
ate frequency (IF) with signal processing.
N
A2

chip
TN )1(


chip
TN )1(


chip
TN )1(

chip
TN)1( 
2
A
)(

R
chip
NT
chip
T

chip
T
chip
NT
chip
T
Figure 5. PRN sequence autocorrelation process.
10 20 30 40 50 60
-1
-0.8
-0.6
-0.4
-0.2
0
0. 2
0. 4
0. 6
0. 8
1
Núm ero d e chi ps
Amplitude
Code PRN
Figure 6. PRN sequence of 63 chips from the Gold code
generator.
10 20 30 40 50 60
-10
0
10
20
30
40
50
60
70
X: 21
Y: 63
Número de c hi ps
Am plitude
Correla ção
X: 41
Y: 15
X: 31
Y: -1
X: 45
Y: -17
Figure 7. PRN code autocorrelation with 21 chips delay.

F. de A. CARVALHO, F. WALTER 377
3. CDMA Transmitter
Before The block diagram of Figure 9 provides an over-
view of the transmitter encoded in baseband and digital
IF. In this diagram, information (voice signal) is digitized
by an A/D converter. The binary sequence provided by
the converter modulates the PRN sequence (in +1 and -1).
This one will convey information. Subsequently, this
signal is heterodyned for an IF after being multiplied by
a digital carrier. If necessary, the transmission rate can be
increased by adding another component in quadrature.
O sinal transmitido é então armazenado em um
arquivo binário para ser lido pelo receptor em software.
The signal transmitted is thereafter stored in a binary
file to be read by the software receiver.
4. CDMA Receirver
After Figure 10 shows a common CDMA receiver with a
block diagram.
After downconverter, the analog IF is sampled, quan-
tized and encoded by an A/D converter. Figure 11 illus-
trates the sampling in the fs frequency (60MHz).
-30 -20-10 010 20
1
2
3
4
5
6
7
8
9
10
11
Freqüênc ia(M Hz )
A m plitude
|C(W)|
Figure 8. Frequency spectrum for the PRN code, already
sampled.
Figure 9. Transmitter block diagram in baseband and digi-
tal IF.
The converter´s output is a digital IF that will be proc-
essed by the correlator using programming techniques.
Figure 12 shows the tracking process executed by the
CDMA receiver [1-4] through a block diagram.
5. Results
The The audio signal captured by the computer's micro-
phone was sampled at a rate of 8 kHz, quantized and
encoded into 16-bit words. Each information bit modu-
lates the phase of a PN code to ± π rad. This one modu-
lates the 10.7 MHz intermediate frequency and stores it
in the accumulator ("buffer"). The signal transmitted is a
BPSK (Binary Phase Shift Keying). For bit timing, it is
Figure 10. Community radio broadcasting CDMA receiver.
2
s
f
s
f
Figure 11. Bandpass sampling process.
Figure 12. Tracking and demodulation step.

F. de A. CARVALHO, F. WALTER
378
transmitted the following preamble 10101010101010101
for every five samples of the audio signal.
For the transmitter screen, we have the following
windows: 1: audio signal (counting from 1 to 5); 2: audio
signal spectrum; 3: signal spectrum in the digital IF; 4:
truncated PRN code ; 5: Doppler on the carrier; 6: digi-
tised IF; 7: error in carrier phase; 8: 16-bit word (quanti-
fied and encoded audio), 9: error in frequency code; 10:
BPSK signal transmitted and 11: error in code phase.
Figure 14 shows the signal stored in binary file and
processed by the receiver.
The station's signal must be weak enough to not affect
other communication systems. In this project, it is trans-
mitted with power next to the thermal noise.
Therefore, the broadcasting station reduces its inter-
ference on other communication systems.
On the receiver´s screen we can list the following
windows 1: Doppler on the carrier; 2: error in carrier
phase; 3: diagram of the constellation; 4: error in the
code frequency; 5: phase error of the code; 6: recovered
bit sequence; 7: correlation amplitude of the advanced
signal; 8: correlation amplitude of the aligned signal, and
9: correlation amplitude of the delayed signal.
0 1 2 34 5
-0.5
0
0.5 Sinal de A udio
t[s]
A
mp
lit
u
d
e
01234
30
40
50
60
70
Freq ncia(KHz)
[dB]
Espectro do Audio
10 20 30 4050 60
2
4
6
8
10
12
Es pec t ro do sinal trans m itido
Freqncia [MHz]
10 20 30 40 5060 7080 90 100
-1
0
1
Amplitude
Codigo da r醖io
20 40 60 80 100
1.07
1.07
1.07
x 107
Amplitude
Doppler so bre a portado ra
10 20 30 40 5060 7080 90 100
-2
0
2
Porta dora na F I
Amplitude
20 40 60 80 100
2
4
6Erro na fase d a portad ora
Amplitude
2468 10 12 14
-1
0
1
Dados B in醨 ios (Tx)
Amplitude
20 40 60 80 100
1.023
1.023
1.023
x 107Doppler sobre o c骴igo
Amplitude
10 20 30 40 5060 7080 90 100
-1
0
1
Sinal BPSK
Amplitude
Amostras 20 40 6080 100
0.08
0.1
0.12
0.14
0.16
Erro na fase do c骴igo
Amplitude
Amostras
11
10
9
8
7
6
5
4
3
1
2
Figure 13. Community radio broadcasting CDMA trans-
mitter. Monitoring interface.
50100 150 200250 300 350
-5
0
5Sinal do F ront-End
Amostras
Amplitude
510 15 20 25 30 354045 50 55 60
5
10
15
20
Es pectro do sinal do F ront - E nd
Freqüência [MHz]
50100 150 200 250300 350
-1
0
1
Amplitude
Sinal Int erno: Codi go P RN
50100 150 200 250300 350
-2
0
2
Sinal Int erno: Com ponent e em F as e
Amplitude
50100 150 200 250300 350
-2
0
2
Sinal Int erno: Componente em Q uadrat ura
Amplitude
Amostras
Figure 14. Tracking step signals: 1: received signal; 2:
spectrum of the received signal; 3: internally generated
PRN code; 4: PRN code and carrier in phase; 5: PRN code
and carrier in quadrature.
-50 -10 010 50
-1 8 0
-1 6 0
-1 4 0
-1 2 0
-1 0 0
-80
Frequency [MHz]
Power [d Bm]

-n dB m
(> -103,98 dB m )
PRN
N oise floor , 20 M Hz BW
RF filter
Figure 15. Thermal noise and power spectral density for the
received signal.
50100 150 200250300350 400
10.5
10.6
10.7
10.8
Freqnc i a da Portadora [MHz ]
Freq ncia
-500 0500
-100
-50
0
50
100
Diagrama de cons t el a玢o
Fase
Quadratura
50100 150 200250300 350 400
10
10.2
10.4 Freqncia do Codi go [MHz ]
Freq ncia
50100 150 200250300 350 400
0
0.01
0.02
0.03 Erro de fas e da Portadora
Amostras
Erro
50100 150 200250300 350 400
-0.5
0
0.5
Erro de fase do C odigo
Amostras
Erro
50100 150 200250 300 350400
-500
0
500
Dados de bin醨ios
50100 150 200250300 350 400
0
200
400
600
800 Correl a玢o (Sinal A D)
Amostras 50100 150 200 250 300 350400
0
200
400
600
800 Correla 玢o (Sinal AL)
Amostras 50100 150 200250300 350 400
0
200
400
600
800 Correla 玢o (Sinal AT)
Amostras
Figure 16. Monitoring interface. Tracking and demodula-
tion. CDMA Community radio broadcasting receiver.
6. Conclusions
Headings, Here is proposed a digital radio broadcasting
system capable of operating in the same frequency bands
allocated for AM /FM radio, but with less transmission
power. It proposes to transmit information with power
below or closer to the thermal noise, and its value will
depend on the bit/second rate chosen for the system, on
the code length and on the range desired.
Many current communication systems use high power
transmitters to ensure service quality. By using PRN
codes, it is possible to transmit signals with low power
without interfering on other existing systems (similarly to
what happens with GPS).
The use of PRN codes expands the use of the receiver,
allowing it to determine its position, once it tracks down
the signals from at least three radio stations. In this case,
the transmitters must be with synchronized time bases.
The positioning service can assist current satellite navi-
gation systems, bringing real benefits for agriculture,
administration and public services. In the aviation sector,
it is able to assist aircraft landings (e.g. category III) and
also serve as a DGPS correction channel DGPS [5] for

F. de A. CARVALHO, F. WALTER
379
GNSS systems.
From the concept of community radio, this prototype
of the source code can be easily adapted to local needs
and real time processing by high performance digital
signal processors such as DSP and FPGAs and it can be
made available for commercial use and for police forces.
Because of its low power consumption and alternative
channels, it can be used by the Fire Department, Civil
Defense, Police, and also the Armed Forces for commu-
nication via walk-talk in public safety services. For the
Armed Forces, the signal can be easily encrypted and
changed periodically.
It must be highlighted that this project would enhance
the national production of transmitters/receivers.
7. Acknowledgements
The Company NavCon Navegação e Controle by allow-
ing the training of its engineers and by being a partner
company of the Instituto Tecnológico de Aeronáutica -
ITA.
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[1] J. B. Y. Tsui, Fundamentals of Global Positioning System
Receivers, A software approach. Wiley Interscience
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Walter, “Receptor GPS em Software. In: 25°Simpósio
Brasileiro de Telecomunicações – XXV SBrT 2007,
Recife, PE, Brasil, set, 03 a 07.
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