Table of Contents1. Introduction...................................................................................................................................2

2. CDMA system................................................................................................................................2

3. Task design....................................................................................................................................3

3.1 Cell Capacity and cell size......................................................................................................3

3.2 Error detection method...............................................................................................................7

3.3 modulation technique.................................................................................................................8

4. Discussion and conclusion...........................................................................................................11

4.1 Discussion............................................................................................................................11

4.2 Conclusion.................................................................................................................................13

5. Reference:.......................................................................................................................................14

1. Introduction.Mobile communication system is worldwide used over the world with different model

such as GSM, CDMA, D-AMPS and so on. Having a fundamental background of CDMA and

its features will give me good start of understanding mobile communication. The target of

this project is designing mobile communication, CDMA system, with particular features such

as cell size, cell capacity, bandwidth, technique of modulation and demodulation and so on

for Orange Island. Orange Island is a distant island with a city, which is Voda. The area of

Voda is 1500 km2 within which there is a densely populated centre of approximately area of

100 km2. The density of user in city centre is 1000 people per km2 and in the suburban areas

100 people per km2. The population density in the rest of island is 5 people per km2

averagely. Government of Orange Island want to set up a mobile communication network

based on CDMA technology. The available bandwidth in the Island is 890 – 915MHz for

uplink and 935-960 MHz for downlink. The government desire to set up wireless

communication and they are considering two options of system. They are:

A voice only network with a required user data rate of 10 kbps

A voice and data network with a voice user, data rate of 10 kbps and the data rate of

other traffic (e.g. internet data) of 150 kbps.

By considering features of these two options, the answer for question “which is suitable

design for Orange Island?” will be given.

2. CDMA system. Code division multiple access (CDMA) is a channel access method used by various radio

communication technology. The basic concept of CDMA technology is allowing several

transmitter send information at the same time over a single communication channels

(bandwidth). It also means that user can share a bandwidth frequency with other users. It is

so-call multiple access. CDMA also use the technology which is spread spectrum. The spread

spectrum technology and using orthogonal coding scheme (idea of orthogonal coding will be

cover in part 3.3) are used to allow many user can use the same bandwidth (same channel).

the spread spectrum is implemented by doing XOR operation between information data with

a pseudo-random bit array which has much higher data generated frequency than information

array. (Information of pseudo-random array is provided in part 3.3).

3. Task design.

3.1 Cell Capacity and cell sizeFirst of all, based on the table 2.2 in lecture note 2, the frequency reuse factor can be

C = 3 because the acceptable SIR (signal to interference ration) can be the value of 7 dB.

Moreover, according to the formula which shows relationship between bandwidth and the

number of channel in one cell, we can get the number of channel with assumption of

EbI 0

=7dB or 5 (dimensionless unit).

Because the frequency reuse factor C= 3, so bandwidth which is used in one cell is

W = 25 MHz / 3 = 8.33 MHz = 8330 KHz. Value of R is the desired data rate of designing

network which is 10 kbps.(requirement of Orange Island Government)

So, K=(8330 :10)5

+1 = 167 channels.

Therefore, with the requirement of 10 kbps, number of channel in one cell is 167 channels.

In the other hand, based on internet statistic website, the average number of calls has

been made a day in US is 3 billion calls [1]. It means that there are 2.08 million calls per

K=(W /R )(Eb / I 0 )

+1

minutes. Furthermore, there are 159 million mobile phone connections in US. It means that

there is 2.08159 call is made in one minute per a mobile phone connection. Therefore, number of

calls are made by one thousand mobile phone connection in one minute is 2.08159

∗1000=13.

In addition, in average, one call last around 3 minutes and 15 seconds [2] in normal time.

It means that the Erlang traffic load is 13*3.25 is approximately 43 per a thousand of mobile

phone connections. The Erlang traffic load describes the number of mobile phone use the

channels of one cell at the same time. The average Erlang traffic load above is 43 a thousand

of user, it mean that there are average of 43 users per a thousand mobile phone connections

use their phone at the same time. In my design, I assume that the probability of user make a

call in busy time is triple this number in the normal time. Then, Erlang traffic load in busy

time is triple Erlang traffic load in normal time, which is estimated as 120.

In my design, i choose the blocking probability is 0.5%. Corresponding to Erlang

table, with number of channel is 167 and blocking probability is 0.005, the value of Erlang

load is 143.39. this number show that if there are 167 available channels in one cell and

network designer want to keep probability of blocking user phone is 0.5% (200 call, one call

is blocked), the Erlang traffic load of this network will be 143.39.

Figure 1: Erlang traffic load figure

In the other hand, i want to keep my network with Erland traffic load per one

thousand users is 120. Therefore, the number of user in one cell in is 143.39

120=1.2t housand

users. Particularly, a mobile phone connection density in city centre is 1000 connection per

km2. It means that the coverage area of one cell in city centre is 1.2 km2.

Furthermore, area of the hexagon is 2.6R2 with R is the radius of the hexagon. We

have an area of hexagonal cell is 1.2 km2 in city centre. So, its radius of hexagon is 0.68 km.

Number of base station in city centre is 10001.2

=833 units.

To sum up what feature we got so far is that in the city centre, the number of user in

one cell is 1.2 thousand people and radius of cell is 0.68 km. In similar manner, the number

of people in one cell in suburban area is still 1.2 thousand people; however, the user density

is 10 times smaller (100 user per one km2). Hence, the radius of cell is 0.68∗√10=¿2.15 km

and number of base station is 1500

12=125 units. The same standard is applied for the rest of

Island. It means that number of user in one cell is 1.2 thousand. Logically, radius of the cell

increases √20 times (compared with suburban area, 5 people/km2 whereas 100 people/km2 in

suburban area, number of user in one cell in suburban is 20 times number of user in

countryside). Therefore, the radius of cell in countryside is 9.6 km.

countryside Suburban City centreBandwidth 8.33MHz 8.33MHz 8.33MHz

cell size (radius) 9600m 2150m 680mNumber of cells ------- 125 833

Table 1: summarizing bandwidth, cell size and number of cell in different area

In the middle of the cell, there is a base station which is used to collect signal from user in its

coverage range. There are three kinds of base station, which are used in pico-cell, micro-cell

and macro-cell. First kind of base station is the macro-cell which provide the main coverage

of a mobile network. Normally, its antenna is mounted on existing building or mast. Because

of its high coverage range, it can be used in countryside, also in suburban and city centre to

collect signal from Pico-cell or micro-cell. Another kind of base station is micro- cell is

usually used to infill the coverage gap in mobile network. It is small and can be mounted

easily on the street, house or on the wall. Typical micro-cell has coverage range is about

300m to 100m apart. Third kind of base station is Pico-cell base station, which has the

smallest coverage range and is the cheapest one. It is normally attached inside building,

shopping centre or station, which has high user density place. This kind of base station is

much proper to use in city centre of Orange Island. The following figure gives generally idea

of base station position in the network and its coverage range.

Figure 2: coverage area of different kind of base station

Now, we will be considering the network with voice network; data rate of 10kbps; and other

traffic network such as internet with data rate is 150 kbps. The obvious thing in this case is

this network has much higher data rate compared with voice only network. In the data term,

the volume of data to be transmitted and received is 16 times compared with voice only

network. Based on rate of data, the coverage area of each base station is 16 times smaller. It

means that the number of base station increase incredibly and make a huge increase of cost to

build the network. Moreover, with the internet network on mobile, the coding scheme is also

totally different because error is not allowed to exist in internet. So, there are much more

extra bits for error correction scheme for internet network. By looking into the economic

term, voice only network is decided to be set up in Orange Island. In the other hand, when the

requirement of data rate is increase or people want to use internet, there are many ways to

increase the network capability. Therefore, in the early state, voice only network is much

suitable to set up in Orange Island. (part 4.0)

3.2 Error detection method.

The block coding is primary method in error detection method in the early communication system

network. By using this method, there are some extra bits are added into the original data to create a

new array of data which is so-called codeword. For example, original data has size of k bit and there

is r additional bit for error detection. So, the size of codeword is n which is sum of k and r.

Because the voice only network can accept the small amount of data errors, the block coding

method is suitable for mobile communication network. The block coding has advantage that in this

method, there are just few extra bits added to the data whereas other method as convolutional

coding add more extra bits for error correction. The error correction is not necessary for voice only

network.

171 bits

160 bits 11 bits

In the design, voice is sampled every 20 miliseconds and data of each sample will be convert into

16-bit data. This 16-bit data will be fed into output 171-bit data frame. This technique is so-call EVRC

(enhanced variable rate CODEC which is speed codec used in CDMS network) [6].

Figure 3: frame of data in CDMA voice only network.

3.3 modulation technique. In communication network, modulation technique is the process of varying the

properties of periodic waveform (so-called carrier signal) to transmit the modulation signal,

which contain information [3]. In the early day of mobile communication network, AMPS

utilize analogue FM modulation technique to transmit signal. The digital AMPS system

moreover use ......technique.

There are many various of modulation techniques which has their own advantages and

disadvantages. One of the most simple modulation method is AM (amplitude modulation). It

is simply switch the amplitude of signal on and off for data ‘1’ or ‘0’ respectively. However,

in fading transmitting environment, modulation technique, which provides constant

amplitude signal, is more suitable. It sound that FSK (frequency shift keying) or PSK (phase

shift keying) is more proper for mobile communication. However, FSK suffer disadvantage

that there is a need to have frequency space (at least the difference between two frequency

which are used to modulate signal) and more elaborate receiving gear required. Furthermore,

FSK and PSK just can transmit one bit per symbol and it can be increased. Therefore, i

decide to choose MSK (minimum shift keying) modulation technique in CDMA system

because of some improvements in transmitted data rate and amplitude fluctuation.

First, briefly let have a look on QPSK modulation technique. Using QPSK modulation

technique, each symbol can carry two bits in modulating date. Signal is transmitted by two

carrier with the same frequency f0 but one in quadrature with the other i.e. cos0t and sin0t.

And the signal output will be in the form

The output signal carries information of two bit which is ai and bi. In term of

transmitted data rate, QPSK is advantageous. However, the case of ai and bi change at the

same time for example ai=1 bi=1 change to ai=-1 bi=-1. So the phase of output signal change

180o which can cause amplitude fluctuation in bandwidth in bandwidth limited system [4].

So, that why people introduce OQPSK modulation technique. It work similar to QPSK but it

introduce an delay of ½ clock cycle. This work will limit the change of phase at once is just

90o. The process to generate output of OQPSK is follow.

si( t )=aih( t )cos (ω0 t )+bih( t )sin(ω0 t )

Figure 4: process to generate OQPSK output

In comparison, the output of QPSK and OQPSK is following.

Figure 5: comparison of QPSK and OQPSK output

And for OQPSK, it use less bandwidth because of less rapid change in signal.

However, to generate the sharp edge of signal, system still need very high bandwidth. To

control the sharp of signal, people make a small improvement when they filtered the signal

before multiply it with carriers. It come with the MSK (minimum shift keying).

Figure 6: a. Process to generate MSK output

b. sharp of signal output of lowpass filter.

This is the modulation technique i choose to transmit signal in CDMA network.

In addition, the signal, which is transmitted by transmitter, is the modulated signal. In

CDMA network, information is combined with an pseudo-random code by XOR function.

The Pseudo-random code is generating by system which is generated with much higher rate

than information rate and used to provide orthogonal code. The idea of orthogonal code is

with particular code, receiver just can get its data from received signal by using its own code

whereas other receiver cannot. The following is an example of orthogonal code [5].

Two users have orthogonal codes defined by the vectors v0 = (1,1) and v1 = (1, -1) and

want to send data (1,0,1,1) and (0,0,1,1) respectively. Firstly, we transform all value 0 in data

to -1. So, data will become (1,-1,1,1) and (-1,-1,1,1). Then multiply the vector of user with

user data.

For example, the user 1 has vector v0 = (1,1) and data (1,-1,1,1) , after multiplication, the

result will be. encode0= vector0.data0 = (1,–1).(1,–1,1,1) and result for user 2 is

encode1=(1,1).(–1,–1,1,1). By doing that, what we got is modulated data of each user

encode0=(1,–1,–1,1,1,–1,1,–1) and encode1=(–1,–1,–1,–1,1,1,1,1) .

then we add these 2 array of value

signal0 + signal 1 = (1,–1,–1,1,1,–1,1,–1) + (–1,–1,–1,–1,1,1,1,1) = (0,–2,–2,0,2,0,2,0).

This is the modulated signal, which we will transmit in the transmitter. In receveir, int he case

that we can retrieve the same information, the reverse process is applied for demodulation.

Step Decode sender0 Decode sender1

0 vector0=(1,–1), pattern=(0,–2,–2,0,2,0,2,0) vector1=(1,1), pattern=(0,–2,–2,0,2,0,2,0)

1 decode0=pattern.vector0 decode1=pattern.vector1

2 decode0=((0,–2),(–2,0),(2,0),(2,0)).(1,–1) decode1=((0,–2),(–2,0),(2,0),(2,0)).(1,1)

3 decode0=((0+2),(–2+0),(2+0),(2+0)) decode1=((0–2),(–2+0),(2+0),(2+0))

4 data0=(2,–2,2,2)=(1,0,1,1) data1=(–2,–2,2,2)=(0,0,1,1)

Therefore, each receiver has its own particular code, which is used to retrieve its own information.

4. Discussion and conclusion

4.1 Discussion. Again, the decision of network which is set up in orange Island is the voice only network.

There are many survey shows that there is a huge demand of internet requirement.

Mobile media users pick up their phone 18 times a day to consume content via apps/browser.[7]

There are currently 13.9 million mobile internet users in the UK. This figure is expected to rise to 17.3 million by 2012 and 18.6 million by 2013 [7].eMarketer, February 2011

“ There are 13.9 million mobile internet user over 62 million people in uk (in 2008). This is high rate of internet user demand. Moreover, internet provider also benefit from their advertising on mobile”.

“1 in 4 respond positively to mobile ads” [7]

“UK Mobile advertising spend is expected to reach £27.5 million by the end of 2010. It is estimated that this figure will rise to £258 million by 2015” [7]

It seems to worry people because of its low data rate while the demand of mobile

network user is increasing gradually. It is not cost efficiency to set up low data rate network

and then replace the entire network by higher data rate network. It is not true because when

user demands increase, the network can be upgrade network capacity instead of replacing the

whole network. There are many way to enhance the capacity of CDMA network base on the

existing network. One easy way is reuse the unused channel in available spectrum block. It

means that the bandwidth of a cell is not exactly the same to another cell. Number of channel

depends on user density. Another way is cell splitting. We can reduce coverage area of old

base station by setting up more base station. For example, we set up the base station in the

middle of the existing base station. The coverage area will reduce by factor of four.

Consequently, the number of cell increase four times. So, it can offer higher data rate as well

as more users.

Figure 4.1: cell splitting method

Another concern is about some reality situation. When i stay in Thrumpton building in

Sutton Bonnington Campus, i cannot make a call from there. It raise a problem that user will

not happy with their network if they cannot use their phone. It seems that they disconnect

with the network. However, people stay in this building still can receive a ring from another

people. It means that the downlink signal is better than uplink signal for the same channel.

From that concern, the cells need to cover all area to keep customer happy. In addition, SIR

(signal interference ratio) of downlink should be higher than SIR of uplink should for the

case as in above example.

4.2 Conclusion.In conclusion, the voice only network 10kbps will be set up in Orange Island. There

are 833, 125 and a few of base station will be installed in city centre, suburban and

countryside area, respectively. The frequency reuse factor is chosen C =3 with acceptable

Signal-to-interference ration is 7 dB. And a cell capacity is provide connection for 1200

users. The MSK (minimum shift keying) technique is utilized in CDMA system. Block

coding scheme will be used in this Orange Island network for error detection.

Even the growth in user internet demands is very quickly, however, we can upgrade

the voice only network to higher data rate network without replacing the entire network.

5. Reference:[1] http://www.planetomni.com/ARTICLES-Cell-Phones-in-USA.shtml

[2] http://www.articlesbase.com/answers/how-many-telephone-calls-are-made-each-day-76162

[3] http://en.wikipedia.org/wiki/Modulation

[4] lecture 4 –mobile communication and network 2010-2011. Prof Morgan.

[5] lecture 5-6 – mobile communication network 2010-2011. Prof Morgan

[6] http://en.wikipedia.org/wiki/Enhanced_Variable_Rate_Codec

[7] http://www.google.co.uk/intl/en/landing/internetstats/