Unit-5 Introduction to CDMA Digital Cellular Standard

Que 1.Draw and explain Basic Architecture block diagram of CDMA System.

1. Code Division Multiple Access (CDMA) is a digital technique for sharing the frequency spectrum.
2. Multiple users are assigned radio resources using spread Spectrum techniques. Although all users are transmitting in the same RF band, individual users are separated from each other via the use of orthogonal codes.
3. CDMA is based on proven Spread Spectrum communications technology.
4. The first CDMA standard for mobile networks is referred to as Interim Standard 95A (IS95A), and is considered to be a 2G technology. CDMA2000 and WCDMA is 3G based technology.
5. Fig shows basic block diagram of CDMA architecture. The main components are mobile station (MS), base station subsystem (BSS), network subsystem (NSS), and other networks like PSTN etc.

1. Mobile System Subsystem

A. Mobile Station (MS)

The MS is the mobile subscriber equipment, which can originate and receive calls and communicate with the BTS. CDMA, on the other hand, employs spread spectrum technology, allowing it to make the best use of the available bandwidth. It allows any user to send data over the full frequency range at any moment.

B. SIM Card

On the other hand CDMA-based devices, do not require a SIM card; instead, ESN is used (Electronic Serial Number). Because it does not utilise a SIM card, switching from one device to another is complicated and necessitates a correct method.

2. Base Station Subsystem (BSS)

A. Base Transceiver Station (BTS):

The BTS transmits and receives radio signals, realizing communication between the radio system and the mobile station.

B. Base Station Controller (BSC):

The BSC implements the following functions:

• Base Transceiver Station (BTS) control and management
• call connection and disconnection
• mobility management
• stable and reliable radio link provision for the upper-layer services by soft/hard handoff
• power control
• radio resource management.

3. Network Subsystem (NSS)

A. Mobile Switching Center (MSC):

The MSC implements the service switching between the calling and called subscribers. One MSC is connected with multiple BSCs. The MSC can also be connected to the PSTN, ISDN or other MSCs. It provides the interface between the radio network and PSTN.

B. Visitor Location Register (VLR):

It is a dynamic database, stores the temporary information (all data necessary to set up call connections) of the roaming subscribers in the local MSC area. VLR is used to store the subscriber information of all the MSs in its local area, which can be used to establish the incoming/outgoing call connections, to support basic services, supplementary services and mobility management.

C. Home Location Register (HLR):

It is a database for mobile subscriber management, the HLR (Home Location Register) is responsible for storing subscription information (telecom service subscription information and subscriber status), MS location information, MDN, IMSI (MIN), etc.

D. Authentication Register (AC):

The AC (Authentication Center) is physically combined with the HLR. It is a functional entity of the HLR, specially dedicated to the security management of the CDMA system. It stores the authentication information. It also prevents unauthorized subscribers from accessing the system and prevents the radio interface data from being stolen.

E. Equipment Identity Registry (EIR):

It is a database that is used to track handset using the IMEI number. It is made up of three sub classes- the white list, the black list and the grey list.

Que 2.Explain in brief forward and reverse CDMA channels.

Frequency allocation: The forward channel carries information from the Base Station to the mobile unit; the reverse channel carries information from the mobile unit to the base station. The forward channels are between 869 and 894 MHz, while the reverse channels are between 824 and 849 MHz. In most of the countries, the channel bandwidth is 1.25 MHz.

1. Forward Channel :

CDMA forward link uses same frequency spectrum as AMPS i.e. 869-894 MHz. One channel bandwidth is 1.25 MHz. Modulation scheme used is QPSK. Orthogonal Walsh codes are used. Walsh codes are called Hadamard codes and they are used in all CDMA techniques. Forward channels are separated from each other using different spreading codes. 64 Walsh codes are used to identify each channel.

The Forward channel is the direction of the communication or mobile-to-cell downlink path. It includes the following channels −

• Pilot Channel − Pilot channel is a reference channel. It uses the mobile station to acquire the time and as a phase reference for coherent demodulation. It is continuously transmitted by each base station on each active CDMA frequency. And, each mobile station tracks this signal continuously.
• Sync Channel − Synchronization channel carries a single, repeating message, which gives the information about the time and system configuration to the mobile station. Likewise, the mobile station can have the exact system time by the means of synchronizing to the short code.
• Paging Channel − Paging Channel’s main objective is to send out pages, that is, notifications of incoming calls, to the mobile stations. The base station uses these pages to transmit system overhead information and mobile station specific messages.
• Forward Traffic Channel − Forward Traffic Channels are code channels. It is used to assign calls, usually voice and signaling traffic to the individual users.

2. Reverse Channel :

The reverse link is separated from the forward link by 45 MHz. It is used by all the mobiles in the cell coverage area to transmit to the base station. The reverse channel as it appears at the base station receiver is a composite of all the outputs from all the mobiles in the base station’s coverage area. The reverse channel structure allows a maximum of 94 channels which include a maximum of 32 different Access channels and 62 different Traffic channels.

The Reverse channel is the mobile-to-cell direction of communication or the uplink path. It consists of the following channels −

• Access Channel − Access channel is used by mobile stations to establish a communication with the base station or to answer Paging Channel messages. The access channel is used for short signaling message exchanges such as call-ups, responses to pages and registrations.
• Reverse Traffic Channel − Reverse traffic channel is used by the individual users in their actual calls to transmit traffic from a single mobile station to one or more base stations.

Que 3.Enlist the features offered by CDMA technology.

Features (attributes) of CDMA are as follows:

1. Frequency usage – Many users of a CDMA system share the same frequency. Either TDD or FDD may be used.
2. Soft Capacity Limit – Increasing the number of users in a CDMA system raises the noise level in a linear manner. Thus, there is no absolute limit on the number of users in CDMA. Rather, the system performance gradually degrades for all users as the number of users is increased, and improves as the number of users is decreased.
   Hence, unlike TDMA or FDMA, CDMA has a soft capacity limit.
3. Multipath Resistance – In CDMA systems, channel data rates are very high. Hence the symbol (chip) duration is very short and usually much less than the channel delay spread. Since PN sequences have low auto correlation, multipath which is delayed by more than a chip will appear as noise. A RAKE receiver can be used to improve
   reception by collecting time delayed versions of the required signal.
4. Soft Handoff – Since CDMA uses co channel cells, it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may choose the best version of the signal at any time without switching frequencies.
5. Self-Jamming – Self Jamming is a problem in CDMA system. Self-Jamming arises from the fact that the spreading sequences of different users are not exactly orthogonal. Hence in the de-spreading of a particular PN code, there is some level of cross correlation. There will be non-zero contributions at the receiver from the transmissions of other users in the system.
6. Near Far effect – The near far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. Signals close to the receiver are received with less attenuation than signals farther away.
7. Fading effects – Multipath may be substantially reduced because signal is spread over a large spectrum.
8. Economical – CDMA is the most cost-effective technology that requires fewer cell sites and no costly frequency reuse pattern. The average power transmitted by CDMA mobile station averages 6 to 7 mill watts(mW), which is significantly lower than the average power transmitted by FM and TDMA phones. Transmitting less power means
   that the average battery life will be longer.

Que 4. Explain in brief the basic technology of IS-95 CDMA and CDMA2000.

1. IS-95 (Interim Standard 1995) CDMA

IS-95 (cdmaOne) is a 2G technology. IS95 or Interim Standard 1995 is a second-generation (2G) cellular technology that introduced the first-ever CDMA-based mobile network.

It uses the spread spectrum technology with a carrier bandwidth of 1.25 MHz to enable voice and data services offering peak data rates of up to 115.2 kbps.

IS95 was introduced to deliver 2G cellular services through the Code Division Multiple Access (CDMA) technology.

The proprietary name for IS-95 is cdmaOne, and it is a digital technology that employs CDMA for its air interface.

IS95 was the first-ever standard in mobile communications that was based on CDMA technology. The standard IS-95 has two variants, IS-95A and IS-95 B. IS95 can operate in two frequency bands: 850 MHz and 1900 MHz.

IS-95 employs Frequency Division Duplex (FDD) and uses separate bands for uplink and downlink. It can use either 824MHz-894MHz or 1850-1990 MHz with separate frequency bands for the uplink and the downlink.

The carrier frequencies used in IS-95 have a bandwidth of 1.25 MHz, and the channel separation (difference between the centre frequencies of two channels) is also 1.25 MHz.

IS95A can enable data rates of up to 14.4 kbps, which can be improved to 115 kbps with the technology enhancements in IS95B.

IS-95 provides capacity advantages for its ability to accommodate more users per MHz of the bandwidth. The power consumption in these networks is low, allowing users to make phone calls in decent quality even when the signal strength is not at its best. Low power consumption also extends the cell coverage, which increases the cell’s size.

IS95 networks also have soft handovers (SHO) that are more robust than the “hard” handovers in TDMA/FDMA technologies.

2. CDMA2000

CDMA2000 is a 3G technology

CDMA2000, also known as CDMA2000 1xRTT or IS-2000, is a technology standard used for delivering 3G cellular services.

CDMA2000 is a successor of IS95 (cdmaOne), and it offers 3G mobile services as specified in IMT2000, enabling peak data rates of up to 153 kbps in the original launch.

Mobile networks worldwide follow two main paths for migrating from 2G to 3G. The first path is for GSM networks that employ the UMTS technology for 3G migration, whereas the other path is CDMA2000 which allows both IS-95 (cdma2000) and D-AMPS (Digital Advanced Mobile Phone System) to migrate to 3G.

3G mobile networks can offer much higher data speeds as compared to the earlier 2G networks. Due to the higher data speeds, 3G networks can support multimedia services, including video streaming.

CDMA2000 networks can operate in both 850 MHz and 1900 MHz frequency bands, just like the earlier IS95 networks with separate frequency bands for uplink and downlink. The uplink in CDMA is called the reverse link, and the downlink is called the forward link.

CDMA2000 is backwards compatible with its predecessor, IS95, making the upgrade from IS95 to CDMA2000 easy and seamless. CDMA2000 has seen several enhancements including CDMA2000 1xEVDO (EVolution Data
Optimised), CDMA2000 1xEVDV (EVolution Data and Voice) and CDMA2000 3 x RTT (Radio Transmission Technology).

It uses the carrier bandwidth of 1.25 MHZ like IS95 and is both circuit-switched and packetswitched.

However, while the standard frequency carrier bandwidth is 1.25 MHz, with 3xRTT, CDMA2000 can aggregate up to three 1.25 MHz channels to get a total bandwidth of 3.75 MHz. For comparison, the UMTS networks have a standard carrier bandwidth of 5 MHz. CDMA2000 1xRTT can support peak data rates of up to 153 kbps in the downlink and the uplink.

With the EVDO Rev B enhancement, CDMA2000 can support maximum data rates of up to 14.7 Mbps.

Que 5. Explain the Difference Between GSM and CDMA.

Parameter	GSM	CDMA
Full-Form	GSM stands for Global System for Mobile communication.	CDMA stands for Code Division Multiple Access.
Basic	It is SIM-specific.	It is handset-specific.
Availability	GSM is highly available and globally used. Over 80% of the entire world’s mobile networks use it.	CDMA is not very common, and it is available in comparatively fewer carriers and countries. These devices are exclusive to Canada, Japan, and the United States.
Technology Used	It uses the Time division multiple access (TDMA) and Frequency division multiple access (FDMA).	It uses the Code division multiple access (CDMA).
Voice and Data	GSM supports the transmission of both voice and data at once.	The CDMA technology does not support any such feature. It cannot transmit voice and data simultaneously.
Rate of Data Speed	It is slower. GSM offers 42 Mbps (Megabytes per second) in 3G (HSPA).	It is faster. CDMA offers 3.6 Mbps (Megabytes per second) in CDMA.
Maximum Downloading Speed	GSM offers a maximum speed of 384 Kbps (Kilobytes per second).	CDMA offers a maximum speed of 2 Mbps (Megabytes per second).
Storing of Customer Information.	GSM stores it in the SIM card.	CDMA stores it in a phone or headset.
Roaming	GSM offers worldwide roaming.	CDMA offers limited roaming.
Network	In every cell, the network tower serves as a mobile phone in that particular area.	It has a physical channel along with dedicated codes for every device present in the network.
Spectrum	The GSM technology follows a wedge spectrum (carrier).	The CDMA technology follows a spread spectrum technology.
Data Transfer Technology	The GSM technology makes use of EDGE.	The CDMA technology uses the EVDO ready data transfer technology. Thus, the data transfer here is also very fast.
Division	The carrier is divided into time slots. Each user gets a different time slot. Therefore, no one else can access the same calling slot until the ongoing call ends.	CDMA allows transmission of each user all the time over the entire frequency spectrum.
Signal Detection	One can detect the GSM signals because they stay concentrated in a narrow bandwidth.	One cannot easily detect signals in CDMA.
Built-in Encryption	GSM does not come with built-in encryption.	CDMA comes with built-in encryption.
Security	The GSM technology offers less security as compared to the CDMA technology.	The CDMA technology offers more security as compared to the GSM technology.
SIM Card	For the working of a GSM device, one would always require a SIM card.	The CDMA phones don’t have any such requirements.
Flexibility	The GSM technology is more flexible compared to CDMA. One can interchangeably use and replace a SIM on multiple devices.	The CDMA technology is not so flexible.
Radiation	On average, the GSM phones emit approx 28 times more radiation than the CDMA ones.	The CDMA devices emit very low radiation.
Wave Pulse	A GSM phone emits a more continuous wave pulse. So, you don’t need to reduce its exposure to electromagnetic fields.	The CDMA devices have no such pulses.

Que 6.How are CDMA-based mobile networks different from GSM?

GSM networks use TDMA/FDMA and require the frequency channels to be planned carefully to avoid co-channel and adjacent channel interference; CDMA networks use spread spectrum technology that employs unique codes for each user and enables the network to use the same frequency carrier in all cells.

CDMA stands for Code Division Multiple Access, and it is a technology that can enable wireless connectivity through the use of specialized codes.

The key parameters needed for a CDMA-based mobile network are frequency carrier (channel) and code (e.g. Walsh code). GSM networks use a combination of FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) for wireless connectivity.

. In GSM networks, the frequency channel allocation needs to be planned very carefully
and requires RF engineers to have specialised skills in frequency planning to avoid cochannel and adjacent channel interference.

That is because the carrier to interference ratio (C/I) needs to be at an acceptable level in TDMA/FDMA based systems like GSM.

It means that the frequency channel your mobile phone is connected to needs to be stronger than other channels using the same frequency (co-channel) by a good margin. That margin is called the acceptable C/I ratio, expressed in dB. Fig. 1 shows four frequency channels are reused to avoid the same frequencies being right next to each
other.

On the other hand, CDMA networks do not have the frequency reuse limitation and use the same frequency carrier (channel) in all cells. Fig. 2 shows the concept of universal frequency reuse. CDMA networks use the spread spectrum technology, which is designed to operate at a very low signal-to-noise ratio (SNR) while using a very large bandwidth.

In CDMA networks, the transmitted signal from each mobile user is spread across the available bandwidth and is assigned a unique code (spreading code). A unique despreading code is applied to retrieve the original signal at the receiver end. Even though all users use the same frequency carrier, the chances of any co-channel interference are minimized since the spreading and de- spreading codes are unique for each user. Both IS95 and CDMA2000 are built upon the Direct Sequence Spread Spectrum (DSSS) technique.

Unlike TDMA/FDMA based networks like GSM, where a handover requires a mobile phone to first disconnect from one cell before connecting to a new cell, CDMA networks use a concept called soft handover (handoff).

Since CDMA networks use the same frequency across all cells, a mobile phone can communicate with multiple cells simultaneously. As a result, the mobile phone can gradually disconnect from the cells that are no longer in range whilst staying connected to other required cells. Soft handover makes the handoff process more robust, making it less likely to drop the calls.

Que 7.Explain in brief Wideband Code Division Multiple Access (WCDMA).

Wideband Code Division Multiple Access (WCDMA) is a third-generation (3G) standard that employs the direct-sequence code division multiple access (DS-CDMA) channel access method and the frequency-division duplexing (FDD) method to provide high-speed and high-capacity service. WCDMA is the most commonly used variant of the Universal Mobile Telecommunications System (UMTS).

• WCDMA stands for Wideband Code Division Multiple Access and is the 3G technology that employs the Direct-Sequence Code Division Multiple Access (DS-WCDMA) channel access method and the Frequency Division Duplexing (FDD) method to provide highspeed and high capacity service.
• Third generation (3G) wireless capability has been developed in response to a growing demand in data services
• There are several different radio access technologies defined within ITU, based on either CDM or TDMA technology. Different regional solutions were proposed as solutions to the requirements of IMT-2000. These included Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) utilizing Frequency Division Duplex (FDD) and Time Division Duplex (TDD)
• The fragmentation of the proposals led to the creation of two working groups. One group is known as the Third Generation Partnership Project (3GPP) which is working on the Unified Mobile Telecommunication Standard (UMTS) based on WCDMA.
• The other group is known as 3GPP2 works on CDMA 2000
• Organization 3rd Generation Partnership Project (3GPP) has continued that work by defining a mobile system that fulfils the IMT-2000 standard, This system a called Universal Mobile Telecommunication System (UMTS)
• ITU finally approved a family of five 3G standards which are part of the 3G framework known as IMT-2000. These standards are:
• WCDMA (UMTS 99)
• WCDMA (HSPA)
• CDMA2000
• TD-CDMA
• TD-SCDMA

WCDMA features two modes:

• Frequency Division Duplex (FDD): Separately users by employing both codes as well as frequencies. One frequency is used for the uplink, while other is used for downlink
• Time Division Duplex (TDD): Separates users by employing codes, frequencies and time wherein the same frequency is used for both uplink and downlink

WCDMA does no need base station timing synchronization

WCDMA provides significant flexibility to provide support of multiple users at independent data rates. The flexibility necessitates the utilization of multiple complex waveforms for validation and test.

Que 8.Explain in brief WCDMA Air Interface.

One of the main air interfaces for 3G system is referred as wide band CDMA (WCDMA). It is one of the air interfaces used with UMTS mobile communication standard which allows communication between UE and Node B. Below table shows some of the key parameters of WCDMA air interface.

Sr. No.	Parameters	Specification
1	Channel Spacing/ Separation / Bandwidth	5 MHz
2	Forward RF channel structure	Direct Sequence Spread Spectrum
3	Chip Rate	3.84 Mcps
4	Coherent Detection	Pilot symbols
5	Reverse Channel Multiplexing	Control and pilot channel time multiplexed, I & Q multiplexing for data and control channel
6	Spreading modulation	Balanced QPSK (Downlink), Dual-channel QPSK (Uplink), Complex spreading circuit
7	Spreading Factors	4 – 256
8	Spreading (Downlink)	Variable length orthogonal sequence for channel separation, Gold sequence for cell and user separation
9	Spreading (Uplink)	Same as forward, different time shifts in I & Q channels
10	Data modulation	QPSK (Downlink), BPSK (Uplink)
11	Multi rate	Various spreading and Multicode
12	Frame Length	10ms with 15 time slots
13	Service multiplexing	Multiple Services with different QoS Requirements Multiplexed on one Connection
14	Number of slots/frames	15
15	Power Control	Open and Close loop (1.6 KHz)
16	Handover	Soft handover

Que 9. Define Near-far effect and explain the power control used in CDMA 2000 and WCDMA.

Near-far effect

In CDMA, the power of multiple users at a receiver determines the noise floor after decorrelation. If the power of each user within a cell is not controlled such that they do not appear equal at the base station receiver then the near-far problem occurs.

The near far problem is undesirable effect in CDMA systems. It occurs when many mobile users share the same channel. In general, strongest mobile signal will capture the demodulator at the base station. In CDMA strongest signal level raise the noise floor at the base station demodulators for the weaker signals thus decreasing the probability that weaker signals will be received. To combat the near-far problem, power control is used in most CDMA implementations.

Power control used in CDMA 2000 and WCDMA

1. In cdma2000 and WCDMA, power control is essential in both forward link and reverse link.
2. The reverse link power control is present right from 2G CDMA standards to curb the near-far effect.
3. Since, in CDMA, all users transmit and receive at same frequency and at same time, interference is always present. In such a situation, if a mobile is close to base station its power at the base station will be more than the mobile which is far. The mobile which is far will lose its power in the journey to base station due to shadowing effects.
4. Thus, there is power control on the reverse link. The power at the base station received from all the mobile stations should be same and satisfy minimum SNR level.
5. In the forward link, the slow power control of 2G CDMA standards fail if the mobile is surrounded by high interfering noise.
6. Hence, forward link power control is employed at high level in both 3G CDMA standards of cdma 2000 and WCDMA.
7. The power control in cdma 2000 and WCDMA is to minimize interference due to presence of other mobile users, fading, log normal shadowing as well as from fast Rayleigh fading.
8. In cdma2000, power control is at the rate of 800Hz in both the links and it is at a rate of 1500Hz in both the links in WCDMA.

Also you can refer : Introduction to GSM