CDMA Interview Questions and Answers

• CDMA2000 is a Multi-carrier code-division multiple access version of IMT-2000 standard
• CDMA2000 is a 3G wireless technology.
• World’s first 3G commercial system was launched by SK Telecom of South Korea using CDMA2000 1X
• CDMA 2000 supports mobile data communications at speeds from 144KBPS to 3MBPS
• The versions of CDMA2000 have been developed by Qualcomm and Ericsson
• There were 250,300,000 subscribes world for CDMA by March 2006.

• CDMA stands for Code Division Multiple Access
• CDMA is a wireless technology used in transmission of signal from places with high Security and noise reduction.
• The principle of Spread Spectrum is used to work with CDMA.
• Spread signal is below the noise level noise and has no effect on the signal.
• CDMA does not frequency specific to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
• CDMA always provides better capacity for voice and data communications
• CDMA is a common platform for 3G technologies
• Analog radio transmission technologies like Advanced Mobile Phone System were used in CDMA at the time of its inception
• A unique code is received by all mobile network users and is allowed continuous network access instead of intermittent and timed access.

CDMA One

• CDMA, one refers original ITU IS-95 CDMA wireless interface protocol. It is considered as a 2G mobile wireless technology
• The version IS-95A protocol employs a 1.25 Mhz carrier and data speeds up to 14.4 Kbps
• The version IS-95B support data speeds up to 115 kbps by employing 800 MHz / 1.9 GHz.

CDMA2000

• Also known as IMT Multi-Carrier, a family of 3G mobile technology standards.
• Used for sending voice, data and signaling between cell sites and mobile pones
• The standards such as CDMA2000 1X, CDMA2000 EV-DO Rev. 0, are approved radio interfaces for ITU IMT-2000
• CDMA 2000 is backward compatible with its previous 2G iteration IS-95.

• DECT is one of the wireless technologies, originated in Europe
• DECT was designed to interwork with various networks such as PSTN, ISDN, GSM etc.
• The initial standard. of DECT focused on developing air interface which is a radio link between cordless telephone and the base stations
• This standard plugged into the telephone socket and the standard protocols for handover between several base stations
• All these are connected to the same office switchboard, typically a PABX
• The first product of its kind was from Olivetti, was a wireless LAN type product, known as NET3.

• HSPA is a combination of 2 mobile telephony protocols – High Speed Downlink Packet Access and High-Speed Uplink Packet Access
• HSPA improves the performance of existing WCDMA protocols.
• HSPA provide increased performance by utilizing schemes of modulation and refining the protocols, through which the base stations and handsets communicate
• HSPA is used for better usage of the available bandwidth provided by WCDMA
• HSPA supports data rates of up to 14 MBit / s in the downlink
• HSPA supports data rates of up to 5.8 MBit / s in the uplink
• HSPA reduces the latency and increases up to 5 times more system capacity during downlink and twice more system capacity during uplink
• HSPA uses 16QAM for yielding higher bit rates
• HSPA rollouts are achieved by implementing software upgrades to existing 3G networks. This gives HSPA a head starts over WiMax.

• EVDO stands for Evolution Data Optimized
• EVDO is a 3G broadband technology used by Verizon, Spring, Alltel
• It provides typical download speeds of 600 to 1400 KBPS
• A personal broadband service for wide range of customers
• It is always On
• EVDO utilizes CDMA signals
• Users can be connected remotely for using email, downloading large files, spreadsheets etc.
• EVDO is relatively low cost with high capacity
• Allows rich web browsing and application usage
• Seamless roaming, internal internet connectivity without relying on other’s connection, accessibility to the corporate Virtual Private Network by customers are some of the advantages over WiFi .

The important differences are:

Data Transfer Speed:

• CDMA is faster than GSM.
• CDMA2000 downstream rate is 2 megabits / second, through EVDO, where GSM downstream rate is up to 384 kilobits / second, through EDGE technology
• Subscriber Identity Module (SIM) cards:
• SIM is tied to the network, rather than the actual phone. Phones with card-enabled can be used with any carrier of GSM
• Proprietary handsets are linked to one carrier only and not card-enabled in CDMA
• Roaming:
• GSM carriers have wider coverage of more rural areas, where as CDMA may not cover rural areas compared to GSM carriers

International Roaming:

• GSM has facility to offer more international roaming, as the number of connections in world market dominate GSM network.
• CDMA phones do not have the capacity; however, there are more countries that use CDMA networks.
• What is the importance of CDMA in today’s cellular world?
• The transition to digital radio through the process of defining standards is implementing about 90% in the cellular industry.
• Smooth transition to digital standards is involved in CDMA technology.
• Most often, it can be viewed as improved and replacement technology of TDMA
• Moving towards 4G technology is compatible with CDMA technology.

CDMA

• Same frequency is used by every user and simultaneous transmission occurs
• Every narrowband signal is multiplied by wideband spreading signal, usually known as codeword
• Every user has a separate pseudo-codeword, i.e., orthogonal to others
• Only the desired codeword is detected by the receivers and others appear as noise
• It is mandatory for the receivers to know about the transmitter’s codeword

FDMA

• When the channel is not in use, it sits simply idle
• Bandwidth of Channel is relatively narrow (30 KHz), known as narrowband system
• Little or no equalization is needed for spreading symbol time
• Analog links are suitable for FDMA
• Framing or synchronization bits are not needed for continuous transmission
• Tight filtering is needed to minimize interference
• Combined with FDD for duplexing

CDMA

• Power limited system
• While people talking, random noise band playing occurs
• Conversation need to be extracted from the background din
• GP is high when people speak different languages, which is easier to distinguish individual speakers
• It is difficult for distinguishing individuals, when GP is low.
• The system performance will be degraded for every user when the number of users increases.
• Fading would be reduced with wide frequency spectrum
• Need to have separate multipath signals with different delays by “chip” unit.

TDMA

• Receiving or transmission is allowed for only one user in a given slot
• All slots are assigned cyclically
• The transmission is non-continuous
• It is essential to use digital data and modulation
• Data rate overhead is between 20% – 30%
• Overhead trade-offs are size of data payload and latency
• Multiple users are shared with single carrier frequency
• Handoff is made simpler by using non-continuous transmission
• All slots are assigned on demand
• Due to reduced inter user interference, the power control is less stringent

The Differences:

• CDMA is one of the types of mobile connections for making calls whereas GPRS is a technology for internet connections through a mobile phone.
• GPRS enables to surf the internet from a mobile phone. GPRS technology is used in other gadgets apart from mobile phones.
• W-CDMA technology is faster than GPRS, since the user cares about the technology that is applied for air interface.
• Average latency on GPRS is around 1.3 seconds, whereas on CDMA is around 400 milli second
• GPRS supports only User Datagram Protocol, whereas CDMA supports both UDP and TCP
• GPRS relies on Network Address Translation and a private IP address is assigned to it. On the other hand, CDMA assigns a public IP address CDMA is better because:
• E-mail is pretty light, until you enter MS-Office.
• Pocket PC’s support for office documents with similar functionality which can be had on Palm OS and Symbian
• Upon availability of APIs, a software developer could write various applications, like voice transfer, data transfer to client’s account, integrating billing amount etc.
• Web services could be utilized.

Pilot Channel

• A forward link channel which is a base to mobile that modulates only by the pilot PN. It spreads common codes to all signals transmitted from a given base station.
• Several critical important functions are provided by the pilot channel for forward links in IS-95 systems
• The pilot channel modulation facilitates the process of time synchronized replica generation at the receiver of the PN spreading sequences. These are utilized at the transmitter for modulating the synchronization, paging and traffic channels. These are transmitted from the same base station.

Sync. Channel

• A base station transmits a Sync-Channel that spreads with Walsh code 32.
• The frame of synch channel is 80/3ms long with frame boundary is aligned to the pilot.
• Sync channel transmits a single message continually, called Sync Channel Message.
• The Sync Channel Message contains network information, including the PN offset that is used by the Base Station sector.
• The length and content of the message is dependent on the P_REV.
• The message transmission rate is 32 bits / frame.
• The message is encoded to 128 symbols
• The message yielding rate is 1200 bits / s.

Soft handoff

• Soft handoff is a feature in which a cellular phone is simultaneously connected to two or more cellular phones during a single call
• It is he overlapping of repeater coverage ones, which enables every cell phone set is always well within the range of a specific repeater.
• More than one repeater can send and receive signals to transmit signals to and from mobiles.
• All repeaters are used with the same frequency channel for each mobile phone set.
• Practically no dead zones and as result, the connections seldom interrupted or dropped.

Softer handoff

• Softer handover is a significant soft handover in which the added and removed links belong to the same node
• Macro diversity with maximum ratio combining could be performed in the same node
• The movement of handoff, when a user can be served in another cell more efficiently (less power emission, less interference), is the most obvious cause for better performance.

• Short form of General Packet Radio System
• GPRS is the next generation of GSM, and the basis of the 3G networks
• A packet-oriented data service available to users of GSM and IS-136 type mobile phones
• It complements the existing services like circuit-switched cellular phone connections and the SMS
• Shortest set-up time is added for ISP connections
• An optimized mobile service for sending and receiving information across a mobile telephone network, with less cost
• GPRS provides data rates of mobile telephony and moderate speed data transfer, using unused TDMA channels
• GPRS does not need dial-up modem connection
• Sending and receiving e-mails, surfing internet are among the features of GPRS

• GPRS uses 2.5 generation of GSM signals
• The radio interface is the same that of GSM
• GPRS uses 900 / 1800 Mhz, frequency band and GMSK modulation
• The bit rates are EGPRS, similar to EDGE
• Separate hardware and ports need to be added and availed.
• How are GPRS terminals classified?
• The GPRS services are classified into 12 service classes as per the duration of the time slots occupied / frame.
• Usually 5 time slots per frame are occupied including transmission time slots and reception time slots
• Increase in the number of occupied time slots certainly causes the increase in the entire duration of the occupied time slots, so that increased amount of data transmission and receipt are enabled
• All the 12 service classes are further classified into higher and lower classes as per the relative duration of time slots
• Higher speed data communication can be transmitted with high-class GPRS service by setting high transmission power and setting the duration of the time slots long.

Multiple Access Coding Scheme:

• Multiple access scheme is used in GPRS based on the FDD and TDMA
• One pair of up-link and down-link frequency channels are assigned to a user during a session
• The Multiple Access Coding Scheme is combined with statistical multiplexing [packet mode communication], that allows several uses to share the same frequency channel
• Downlink uses first-come first-served packet scheduling and uplink uses a scheme that is similar to reservation ALOHA.
• Slotted ALOHA is utilized for reservation inquiries in a contention phase, followed by transferring data by utilizing dynamic TDMA with first-come, first-served scheduling

Channel Encoding Scheme

• Channel Encoding Scheme is based on a convolution code at various code rates and GMSK modulation
• The table below summarizes the options:

Coding Scheme                                 Speed

CS-1                                                 8.0

CS-2                                                 12.0

CS-3                                                 14.4

CS-4                                                  20.0

• The least robust and fasted coding scheme is CS-4 and is available near a Base Transceiver Station
• The most robust coding scheme is CS-1 and is used in case of mobile station is further away from a BTS
• A speed of 20.0 Kbit/s/time slots can be achieved with CS-4 and has 25% normal cell coverage
• A speed of 8.0 Kbit/s/time slots can be achieved with CS-1 and has 98% of normal coverage