CDMA Interview Questions and Answers | Basic and Advanced Levels

Q1. What is CDMA?

Ans: CDMA stands for Code Division Multiple Access. It is a wireless technology used in transmission of signals 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 and noise has no effect on the signal. CDMA is not a frequency specific to each user, instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. A unique code is received by all mobile network users and is allowed continuous network access instead of intermittent or timed access.

Q2. How Many Channels Are There In Cdma Forward Channels?

Ans: Forward channel consists of four channels which includes:

• Pilot Channel
• Sync Channel
• Paging Channel and
• Forward Traffic Channels.

Q3. What is FDMA? Explain?

Ans: Frequency Division Multiple Access (FDMA) is one of the most common analogue multiple access methods. The frequency band is divided into channels of equal bandwidth so that each conversation is carried on a different frequency. Guard bands are used between the adjacent signal spectra to minimize crosstalk between the channels.

Advantages:
In FDMA when the channel is not used, it is the channel bandwidth while rest simply is relatively narrow (30 KHz), known as System narrowband. Little or no equalization is needed. For broadcasting, time symbols are suitable analogue links. Framing for FDMA or synchronization bits are not needed for the tight filter streaming. It is required to minimize the combined interference of FDD.

• Disadvantages:
  It does not differ significantly from analog systems; improving the capacity depends on the signal-to-interference reduction, or a signal-to-noise ratio (SNR).
• The maximum flow rate per channel is fixed and small.
• Guard bands lead to a waste of capacity.
• Hardware implies narrowband filters, which cannot be realized in VLSI and therefore increases the cost.

Q4. Differentiate between CDMA and FDMA?

Ans:

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

Q6. What is Forward Channels in CDMA?

Ans: The forward channel CDMA is the direction of the communication or mobile-to-cell downlink path.Forward channel consists of four channels which includes:

Pilot Channel: Pilot channel is a reference channel using 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. Each mobile station tracks this signal continuously.

Sync Channel: Synchronization channel carries a single repeating message and transmits the synchronization configuration information and the system of the mobile station in the CDMA system.

Paging Channel: Paging Channels’ 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 Channels: Forward Traffic Channels are code channels and used to assign calls, usually voice and signaling traffic to the individual users.

Q7. What is Scrambling Code?

Ans: Scrambling-code identification
During the third and last step of the cell search  procedure, the UE determines the exact primary scrambling  code used by the found cell. The primary scrambling code is  typically identified through symbol-by-symbol correlation  over the CPICH with all codes within the code group  identified in the second step. After the primary scrambling code has been identified, the Primary CCPCH can be detected and the system- and cell specific BCH information can be read. If the UE has received information about which scrambling codes to search for, steps 2 and 3 above can be simplified.

Ans:

• For 1xRTT has four forward and two reverse channels, where as in EVDO channel structure is different.
• 1x RTT supports upto 153.2 kbps of data, where as EVDO  supports upto 3.1Mbps.
• In 1x RTT during measurement pilot power ratio , Traffic power ratio are also taken into consideration; where as in  EVDO FTAP , RTAP with Slots are taken into consideration