CDMA Interview Questions and Answers

Q1. What is CDMA?

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?

Forward channel consists of four channels which includes

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

Q3. What is FDMA? Explain?

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?

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

Q5. Explain the following two types of hand off in CDMA system: a.) Soft handoff b.) Softer handoff

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.

Q6. What is Forward Channels in CDMA?

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?

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

Q8. Do you know What is the main difference between 1x RTT and 1X EVDO?

1.For 1xRTT has four forward and two reverse channels, where as in EVDO channel structure is different.
2.1x RTT supports upto 153.2 kbps of data, where as EVDO  supports upto 3.1Mbps.
3.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

Q9. Which is the weaker Link: FWD or Reverse in CDMA?

Reverse link is weak in CDMA because of use of PN Long codes compared to Walsh codes in the forward link. Since, walsh codes are perfectly orthogonal in nature there is less interference in the forward link. PN long codes on the other hand are non-orthogonal in nature. However, we cant use walsh codes for the reverse link as it may seem from the above as the best possible solution. This is because walsh codes are very small in number (64/128/512 max), and hence are not sufficient for identification of large number of users in the reverse link for which the PN long codes are employed.

Q10. Explain the underlying functional concept of gsm and cdma?

GSM refers to Global System for Mobile communications while CDMA refers to Code Division Multiple Access. Both are two diffrent technologies in Telcommunications and
behave diffrently in terms of giving voice and data services to the customer.Though GSM is globally used teschnology – CDMA technology has more benefits in terms of call clarity, faster data speeds etc while GSM covers most of the network areas and has got variety of handsets to choose from, people prefer this technology. Other than these 2 technologies we have other technologies like TDMA, WCDMA etc.

Q11. Describe the Centralized Methods in CDMA?

– The band used in CDMA is 824 MHz to 894 MHz (50 MHz + 20 MHz separation);
– Frequency channel is divided into code channels; and
– 1.25 MHz of FDMA channel is divided into 64 code channels.

Q12. What is 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.

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