CDMA - Questions and Answers

1. 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.

2. Explain CDMA Development Group (CDG).

CDG is comprised of service providers, infrastructure manufacturers, device vendors, test equipment vendors, application developers, and content providers. Its members jointly define the technical requirements for the development of complementary systems CDMA2000 and 4G and interoperability with other emerging wireless technologies to increase the availability of wireless products and services to consumers and businesses worldwide.

3. What is Forward Channels in CDMA?

The forward channel CDMA is the direction of the communication or mobile-to-cell downlink path.

4. 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.

5. Explain 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.

6. Explain 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.

7. Explain 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.

8. Explain Forward Traffic Channel.

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

9. What is Reverse Channels in CDMA?

The Reverse CDMA channel is the mobile-to-cell direction of the communication or uplink path.

10. How many Channels are there in CDMA Reverse Channels?

Reverse channel is consists of two channels which includes −

• Access Channels and
• Reverse Traffic Channels.

11. Explain Access Channels.

Access channels are used by mobile stations to establish communications 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.

12. Explain Reverse Traffic Channels.

Reverse traffic channels are used by individual users in their actual calls to transmit traffic from a single mobile station to one or more base stations.

13. Explain the CDMA Capacity.

The factors deciding capacity are −

• Processing Gain,
• Signal to Noise Ratio,
• Voice Activity Factor, and
• Frequency Reuse Efficiency.

Capacity in CDMA is soft, CDMA has all users on each frequency and users are separated by code. This means, CDMA operates in the presence of noise and interference. In addition, neighboring cells use the same frequencies, which means no re-use. So, CDMA capacity calculations should be very simple. No code channels in a cell, multiplied by no cells. But it is not that simple. Although not available code channels are 64, it may not be possible to use a single time, since the CDMA frequency is the same. Flexible capability means that all the code channels can be pursued at a time, but at the expense of quality.

14. 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.

15. Explain Processing Gain in CDMA.

P (gain) = 10log (W/R)

W is Spread Rate

R is Data Rate

For CDMA P (gain) = 10log (1228800/9600)

= 21dB

Actual processing gain = P (gain) - SNR

= 21 – 7 = 14dB

CDMA uses variable rate coder

The Voice Activity Factor of 0.4 is considered = -4dB.

CDMA has 100% frequency reuse. Use of same frequency in surrounding cells causes some additional interference.

In CDMA, frequency reuse efficiency is 0.67 (70% eff.) = -1.73dB

16. What are the CDMA Identities?

Network Identities −

• SID (System Identity)
• NID (Network Identity)

Mobile Station Identities −

• ESN (Electronic Serial Number)
• Permuted ESN
• IMSI (International Mobile Station Identity)
• IMSI_S
• IMSI_11_12
• Station Class Mark

17. What is ESN (Electronic Serial Number)?

The ESN is a 32-bit binary number that uniquely identifies the mobile station in a CDMA cellular system.

18. What is Permuted ESN? Explain.

CDMA is a spread spectrum technique where multiple users to access the system at the same example in a cell, and of course on the same frequency. Therefore, discriminate the users on the reverse link (i.e. information from MS to the base station). It spreads information using codes that are unique to the mobile station in all the CDMA cellular systems. This code has an element that is the ESN. But it doesn’t use the ESN in the same format; instead, it uses an ESN swapped.

19. What is International Mobile Station Identity (IMSI)?

• MCC: Mobile Country Code
• MNC: Mobile Network Code
• MSIN: Mobile Station Identification
• NMSI: National Mobile Station Identity

20. What is the Function of IMSI?

The mobile stations are identified by the identity of the international mobile station (IMSI). The IMSI consists of up to 10 tons - 15 numerical characters (0-9). The first three digits of the IMSI is the country code of the mobile (MCC), the remaining digits are the National NMSI mobile station identity.

The NMSI consists of the mobile network code (MNC) and the mobile station identification number (SIDS). An IMSI is 15 digits in length is called a class 0 IMSI (NMSI is the 12 digits in length). IMSI, which is less than 15 digits in length, is a class called IMSI (NMSI the length is less than 12 counts).

For CDMA operation, the same IMSI may be registered in multiple mobile stations. Individual systems may or may not allow these capabilities. The management of these functions is a function of the base station and the system operator.

21. What is FDD and what are the Frequencies it uses?

Frequency Division Duplex is one of the multiple access methods in wireless technology; it uses following frequency bands −

Uplink: 1920 MHz - 1980 MHz and

Downlink: 2110 MHz - 2170 MHz.

22. What is TDD and what are the Frequencies it uses?

TDD is Time Division Duplex. A duplex method by which the Uplink and the Downlink transmissions are carried over the same frequency using synchronized time intervals. The carrier uses a 5 MHz band, although there is a low chip rate solution under study by the 3GPP (1.28 Mcps). The available frequency bands for TDD will be 1900-1920 MHz and 2010-2025 MHz.

23. 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.

24. What are the Advantages of FDMA?

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.

25. What are the Disadvantages of FDMA?

• 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.

26. What is TDMA? Explain.

Time Division Multiple Access (TDMA) is a complex technology, because it requires a very accurate synchronization between the transmitter and the receiver. The TDMA is used in digital mobile radio systems. The individual mobile stations are assigned cyclically a frequency for exclusive use for the duration of a time interval.

27. What are the Advantages of TDMA?

• It permits flexible rates (i.e., several slots can be assigned to a user, for example, each time interval translates 32Kbps, a user is assigned two 64 Kbps slots per frame).

• It can withstand gusty or variable bit rate traffic. Number of slots allocated to a user can be changed frame by frame (for example, two slots of the frame 1, frame 2 of the three slots, one slot in the frame 3, frame 0 of the notches 4, etc.)

• No guard bands required for the wideband system.

• No narrowband filters required for the wideband system.

28. What are the Disadvantages of TDMA?

• The high data rates of broadband systems require complex equalization.

• Due to the burst mode, a large number of additional bits for synchronization and supervision are needed.

• Call time is needed in each slot to accommodate time to inaccuracies due to clock instability.

• Electronics operating at high bit rates increase energy consumption.

• Complex signal processing is required to synchronize within short slot.

29. What is CDMA? Explain.

Code Division Multiple Access system is very different from time and frequency multiplexing. In this system, a user has access to the whole bandwidth for the entire duration. The basic principle is that different CDMA codes are used to distinguish between different users. Forms generally used are direct sequence spread spectrum modulation (DS-CDMA), frequency hopping or mixed CDMA detection (JDCDMA). Here, a signal is generated which extends over a wide bandwidth. A code called spreading code is used to perform this action. Using a group of codes, which are orthogonal to each other, it is possible to select a signal with a given code in the presence of many other signals with different orthogonal codes.

30. What are the Advantages of CDMA?

• CDMA has a soft capacity. The greater the number of codes, the more number of users. However, many codes are used S/I drops and the BER (Bit Error Rate) will increase for all users.

• CDMA requires a tight power control as it suffers from near-far effect. In other words, a user near the base station transmits the same power as a user later will drown the latter signal. All signals must have more or less equal power at the receiver.

• Rake receivers can be used to improve signal reception. Delayed versions of time (a chip or later) of the signal (multipath signals) can be collected and used to make decisions at the bit level.

• Flexible transfer may be used. Mobile base stations can switch without changing operator. Two base stations receive mobile signal and the mobile receives from two base stations.

• Transmission Burst - reduces interference.

31. What are the Disadvantages of Code Division Multiple Access?

• The code length must be carefully selected. A large code length can induce delay or may cause interference.

• Time synchronization is required.

• Gradual transfer increases the use of radio resources and may reduce capacity.

• As the sum of the power received and transmitted from a base station needs constant tight power control. This can result in several handovers.

32. What are the Differences between CDMA and FDMA?

33. What is Spread Spectrum Technique?

Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have, if its frequency were not varied. In other words, the transmitted signal bandwidth is greater than the minimal information bandwidth needed to successfully transmit the signal. Some function other than the information itself is being employed to determine the resultant transmitted bandwidth.

34. How many types of Spread Spectrum Techniques are used in CDMA?

Following two types of spread spectrum techniques are used −

• Direct Sequence and
• Frequency Hopping.

35. What is Frequency Hopping?

Frequency hopping is a spread spectrum in which the propagation takes place by hopping in frequency over a wide band. The precise order in which the break occurs is determined by a hopping table generated by using a pseudo-random code sequence.

36. What are the Advantages of Spread Spectrum?

• Since the signal is spread over a wide frequency band, the power spectral density becomes very low, so other communications systems do not suffer from this kind of communication. However, the Gaussian noise increases.

• Multipath can be agreed with, as a large number of codes can be generated, allowing a large number of users.

• The maximum number of users have not limited spectrum or resource, as other access systems such as FDMA, here they have only limited interference.

• Security − without knowing the spreading code, it is almost impossible to recover the transmitted data.

• Descending rejection − as large bandwidth is used the system, it is less susceptible to deformation.

37. What is PN Sequence in CDMA? Explain.

The DS-CDMA system uses two types of spreading sequences − PN sequences and orthogonal codes. The PN sequence is generated by the pseudo-random noise generator which is simply a binary linear feedback shift register, consisting of XOR gates and a shift register. This PN generator has the ability to create a sequence identical for both the transmitter and the receiver, and while retaining the desirable properties of the noise randomness bit sequence.

38. What is Multi-path Fading? Explain.

In wireless communications, fading is the deviation of the signal attenuation affecting a certain propagation media. Discoloration may vary with time, the geographical position or frequency of the radio, which is often modeled as a random process. A fading channel is a communication channel experiencing fading. In wireless systems, fading can be due to multipath, called multi-path fading.

39. What is Rake Receiver?

CDMA system uses a signal fast chip rate for spreading spectrum and it has a high time resolution. For this reason, CDMA is able to recognize by decomposing each of the path to reach with the time difference. From this, it receives a different paths of signal each one separately, by summing later, it can prevent signal degradation. This is called as the RAKE receiver.

40. What is Walsh Code? Explain.

Walsh Codes are most commonly used in CDMA applications orthogonal codes. These codes correspond to lines of a special square matrix called the Hadamard matrix. For a set of Walsh codes of length N, it consists of n lines to form a square matrix of n × n Walsh code. The IS-95 system uses 64 Walsh function matrix 64. The first line of this matrix contains a string of all zeros with each of the following lines containing different combinations of bit 0 and 1. Each line is orthogonal and equal representation for binary bits. When implemented with the CDMA system, each mobile user uses one of the 64 sequences of rows in the matrix as a spreading code, providing zero cross-correlation among all the other users.

41. What is Soft Handover/Handoff?

Cellular system tracks mobile stations in order to maintain their communication links. The mobile station goes to neighbor cell and communication link switches from current cell to the neighbor cell which is called as soft handover.

• 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 the 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.

42. What is Hard Handover? Explain.

In FDMA or TDMA cellular system, new communication establishes after breaking current communication at the moment doing handoff. Communication between MS and BS breaks at the moment switching frequency or timeslot which is known as hard handover.

43. What is Power Control?

Power control is the intelligent selection of transmit power in a communication system for achieving best performance within the system. The performance depends on context and there are chances to include optimizing metrics like link data rate, network capacity, geographic coverage, and range. A higher transmit power translates into a higher signal power at the receiver.

44. What is Reverse Link Power Control? Explain.

The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile transmitted power is controlled by the base station. For this purpose, the base station continuously monitors the reverse link signal quality. If the quality of the connection is poor then the base station increases the power. Similarly, if the quality of the link is very high then the mobile base station controller reduces the power. This is called as reverse link power control.

45. What is Forward Link Power Control? Explain

Similar to reverse link power control, forward link power control is also necessary to maintain the forward link quality to a specified level. This time, the mobile monitors the forward link quality and indicates to the base station to turn on or off, this power control has no effect on the near-far problem because all the signals are blurred together the same level of power when they get to the mobile. In short, there is no near-far problem in the forward link.

46. Explain the Effects of Power Control.

• Power control is capable of compensating the fading fluctuation.
• Received power from all MS are controlled to be equal.
• Near-Far problem is mitigated by the power control.

47. Explain the Frequency Allocation Concept

In FDMA or TDMA, radio resource is allocated not to interfere among neighbor cells −

• Neighbor cells cannot use the same (identical) frequency band (or timeslot).

• The left figure shows the simple cell allocation with seven bands of frequency.

• In actual situation, because of complicated radio propagation and irregular cell allocation, it is not easy to allocate frequency (or timeslot) appropriately.

CDMA system is against this, since all users share the same frequency, the arrangement of the frequency is not an issue. This is in designing the system, which will be a very big advantage.

48. What are the Interferences in CDMA?

There are four main interferences in CDMA as given below −

• Noise sources,
• Signal processing,
• Frame error rate, and
• Power per Walsh code.

49. Explain the CMDA Interference “Frame Error Rate.”

The number of transmission errors, measured in terms of a frame error rate (FER). It increases with the number of calls. To overcome this problem, the minicell and mobile site can increase the power until either the mobile or the minicell site can power up more further to reduce FER to an acceptable amount. This event provides a soft limit calls from a particular minicell.