UNIT 6
CDMA Based Mobile Systems
- It works in 850 MHz frequency band. In CDMA every device get served in network with its unique and physical channel with help of unique code signal gets multiplexed and same physical channel used to send signal.
- Main advantage of CDMA is privacy as signals are transmitted after coding.
- In wireless n/w
Wireless PANs (sensor n/w, Bluetooth, UWB: Ultra Wide Band)
Wireless LANs (Wi-fi, 802.11 a, b, g, n)
Wireless MAN (Wireless local loops and free space optics are also popular.
Wireless WANs (cellular n/w, satellite system)
Fig.: CDMA users are separated by codes
Fig. : General spread spectrum structure
- Spread spectrum is specially used for wireless communication signal spreading. Transmitted signals frequency varies deliberately.
- Frequency hopping and direct sequence are two popular spread spectrums.
- In frequency hopping, signals are broadcast over any random series of frequency while in direct sequence each bit is in order of multiple bit of transmitting signal it uses chipping code.
Frequency Hoping Spread Spectrum (FHSS)
Fig.: Channel assignment
Fig.: Channel use
Fig.: FHSS (Transmitter)
In FHSS, according to PN sequence RF carries frequency used to get change.
FHSS has two types
1. Fast hopped FHSS.
2. Slow hopped FHSS.
Hopping is done with faster rate compared to bit rate in fast hopped FHSS while slow rate compared to message bit rate in slow hopped FHSS.
FHSS systems rely on changes in RF carrier frequencies which turn into burst errors.
Fig.: FHSS (Receiver)
Direct Sequence Spread Spectrum (DSSS)
Fig.: DSSS (Transmitter)
- Base of DSSS is key to a successful recovery of message is the knowledge about PN sequence which is used at the transmitter.
- Information bits are spread across frequency as well as time, it result by minimizing effects of interference and fading. It somehow prone to error comparably FSSS system.
Fig.: DSSS (Receiver)
- Transmitted signal is received at receiver as time delayed multiple versions of transmitted signal due to propagation delay. RAKE receiver combines all multipath components of original transmitted signal in order to improve signal to noise ratio at receiver. It provides separate correlation receivers for each multipath component to combine all multipath components.
- RAKE receiver is diversity receiver designed for CDMA, where the diversity is provided by the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed a chip period.
- A RAKE receiver utilizes multiple correlators to separately detect the M strongest multipath components. The outputs of each correlator are weighted to provide a better estimate of the transmitted signal than is provided by a single component. Demodulation and bit decision is based on weighted outputs of the M correlators.
- Basic idea of RAKE receiver was proposed by Price and Green. In outdoor environments, the delay between multipath components is usually large and, if the chip rate is properly selected the low auto correlation properties of CDMA spreading sequence can assure that multipath components will appear nearly uncorrelated with each other.
- If only one correlator is used the receiver, once the output of the single correlator is corrupted by fading, the receiver cannot correct the value. Bit decision based on only a single correlation may produce a large bit error rate. In RAKE receiver, if the output from one correlator is corrupted by fading, other signals can be used to recover the original signal and corrupted signal is not counted through weighing process. Decision based on the combination of the M separate decision statistics offered by the RAKE provides a form of diversity which can overcome fading and thus improve CDMA reception.
- The M decision statistics are weighted to form an overall decision statistics as shown in above figure. The outputs of the M correlators are denoted by Z1,Z2..and ZM.
- They are weighted by α1,α2,……… and αM respectively.
- The weighting coefficients are based on the power or the SNR from each correlator output. If the power or SNR is small out of a particular correlator, it will be assigned a small weighting factor. In case of a maximal ratio combining diversity scheme, the overall signal Z is given by,
The weighting coefficients are normalized to the output signal power of the correlator in such a way that coefficients sum to unity.
Choosing weighting coefficient based on actual outputs of correlators yields good RAKE performance.
IS-95 was the first CDMA mobile phone system to gain widespread use and it is found widely in North America. Its brand name is cdmaOne and the initial specification for the system was IS95A, but its performance was later upgraded under IS-95B. It is this later specification that is synonymous with cdmaOne. Apart from voice the mobile phone system is also able to carry data at rates up to 14.4 kbps for IS-95A and 115 kbps for IS-95B.
IS95 / cdmaOne was the fist cellular telecommunications system to use the CDMA - code division multiple access system. Previous systems had used FDMA - frequency division multiple access or TDMA - time division multiple access. With IS-95 being a second generation - 2G system and all the later 3G systems using CDMA as their access system, this meant that IS95 / cdmaOne was a pioneering system.
The CDMA or code division multiple access system used for IS-95 is very different to other multiple access schemes used in previous cellular systems. However it offers a number of advantages and as a result has been widely used in many cellular technologies.
The part of electromagnetic spectrum defined as radio wave and microwaves is divided into eight ranges. These are regulated by government authorities and called as bands.
Table: Bands
Range | Band | Propagation type | Application |
3 – 30 KHz | VLF (Very Low Frequency) | Ground | Long range radio navigation. Ex. Navy military communication |
30 – 300 KHz | LF (Low Frequency) | Ground | Navigational locators Ex. Aeronautical |
300 KHz – 3 MHz | MF(Middle Frequency ) | Sky | AM radio Broad Cast |
3 – 30 MHz | HF (High Frequency) | Sky | Air craft communication and short wave transmission |
30 – 300 MHz | VHF (Very High Frequency) | Sky and LOS | FM radio, TV |
300 MHz -3 GHz | UHF (Ultrahigh Frequency) | Line of sight | VHF TV, Paging, cellular phone |
3 – 30 GHz | SHF (Super High Frequency) | Line of sight | Satellite communication |
30 – 300 GHz | EHF (Extremely High Frequency) | Line of sight | Satellite and radar. |
When all mobile stations transmit the signals at the same power (MS), the received levels at the base station are different from each other, which depend on the distances between BS and MSs.
The received level fluctuates quickly due to fading. In order to maintain the received level at BS, a suitable power control technique must be employed in CDMA systems.
We need to control the transmission power of each user. This control is called the transmission power control (Control Power). There are two ways to control the transmission power. First is the open-loop (Open Loop) control and second is closed-loop (Closed Loop) control.
Reverse Link Power Control
In CDMA, each user's transmission power is allocated by the control power to achieve the same power (Pr) which is received by the base station/BTS with access probe with low power. The mobile sends its first access probe, then waits for a response from the base station. If it receives no response, then the second access probe is sent with a higher power.
The process is repeated until the base station responds. If the signal answered by the base station is high, then the mobile gets connected with the base station which is closer to the mobile cell with low transmission power. Similarly, if the signal is weak, the mobile knows the path loss is greater and transmits high power.
The process described above is called open loop power control since it is controlled only by the mobile itself. Open loop power control starts when the first mobile attempts to communicate with the base station.
This power control is used to compensate for the slow variables shading effects. However, since the rear and forward links are on different frequencies, the estimate transmit power does not give accurate solution for the power control because of the path loss to the front of the base station. This power control fails or too slow for fast Rayleigh fading channels.
The power of closed loop control is used to compensate for the rapid Rayleigh discoloration. This time, the mobile transmit 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 low, it tells the mobile to increase its power; and if the quality of the connection is very high, the mobile base station controller reduces its power.
Forward Link Power Control
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. All the signals are blurred together at the same level of power when they get to the mobile. In short, there is no near-far problem in the forward link.
Effect of Power Control
By transmission power control, the user can obtain a constant communication environment regardless of the location. The user who is far from the base station sends a higher transmission power than the user who is nearer to the base station. Also by this transmission power control, you can reduce the effects of fading. This means that the variation of the received power due to fading can be suppressed by the transmission 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.
- As growing popularity of communication new requirements raising.
- A single global frequency band and new standard for digital cellular systems comes in Third Generation (3G) system in late 1990s lade by international Telecommunication Union (ITU).
- Voice services, data services for various applications like internet browsing, high quality audio and video entertainment, video call, etc; made renamed to standard as Internation Mobile Telephone 2000 (IMT 2000)
- IMT 2000 standards i.e. 3G has to support one of two competing standard i.e.
(a) CDMA 2000
It is supported by third generation partnership project 2 (3GPP2). CDMA 2000 is backward compatible i.e. CDMA one.
(b) Wideband CDMA
It is supported by third Generation Partnership Project 1(3GPP1). W-CDMA compatible with IS – 136 and GSM.
Table: 3G Digital Cellular Phones Standards
3G standard | CDMA 2000 | W – CDMA | |||||
Subclass | 1x | 1XEV - DO | 1XEV - DV | 3x | UMTS | FOMA | i – phones |
Channel bandwidth | 1.25 | 1.25 | 3.75 | 5 | |||
Pick data rate | 0.144 | 2.4 | 4.8 | 5.8 | 2.4 (8-10 with HSPDA) | ||
Modulation | QPSK (DL), BPSK (UL) |
References
1. Theodore S Rappaport, “Wireless Communications Principles and Practice” Second Edition, Pearson Education
2. John C. Bellamy, “Digital Telephony”, Third Edition; Wiley Publications
3. ThiagarajanVishwanathan, “Telecommunication Switching Systems and Networks”; PHI Publications
4. Wayne Tomasi, “Electronic Communications Systems”; 5th Edition; Pearson Education
5. Vijay K Garg, Joseph E Wilkes, “Principles and Applications of GSM” Pearson Education
6. Vijay K Garg, Joseph E Wilkes, “IS-95CDMA and CDMA 2000 Cellular/PCS Systems Implementation” Pearson Education
7. Mischa Schwartz, “Mobile Wireless Communications”, Cambridge University Press