Monday, October 11, 2010

Spread Spectrum Signals - CDMA

Spread Spectrum uses wide band, noise-like signals. Because Spread Spectrum signals are noise-like, they are hard to detect. Spread Spectrum signals are also hard to Intercept or demodulate. Further, Spread Spectrum signals are harder to jam (interfere with) than narrowband signals. These Low Probability of Intercept (LPI) and anti-jam (AJ) features are why the military has used Spread Spectrum for so many years. Spread signals are intentionally made to be much wider band than the information they are carrying to make them more noise-like.

Spread Spectrum signals use fast codes that run many times the information bandwidth or data rate. These special "Spreading" codes are called "Pseudo Random" or "Pseudo Noise" codes. They are called "Pseudo" because they are not real gaussian noise.

Spread Spectrum transmitters uses similar transmit power levels to narrow band transmitters. Because Spread Spectrum signals are so wide, they transmit at a much lower spectral power density, measured in Watts per Hertz, than narrowband transmitters. This lower transmitted power density characteristic gives spread signals a big plus. Spread and narrow band signals can occupy the same band, with little or no interference. This capability is the main reason for all the interest in Spread Spectrum today.

Since the development of CDMA technology there has been many new releases and platforms. The original CDMA is now referred to as CDMAone. Several different variants of CDMA technology been developed continuously improving quality and data transfer speeds. Third generation CDMA technology, commonly referred to as CDMA2000 encompasses a wide variety of different standards, each continually improving upon the first including; 1X EV, 1XEV-DO, and MC 3X. CDMA2000 is the current standard used by most US carriers today. The first release of CDMA2000 was refereed to as either 3G1X, 1XRTT, or X.Designed to provide data transmissions of ten times faster then the previous technology and double the voice capacity of CDMAone.

Depending on the phone you have and its capabilities you will notice symbols in the default screen of your phone reading either 1X, 1XEV-DO or some variation of the two. This symbol defines the CDMA2000 standards your phone is operating on. Newer phones will display EV or EV-DO using the newer faster, more reliable CDMA technology.

Qualcomm the original developer of CDMA owns patents of this technology. They have granted royalty-bearing licenses to over 100 network operators.

Friday, October 8, 2010

PN Offset in CDMA

Offset is one of the 512 short code sequences used to differentiate sectors on base stations for communication with mobile units. PN stands for pseudo random noise that appears in a repetitive manner. The PN sequence forms a “short” code that is 32,768 chips in length and repeats every 26.666 milliseconds. This short code is combined with the data and transmitted in each of the forward channels. 512 points within the sequence have been selected as the PN offsets (from 0-511). Each base station uses a different point in the sequence to create a unique PN offset or identifier in its pilot signal which can be used to identify the base station sector.

For CDMA networks, the most common form of interference is pilot pollution. Each base-station sector is assigned an identifier called a PN offset, which is a timing offset based on the GPS even-second clock. Since each base station assigned to a particular frequency carrier operates at the same center frequency, the PN offset is used to distinguish base stations from one another.

When a CDMA phone searches for the strongest base-station signal, it identifies the PN offset of each signal it receives. It only looks for PNs for which the network tells it to search. This list of PNs, the neighbor list, constantly is changing since it depends on the phone's current location. Pilot pollution occurs when the CDMA mobile phone's rake receiver receives more than three (four for newer phones) pilot signals having approximately the same Ec/Io relative power levels.

Each base station sector in a cdmaOne network may transmit on the same frequency, using the same group of 64 Walsh codes for pilot, paging, sync and forward traffic channels. Therefore, another layer of coding is required so that a mobile phone can differentiate one sector from another.

The PN offset plays a key role in this code layer. The abbreviation “PN” stands for pseudo-random noise – a long bit sequence that appears to be random when viewed over a given period of time, but in fact is repetitive. In cdmaOne transmissions, the entire PN sequence is defined to form a short code that is 32,768 chips in length and repeats once every 0.027 seconds. The short code is exclusive OR’d with the data and transmitted in each of the forward channels (pilot, paging, sync, and traffic). Within the 32,768 chip sequence, 512 points have been chosen to provide PN offsets. Each base station transceiver uses a different point in the sequence to create a unique PN offset to the short code in its forward link data. As a result, a mobile phone can identify each base station sector by the PN offset in the received signal.

Each base station transmits a version of the long code that is shifted in time by a different multiple of the chip time. The PN offset represents the number of chip times by which a particular base station delays transmission of the long code. The cellular telephone receives the long code offset when the cellular telephone enters a cell or powers on, and stores the PN offset in a nonvolatile portion of memory.

Distance-based Location Update & RouteUpdateRadius in CDMA 1X EV-DO

  • Introduction
Idle-mode mobility management in cellular systems involve location updates and paging. Idle-mode mobility is not tracked at the granularity of individual cells. Instead, it is tracked at a coarser granularity of a group of contiguous cells termed as a "location area". A location update mechanism involves the reporting of this location area information by an idle mode mobile to the network, whenever it moves from one location area to another. Because the network knows the location of the mobile only at the location area-level, when there is an incoming call for the mobile, the network needs to page the mobile in all the cells in the location area. Both the location update mechanism and paging will generate signaling load on the network, and reducing the load due to one of these would involve an increased load due to the other.

In CDMA 1X EV-DO, specified in 3GPP2 C.S0024-A v1.0 (2004), a dynamic distance-based location update mechanism is used. In this scheme, a mobile makes a location update if the distance between the BTS in which it is currently camped, and the BTS where it made its last location update is greater than a parameter called RouteUpdateRadius. This scheme provides a significant performance benefit over the static location update mechanism used in GSM/GPRS/UMTS networks. However, the distance-based mechanism does not utilize the knowledge of the direction of mobiles’ movement.

  • 1X EV-DO Route Update Protocol

In CDMA 1X EV-DO, the idle-mode mobility management procedures are handled by the route update protocol, and it uses the distance-based location update approach. In 1X EV-DO, each cell broadcasts its latitude, longitude, and a parameter called RouteUpdateRadius. An idle mode mobile as it moves from cell to cell, monitors these three parameters. after each cell change, the mobile computes the distance between the site locations of the current cell and the cell in which it last sent a location update message. If this distance is greater than the RouteUpdateRadius parameter broadcast in the cell in which it last sent a location update message, the mobile sends another update to the network.

Otherwise mobile does not send a location update message. To perform this operation, the mobile would have to store the latitude, longitude, and RouteUpdateRadius parameters of the last cell in which it did a location update operation.

The distance computed is the distance between the site locations, and it does not depend on the location of mobile within the serving site. The latitude and longitude information is broadcasted by each cell is used for computing the distance.

Because the mobile sends a location update message only after it moves to a cell that is sufficiently far apart from the cell from which the mobile last sent a location update, the problem of ping-ponging is eliminated. Essentiall, as soon as a mobile sends a location update, it draws a circle around the serving cell of radius RouteUpdateRadius and sends the next location update only if it goes outside that circle. Clearly, this approach eliminates the ping-ponging problem of the static location area approach.

CDMA Introduction

Code Division Multiple Access (CDMA) has gained widespread international acceptance by cellular radio system operators as an upgrade that will dramatically increase both their system capacity and the service quality. CDMA is a "spread spectrum" technology, allowing many users to occupy the same time and frequency allocations in a given band/space. As its name implies, CDMA (Code Division Multiple Access) assigns unique codes to each communication to differentiate it from others in the same spectrum.

In a world of finite spectrum resources, CDMA enables many more people to share the airwaves at the same time than do alternative technologies. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to point communication at the same data rate.

1. INTRODUCTION

CDMA stands for Code Division Multiple Access, but was originally known as IS-95. Qualcomm was the first to created this technology and by 1993 it was adopted by the Telecommunication Industry Association. Later this technology was enhanced and refined by Ericsson.The world is demanding more from wireless communication technologies than ever before as more people around the world are subscribing to wireless. Add in exciting Third-Generation (3G) wireless data services and applications - such as wireless email, web, digital picture taking/sending, assisted-GPS position location applications, video and audio streaming and TV broadcasting - and wireless networks are doing much more than just a few years ago. This is where CDMA technology fits in. CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built.

The CDMA air interface is used in both 2G and 3G networks. 2G CDMA standards are branded cdmaOne and include IS-95A and IS-95B. CDMA is the foundation for 3G services: the two dominant IMT-2000 standards, CDMA2000 and WCDMA, are based on CDMA.

1.1 CDMAONE: The Family of IS-95 CDMA

Technologies cdmaOne describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard, including IS-95A and IS-95B revisions. It represents the end-to-end wireless system and all the necessary specifications that govern its operation. cdmaOne provides a family of related services including cellular, PCS and fixed wireless (wireless local loop).

1.2 CDMA2000: Leading the 3G revolution

CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes CDMA2000 1X and CDMA2000 1xEV technologies. They deliver increased network capacity to meet growing demand for wireless services and high-speed data services. CDMA2000 1X was the world's first 3G technology commercially deployed (October 2000).

2. SPREAD SPECTRUM COMMUNICATIONS

CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:

1. The signal occupies a bandwidth much greater than that which is necessary to send the
information. This results in many benefits, such as immunity to interference and jamming and multiuser access, which we'll discuss later on.

2. The bandwidth is spread by means of a code which is independent of the data. The independence of the code distinguishes this from standard modulation schemes in which the data modulation will always spread the spectrum somewhat.

3. The receiver synchronizes to the code to recover the data. The use of an independent code and synchronous reception allows multiple users to access the same frequency band at the same time.
In order to protect the signal, the code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo random code is also called pseudo-noise (PN).

There are three ways to spread the bandwidth of the signal:

• Frequency hopping. The signal is rapidly switched between different frequencies within the hopping bandwidth pseudo randomly, and the receiver knows before hand where to find the signal at any given time.

• Time hopping. The signal is transmitted in short bursts pseudo-randomly, and the receiver knows beforehand when to expect the burst.

• Direct sequence. The digital data is directly coded at a much higher frequency. The code is generated pseudo-randomly, the receiver knows how to generate the same code, and correlates the received signal with that code to extract the data.

What is Ec/I0

In CDMA refers to the portion of the RF signal which is usable. It's the difference between the signal strength and the noise floor.

Ec/Io (pronounced "ee-see over eye-not") is basically a measure of how well your phone can hear the tower over all the other traffic on the channel.

A reading near 0.0 is very good. You can find low readings late at night on weekdays when traffic is low. When the reading is high (-12.0 to -15.0), quality will drop and you may even lose the call.

Friday, October 1, 2010

BTS – Base Transceiver Station


The base transceiver station, or BTS, contains the equipment for transmitting and receiving radio signals (transceivers), antennas, and equipment for encrypting and decrypting communications with the base station controller (BSC). Typically a BTS have several transceivers (TRXs) which allow it to serve several different frequencies and different sectors of the cell (in the case of sectorised base stations).

A BTS is controlled by a parent BSC The BTSs are equipped with radios that are able to modulate layer 1 of interface Um; for GSM 2G+ the modulation type is GMSK, while for EDGE-enabled networks it is GMSK and 8-PSK.

Frequency hopping is often used to increase overall BTS performance; this involves the rapid switching of voice traffic between TRXs in a sector. A hopping sequence is followed by the TRXs and handsets using the sector. Several hopping sequences are available, and the sequence in use for a particular cell is continually broadcast by that cell so that it is known to the handsets. A TRX transmits and receives according to the GSM/CDMA standards.

In short BTS

1. Encodes,encrypts,multiplexes,modulates and feeds the RF signals to the antenna.
2. Frequency hopping
3. Communicates with Mobile station and BSC
4. Consists of Transceivers (TRX) units