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.

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.