Digital Cellular System and Multiple Access Scheme :
The area served by mobile phone systems is divided into small areas known as the cell . Each cell contains
the base station that communicates with mobiles in the cell by transmitting and receiving signals on radio
links. The transmission from the base station to a mobile is typically referred to as downstream, forwardlink,
or the down link. Corresponding terms for the transmission from a mobile to the base are upstream,
reverse-link, and up link. Each base station is associated with a mobile switching center (MSC) that
connects the calls to & from the base to mobiles in other cells and the public switched telephone network.
A typical setup depicting a group of base stations to a switching center. In this section terminology associated with cellular systems is introduced with a brief description to understand how
these systems work.
The CDMA scheme is a direct sequence (DS), spread-spectrum method. It uses linear modulation with
wideband pseudonoise (PN) sequences to generatethe signals. These also known as the codes, spread
the spectrum of the modulating signal over the large bandwidth, simultaneously reducing spectral
density of the signal. Thus, various CDMA signals occupy the same bandwidth and appear as noise to
each other. More details on DS spread-spectrum may be found .
In the CDMA scheme, each user is assigned an individual code at the time of the call initiation. These code
are used both for spreading signal at time of transmission and the despreading the signal at the time
of reception. Cellular systems using CDMA schemes use the FDD, thus employing 2 frequency channels
for forward and the reverse links.
In CDMA systems:
If a user doesn’t has anything to send, it causes less interference to the other users of the system
Typically, each user needs to transmit less than half the time
Since the interference limited, this doubles capacity.
CDMA have overcome most cynicism to dominate worldwide wireless voice market
• What about the data services? Scheduling vs. thee Inteference Averaging
• CDMA appears to be an underdog for the 4 Generation , but still may win
• Ongoing research on CDMA
Increase the capacity by joint decoding Applying CDMA to other applications: the optical CDMA, the ad hoc networks, the dense wireless LANs “MultiCDMA”:multicarrier CDMA, multiple antenna CDMA, multicode CDMA.
Drawbacks of CDMA:
1 . Tight synchronization is required to use the orthogonal codes, that then break in a multipath channel anyway
2. The Quasi orthogonal codes cause the self interference, that dominates the performance in most CDMA systems
3. Near far problem is the serious hindrance, requiring fast & accurate power control (that uses up bits we could otherwise send information with)
4. And for all this, the required bandwidth is now J times larger than it was previous , so there doesn’t appear to be a capacity gain.
The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, & the originally described digital, circuit switched network optimized for the full duplex voice telephony. These were expanded over the time to include the data communications, 1st by circuit switched transport, then the packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).
Several open-source software projects exist that provide certain GSM features:
• gsmd daemon by Openmoko.
• OpenBTS develops a Base transceiver station
• The GSM Software Project aims to build a GSM analyzer for less than $1000.
• OsmocomBB developers intend to replace the proprietary baseband GSM stack with a free software implementation.
Q1: What is a cell phone ? and Why is it called Cellular?
Ans: A cell phone is a radio like a walkie-talkie
Walkie Talkie is simplex Only 40 channels to choose from 5 Watts can go about 4 miles
Cell phone is Duplex
There are around 1664 channels
Has few milliWatts only limited range
An antenna tower handles calls from a short distance from it (Base Station)
More antenna towers means more users can talk on cell phones
Lower power the cell phone emits
Repeat the frequencies used
The antennas form small cells of coverage around the city so it is called cell phone .
Q2:Give an example of Multiple Access in cellular?
Ans: Cellular Example: Many people want to talk on their cell phones. Each phone must communicate with a base station.
Q3: What are the Cell Phone Identity Systems ?
Ans: Every cell phone has 2 unique identifications:
MIN – Mobile Identification Number A 10 digit telephone number 309-555-1212
ESN – Electronic Serial Number 32 bit binary number Built into the cell phone.
Q4: Explain the Channel Characteristics ?
Ans: An understanding of propagation conditions and channel characteristics is important for an efficient use
of a transmission medium. Attention is being given to understanding the propagation conditions where
a mobile is to operate and many experiments have been conducted to model the channel characteristics.
Q5: What are the network structure of GSM ?
Ans: The network is structured into a number of discrete sections:
• The Base Station Subsystem (the base stations and their controllers).
• The Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
• The GPRS Core Network (the optional part which allows packet based Internet connections).
• The Operations support system (OSS) for maintenance of the network.