Multiple Access


To transmit a signal from one user to another, the information must be sent over a channel that is possibly shared with many other users simultaneously transmitting their own channels

  • TDMA – Time Division Multiple Access
  • FDMA – Frequency Division Multiple Access
  • CDMA – Code Division Multiple Access




WHAT IS TDMA ?

  • Time-division multiple access (TDMA) is a channel access method for shared-medium networks.
  •  It allows several users to share the same frequency channel by dividing the signal into different time slots. 
  • In other words each user uses the whole channel BW for a fraction of time.
  • The users transmit in rapid succession, one after the other, each using its own time slot.
  •  This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. 
  • TDMA is a type of time-division multiplexing (TDM), with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters.



Advantages of TDMA

  • Permits a flexible bit rate
  • No frequency guard band required
  • Shares single carrier freq. with multiple users
  • Occupy the entire system BW
  • Slots can be assigned on demand
  • Extended battery life
  • BW supplied on demand
  • Less power control needed


Disadvantages to using TDMA

  • Propagation time for a signal from a MS to a BS varies with its distance to the BS
  • Demands high peak power in transmit mode (uplink) that shortens the battery life
  • Multipath distortion
  • Equalization necessary for high data rates


WHAT IS FDMA ?

  • In this BW of the available spectrum is divided into separate channels, each individual channel frequency being allocated to a different user
  • FDMA allows multiple users to send data through a single communication channel by dividing the bandwidth of the channel into separate non overlapping  frequency channels and allocating each sub-channel to a separate user.
  • It is used in satellite communication systems and telephone trunk lines





Advantages of FDMA

  • Individual Channel to individual user
  • If channel is not in use, it sits idle
  • Channel bandwidth is relatively narrow (30kHz)
  • Best suited for analog links
  • Fairly efficient when the number of stations is small and the traffic is uniformly constant
  • No need for network timing
  • No equalization required
  • No framing or synchronization bits needed
  • Simplest system


Disadvantages to using FDMA

  • Requires high performance RF filters to minimize adjacent channel interference
  • Maximum bit rate per channel is fixed 
  • Handles only voice transmission. It can’t manage other form of data.
  • FDMA is a wasteful of BW



CDMA

  • Decades ago, the military organization of the world adopted the CDMA scheme to meet the need for highly secure communication.
  • CDMA uses a spreading code to mix up the data & spread the BW.
  • CDMA allows every one to transmit at the same time.
  • Each user’s signal is spread over the entire BW by a unique spreading code.
  • At the receiver the unique spreading code is used to recover the signal.
  • In CDMA each couple talk at the same time(in a room full of people), but they all use a different language that does not cause a real problem



Advantages of CDMA

  • Many users of CDMA use the same frequency, TDD or FDD may be used
  • Multipath fading may be substantially reduced because of large signal bandwidth
  • No absolute limit on the number of users
  • Easy addition of more users
  • Impossible for hackers to decipher the code sent
  • Better signal quality


Disadvantages to using CDMA

  • As the number of users increases, the overall quality of service decreases
  • Self-jamming
  • Near- Far- problem arises
  • Complex Hardware required
  • High BW is required
  • Synchronization Problem




Comparison b/w FDMA, TDMA & CDMA





Spread Spectrum

  • It is a technique that increases signal BW beyond the minimum required for data communication.
  •       Principle of SS:
  • The signal occupies a BW much larger than the required.
  • SS modulation is done using a spreading code.
  • De-spreading at the receiver is done by correlating the received signal with a synchronized copy of the spreading code.


 W-CDMA/UMTS

  • Wideband CDMA is a 3G wireless standard which allows use of  both voice & data and offers data speeds of up to 384 Kbps.
  • WCDMA is also called UMTS (Universal Mobile Telecommunications System) and the two terms have become interchangeable.
  • UMTS is a third generation mobile cellular system for networks based on the GSM standard, developed and maintained by the 3GPP (3rd Generation Partnership Project)
  • W-CDMA uses the DS-CDMA channel access method with a pair of 5 MHz wide channels.


WCDMA

The specific frequency bands originally defined by the UMTS standard are 1885–2025 MHz for the base-to-mobile (uplink) and 2110–2200 MHz for the mobile-to-base (downlink).

In the US, 1710–1755 MHz and 2110–2155 MHz are used instead, as the 1900 MHz band was already used. 


Architecture of UMTS Network





  • As shown in the figure there are three main components in UMTS network architecture. 
  • User Equipments is composed of Mobile Equipment (ME) and USIM. 
  • Radio Access Network is composed of NodeB and RNC. 
  • Core Network is composed of circuit switched and packet switched functional modules. 
  • For Circuit switched (CS) operations MSC and GMSC along with database modules such as VLR, HLR will be available. 
  • For packet switched (PS) operations SGSN and GGSN will serve the purpose. 
  • GMSC will be connected with PSTN/ISDN in CS case. 
  • GGSN is connected with Packet data Network (PDN) for PS case. 

HSPA

  • High Speed Packet Access (HSPA) is an combination of two mobile protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), that extends and improves the performance of existing 3G mobile telecommunication networks using the WCDMA protocols.
  • The system provides an enhancement on the basic 3G WCDMA / UMTS cellular system, providing much greater data transfer rates.
  • The system provides many advantages for users over the original UMTS system


The two technologies are summarised below

HSDPA - High Speed Downlink Packet Access:   HSDPA provides packet data support, reduced delays, and a peak raw data rate (i.e. over the air) of 14 Mbps. It also provides around three times the capacity of the 3G UMTS technology defined in Release 99 of the 3GPP UMTS standard.
HSUPA - High Speed Uplink Packet Access:   HSUPA provides improved uplink packet support, reduced delays and a peak raw data rate of 5.74 Mbps. This results in a capacity increase of around twice that provided by the Release 99 services.

LTE (Long Term Evolution)

In telecommunications, Long-Term Evolution (LTE) is a standard for wireless broadband communication for mobile devices and data terminals, based on the GSM/EDGE and UMTS/HSPA technologies.


LTE Architecture

  • The high-level network architecture of LTE is comprised of following three main components:
  • The User Equipment (UE).
  • The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
  • The Evolved Packet Core (EPC).
  • The E-UTRAN handles the radio communications between the mobile and the evolved packet core and just has one component, the evolved base stations, called eNodeB or eNB.
  • Each eNB is a base station that controls the mobiles in one or more cells. The base station that is communicating with a mobile is known as its serving eNB.
  • LTE Mobile communicates with just one base station and one cell at a time and there are following two main functions supported by eNB:
  • The eBN sends and receives radio transmissions to all the mobiles using the analogue and digital signal processing functions of the LTE air interface.
  • The eNB controls the low-level operation of all its mobiles, by sending them signalling messages such as handover commands.
  • EPC (Evolved Packet Core) contains information about all the network operator's subscribers, communicates with the outside world, forward the data b/w base station & gateways