Lect 5 Flashcards
Radio band
Small contiguous section of the frequency spectrum.
Multiplexing/multiple access
Method of sharing a channel to many users.
Channel
System resource allocated to a mobile user enabling the user to establish communication with the network.
Classes of multiple access techniques
- Reservation based
- Random
Types of reservation based
- FDMA
- TDMA
- CDMA
Types of Random
- CSMA
- ALOHA
- ISMA
Freq Division Duplexing channels
- Uplink
- Downlink
What’s a Narroband system
System relating bandwidth of a single channel to the entire bandwidth.
Narrowband FDMA
User is assigned a channel which isn’t shared by others.
Narrowband TDMA
Users share a channel but allocates unique timeslot to each user in a cyclical fashion.
CDMA
Allows all transmiters to access the channel at the same time.
Guard band
Narrow frequency band between adjacent frequency channels to avoid interference employed in FDMA systems.
Features of FDMA
- BS and MS transmit simultaneously and continuously.
- Uses duplexers
- Implemented in narrowband systems.
- Less complex.
- Higher cell site system costs.
FDMA disadvantages
- Low capacity in terms of channel availability.
2.Frequency wastage due to guardbands.
Number of channels in FDMA syst
N = (Bt - 2Bguard)/Bc
Where Bt = total bandwidth
Bc = channel bandwidth
Bguard = guard bandwidth
TDMA Co-channel interference
When 2 users overlap in time.
Components of TDMA frame
- Preamble
- Info message
- Tail bits
TDMA
whole channel available to users but each user uses it for a fraction of time.
TDMA features
- Transmission is not continuous
- Handoff is simpler
- Duplexers not required
- High sync overhead is required
- Larger overheads
TDMA advantages
- Single freq can be used by many users.
- No interference due to time gap.
- No guard band needed so all freqs are used.
TDMA disavantages
1.User needs precise synchronization.
2. Reduced data rate.
Efficiency of TDMA
Measure of percentage of transmitted data that contains info as opposed to providing overhead.
Frame efficiency
Percentage of bits per frame which contain transmitted data.
No. Of overhead bits per frame
Boh = NrBr + NrBg + NtBp + NtBg
Where,
Nr = no. of reference burst/frame
Nt = no. of traffic burst/frame
Br = no. of bits/reference burst
Bp = no. of bits/preamble
Bg = no. of bits/guard time
Total no. of bits per frame
bT = Tf*R
Tf = frame duratiion
R = channel biy rate
Efficiency of TDMA frame
= (1 - bOH/bT) * 100%
No. Of channels in TDMA system
N = m(Bt - 2Bguard)/Bc
m = max users per channel
CDMA
The narrowband message signal is multiplied by very large bandwidth signal called spreading signal.
Orthogonality
Property that allows multiple info to be transmitted over a common channel with succesful detection.
Advantages of CDMA
- Improvement in network capacity
- Attractive for 3G high speed internet use
- Easy for a terminal to communicate with two base stations
Disadvantages
- Can’t offer roaming.
- Perfomance degrades with increase in number of users.
3.CDMA network is not matured.
Application of CDMA
- Adopted by 3G cellular telecom systems.
- Merged with GSM to give high speed 4G internet services.
Spectral efficiency depends on.
- Channel spacing.
- Freq reuse factor.
- Cell area.
- Modulation techniques.
- Multiple access techniques.
Spectral efficiency of modulation
= 1/(BcNAc) Channels/MHz/Km sqr
Bc = channel spacing in MHz
N = freq reuse factor
Ac = area covered by a cell in km sqr
Multiple access Spectral efficiency
Ratio of total time or freq dedicated for traffic transmission to total time or freq available to the system.
FDMA multiple access spectr effi.
na = (Bc*Nt)/Bw
na = multiple access spec. effi.
Nt = total no. of traffic channels
Bc = channel spacing
Bw = sys bandwidth
TDMA spec efficiency
na = t*Mt/Tf
t = duration of timeslot that carries data
Tf = frame duration
Mt = no. of time slots/frame
Overall Spectral efficiency
n = nm * na
