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Mobile Communications > Lect 5 > Flashcards

Lect 5 Flashcards

1
Q

Radio band

A

Small contiguous section of the frequency spectrum.

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2
Q

Multiplexing/multiple access

A

Method of sharing a channel to many users.

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3
Q

Channel

A

System resource allocated to a mobile user enabling the user to establish communication with the network.

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4
Q

Classes of multiple access techniques

A
  1. Reservation based
  2. Random
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5
Q

Types of reservation based

A
  1. FDMA
  2. TDMA
  3. CDMA
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6
Q

Types of Random

A
  1. CSMA
  2. ALOHA
  3. ISMA
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7
Q

Freq Division Duplexing channels

A
  1. Uplink
  2. Downlink
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8
Q

What’s a Narroband system

A

System relating bandwidth of a single channel to the entire bandwidth.

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9
Q

Narrowband FDMA

A

User is assigned a channel which isn’t shared by others.

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10
Q

Narrowband TDMA

A

Users share a channel but allocates unique timeslot to each user in a cyclical fashion.

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11
Q

CDMA

A

Allows all transmiters to access the channel at the same time.

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12
Q

Guard band

A

Narrow frequency band between adjacent frequency channels to avoid interference employed in FDMA systems.

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13
Q

Features of FDMA

A
  1. BS and MS transmit simultaneously and continuously.
  2. Uses duplexers
  3. Implemented in narrowband systems.
  4. Less complex.
  5. Higher cell site system costs.
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14
Q

FDMA disadvantages

A
  1. Low capacity in terms of channel availability.
    2.Frequency wastage due to guardbands.
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15
Q

Number of channels in FDMA syst

A

N = (Bt - 2Bguard)/Bc

Where Bt = total bandwidth
Bc = channel bandwidth
Bguard = guard bandwidth

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16
Q

TDMA Co-channel interference

A

When 2 users overlap in time.

17
Q

Components of TDMA frame

A
  1. Preamble
  2. Info message
  3. Tail bits
18
Q

TDMA

A

whole channel available to users but each user uses it for a fraction of time.

19
Q

TDMA features

A
  1. Transmission is not continuous
  2. Handoff is simpler
  3. Duplexers not required
  4. High sync overhead is required
  5. Larger overheads
20
Q

TDMA advantages

A
  1. Single freq can be used by many users.
  2. No interference due to time gap.
  3. No guard band needed so all freqs are used.
21
Q

TDMA disavantages

A

1.User needs precise synchronization.
2. Reduced data rate.

22
Q

Efficiency of TDMA

A

Measure of percentage of transmitted data that contains info as opposed to providing overhead.

23
Q

Frame efficiency

A

Percentage of bits per frame which contain transmitted data.

24
Q

No. Of overhead bits per frame

A

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

25
Q

Total no. of bits per frame

A

bT = Tf*R

Tf = frame duratiion
R = channel biy rate

26
Q

Efficiency of TDMA frame

A

= (1 - bOH/bT) * 100%

27
Q

No. Of channels in TDMA system

A

N = m(Bt - 2Bguard)/Bc

m = max users per channel

28
Q

CDMA

A

The narrowband message signal is multiplied by very large bandwidth signal called spreading signal.

29
Q

Orthogonality

A

Property that allows multiple info to be transmitted over a common channel with succesful detection.

30
Q

Advantages of CDMA

A
  1. Improvement in network capacity
  2. Attractive for 3G high speed internet use
  3. Easy for a terminal to communicate with two base stations
31
Q

Disadvantages

A
  1. Can’t offer roaming.
  2. Perfomance degrades with increase in number of users.
    3.CDMA network is not matured.
32
Q

Application of CDMA

A
  1. Adopted by 3G cellular telecom systems.
  2. Merged with GSM to give high speed 4G internet services.
33
Q

Spectral efficiency depends on.

A
  1. Channel spacing.
  2. Freq reuse factor.
  3. Cell area.
  4. Modulation techniques.
  5. Multiple access techniques.
34
Q

Spectral efficiency of modulation

A

= 1/(BcNAc) Channels/MHz/Km sqr

Bc = channel spacing in MHz
N = freq reuse factor
Ac = area covered by a cell in km sqr

35
Q

Multiple access Spectral efficiency

A

Ratio of total time or freq dedicated for traffic transmission to total time or freq available to the system.

36
Q

FDMA multiple access spectr effi.

A

na = (Bc*Nt)/Bw

na = multiple access spec. effi.
Nt = total no. of traffic channels
Bc = channel spacing
Bw = sys bandwidth

37
Q

TDMA spec efficiency

A

na = t*Mt/Tf

t = duration of timeslot that carries data
Tf = frame duration
Mt = no. of time slots/frame

38
Q

Overall Spectral efficiency

A

n = nm * na

Mobile Communications (1 decks)

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