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Welcome to the Directed Remote Learning session of
LTE Air Interface Basics
During this course, we will:
Compare the LTE air interface in terms of Frequency and Time with
GSM and UMTS
Compare the capacity of the LTE air interface with GSM and UMTS
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In GSM, the Radio Network portion of the mobile telephone network is called the BSS
(Base Station Subsystem).
2G (GSM) and 2.5G (GERAN)
It contains:
BTS BSC
BTS
BTS
TRC To Core
BTSs (Base Transceiver Stations)
BSCs (Base Station Controllers) TRCs (TRanscoder Controllers)
BSS
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3G UMTS/WDMA
It contained:
RNC
Node-B
To Core
Node-Bs
RNCs (Radio Network Controllers)
In UMTS, the Radio Network portion is called the UTRAN or, more simply, the RAN
(Radio Access Network).
Node-B
Node-B
RAN
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4G (almost) EPS
It contains:
e-Node B
To Core
eNBs
In EPS (Evolved Packet System), the Radio Network portion is called the e-UTRAN
(where e stands for evolved) or LTE (Long Term Evolution).
e-Node B
e-node B
LTE or e-UTRAN
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Time Domain
All three technologies utilize Time Slots (TSs)
GSM
WCDMA
LTE
667 sec
577 sec
500 sec
Drawings not to scale.
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Frames - GSM
In GSM, there are 8 TSs in a frame. Normally, each
connection gets its own TS on a carrier of 200KHz
1 frame = 8 TSs
4.616 ms
Carrier 1
Carrier 2
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Frames - UMTS
In UMTS, there are 15 TSs in a frame. Everyone has a connection on every TS.
There is a single carrier having a bandwidth of 5 MHz
1 frame = 15 TSs
1 msec
Carrier 1
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Frames LTE
In LTE, two TSs make a subframe and 10 subframes make a frame.
1 frame = 10 Subframes = 20 TSs
10 msec
TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS
10
Subframe
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TS
Frames LTE
In LTE, two TSs make a subframe and 10 subframes make a frame.
1 frame = 10 Subframes = 20 TSs
10 msec
TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS
Subframe
The minimumallocation for a
connection is one
subframe. Both TSs
must be allocated.
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Frames LTE
In LTE, two TSs make a subframe and 10 subframes make a frame.
1 frame = 10 Subframes = 20 TSs
10 msec
TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS
Subframe
A connection may use
several subframes
during a frame.
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Frames LTE
In LTE, two TSs make a subframe and 10 subframes make a frame.
1 frame = 10 Subframes = 20 TSs
10 msec
TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS TS
Subframe
Not all subframes
have to be allocated
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1. How many subframes can be transmitted in a second?
Exercise
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Frequency GSM and UMTS
In GSM and UMTS, a subscriber is limited to using a single carrier at any point in time.
Carrier 1
Carrier 2
X
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Frequency LTE 2TSs1 subframe
1 msec
Frequency Block
12 subcarriers
@ 15 kHz each
In LTE, the smallest possible
frequency allocation is a
Frequency Block
A Frequency Block consists of
12 contiguous subcarriers at
15 kHz each
freq
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Frequency LTE
A subscriber may be
allocated more than
one Frequency Block
during a subframe*
2TSs
1 subframe
1 msec
Frequency Block
12 subcarriers
@ 15 kHz each
In LTE, the smallest possible
frequency allocation is a
Frequency Block
A Frequency Block consists of
12 contiguous subcarriers at
15 kHz each
freq
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Frequency LTE
A subscriber may be
allocated more than
one Frequency Block
during a subframe*
2TSs
1 subframe
1 msec
Frequency Block
12 subcarriers
@ 15 kHz each
In LTE, the smallest possible
frequency allocation is a
Frequency Block
A Frequency Block consists of
12 contiguous subcarriers at
15 kHz each
freq
*On the UL, the Frequency Blocks for a
single connection must be contiguous. Onthe DL, they do not have to be contiguous
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Frequency LTE 2TSs1 subframe
1 msec
Not all Frequency Blocks
have to be allocated.
Frequency Block
12 subcarriers
@ 15 kHz each
In LTE, the smallest possible
frequency allocation is a
Frequency Block
A Frequency Block consists of
12 contiguous subcarriers at
15 kHz each
freq
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2. What is the bandwidth of a Frequency Block? How does this compare with a single GSM
carrier? How does this compare with a UMTS carrier?
Exercise
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Operator Spectrum
freq
An operator has
flexibility in how
much spectrum theycan allocate for LTE.
Possible allocations
are:
1.4 MHz
3 MHz 5 MHz
10 MHz
15 MHz
20 MHz
1.4
3
5
10
15
20
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3. Calculate the number of Frequency Blocks in each allocation that an operator is allowed to use
(i.e., 1.4, 3, 5, 10, 15 and 20 MHz). In each case, round down and remove one Frequency Block
for guard band purposes.
Exercise
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Scheduling Block LTEIn LTE, the smallest possible allocation
for a connection is a Scheduling
Block
Frequency Block12 contiguous subcarriers
@ 15 kHz each
2 TSs = 1 subframe = 1 msec
Subcarrier 1
Subcarrier 2
Subcarrier 3
Subcarrier 4
Subcarrier 5
Subcarrier 6
Subcarrier 7
Subcarrier 8
Subcarrier 9
Subcarrier 10
Subcarrier 11
Subcarrier 12
Subcarrier 1
Subcarrier 2
Subcarrier 3
Subcarrier 4
Subcarrier 5
Subcarrier 6
Subcarrier 7
Subcarrier 8
Subcarrier 9
Subcarrier 10
Subcarrier 11
Subcarrier 12
time
freq
Each subcarrier is
15kHz wide
`
A Scheduling Block consists of :
1 Frequency Block
1 subframe
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Frequency LTEA possible allocation for 4 subscribers over 5 subframes
time
freq
2 TSs = 1 subframe = 1 msec
Frequency Block
12 subcarriers
@ 15 kHz each
Subscriber 1
Scheduling
Block
Subscriber 2
Subscriber 3
Subscriber 4
Unallocated
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A closer look at a Time Slot
12 subcarriers
Frequency Block
1 Timeslot = 1/2 subframe = 0.5 msec
time
freq
Symbol 1 Symbol 2 Symbol 3 Symbol 4 Symbol 5 Symbol 6 Symbol 7
A Time Slot on one SubCarrier has 7 Resource Elements
Each Resource Element carries one symbol (see next slide)
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Resource Elements
12 subcarriers
Frequency Block
1 msec = 1 subframe = 2 TSs
time
freq
The smallest unit of data transmission is a Resource Element
Scheduling
Block
It consists of:
1 symbol on
1 subcarrier
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4. How many Resource Elements are in a Scheduling Block?
5. For a single Frequency Block, how many Resource Elements can be transmitted in one
second?
Exercise
Bi R d M d l i
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Bit Rates and Modulation
Symbol
1
A symbol (can carry: 2 bits (if the modulation is QPSK) or
4 bits (if the modulation is 16QAM) or
6 bits (if the modulation is 64QAM)
Symbol
2Symbol
3Symbol
4Symbol
5Symbol
6Symbol
7Symbol
8Symbol
9Symbol
10Symbol
11Symbol
12Symbol
13Symbol
14
1 msec = 1 subframe = 2 TSs
One subcarrier
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6. If QPSK is used, how many bits per second can be transmitted on a single Frequency Block?
7. If 16QAM is used, how many bits per second can be transmitted on a single Frequency Block?
8. If 64QAM is used, how many bits per second can be transmitted on a single Frequency Block?
9. The highest bit-rate possible with EDGE in GSM is approximately 60 kbits/sec per TS. Using all
8 TSs, what bit-rate could EDGE achieve for a single carrier? How does this compare with the
three bit-rates of a single Frequency Block on EPS-LTE?
10. Using your answer from question 3, what bit rate could EPS-LTE achieve with 5 MHz of
bandwidth using QPSK? Using 16QAM? Using 64QAM?
11. A single DL carrier in UMTS (P5) can achieve a bit-rate of approximately 14 Mbits/sec. Using
your answer from question 10, how do the 3 bit-rates compare with the bit-rate of UMTS?12. A voice call, including all error-coding and signaling, requires approximately 33 kbits/sec to
support. Approximately how many voice calls could be supported on a single Frequency Block
in EPS-LTE? Assume QPSK modulation.
13. How does the answer in question 12 compare with the number of voice calls supported by a
single GSM carrier?
14. How many voice calls could be supported with 5 MHz of bandwidth in EPS-LTE? How does that
compare with UMTS?
Exercise