WMC: Unit IV

1
WMC Unit IV

• Summarized by
• Neetesh Purohit
• Lecturer, IIIT,
• Allahabad, UP, India
• http//profile.iiita.ac.in/np/

2
Speech coding

• It has two goals
• Highest quality voice
• (either high data rate or higher processing
  delay with greater complexity is required)
• Least possible channel capacity
• (In general, quality degrades if data rate is
  reduced)

3
Types of coders

• Waveform coders
• Performance depends upon voice waveform but not
  on the voice source.
• Source coders (Vocoder)
• (i) Performance of coder depends upon voice
  source
• (ii) Voice is not transmitted but it is analyzed,
  some parameters are extracted and transmitted. At
  Rx voice is synthesized using these parameters.

4
Noise source (for unvoiced)
Vocal tract filter
Periodic Pulse Source (for voiced)
Rx Parameters

• Voiced sounds Periodic air pulses pass through
  vibrating vocal chords. (freq of periodic pulse
  train should be equal to pitch)
• Unvoiced sounds Force air through a constriction
  in vocal tract, producing turbulence.
• Two methods for voiced/ unvoiced decision
• (i) Zero Crossing unvoiced signals have much
  faster zero crossing as compared to voiced.
• (ii) Power unvoiced signal has very low power
  as compared to voiced

5

• Waveform coders
• Time domain
• Non Differential (PCM)
• Differential (ADM, ADPCM)
• Frequency domain
• Sub Band Coding (SCB)
• (Bandpass filters are used to divide the signal
  in to various bands. Different bands are
  quantized with different precision)
• Adaptive Transform Coding (ATC)
• (sampling ? DCT ? adaptive bit allocation for
  quantizing the coefficients ? total number of
  bits in a frame must be constant)

6

• Source coders
• Frequency domain
• Channel Vocoders (sub banding ? from each band
  (also called channel) , one or two samples,
  voiced/unvoiced decision and pitch frequency is
  transmitted)
• Formant Vocoder (same as channel vocoder but
  instead of signal sample, position of highest
  peak (formant) is transmitted)
• Cespstrum Vocoder (sampling?Fourier
  Transform?log10?inverse Fourier transform to get
  Cepstrum coefficients for transmission)
• Voice excited vocoders (x(t) is divided into two
  bands LF and HF. LF is transmitted using PCM and
  HF is transmitted using Channel vocoder without
  sending pitch and voiced/unvoiced decision. These
  are available in LF)

7

• Linear Predictive Coders (LPC) Time domain source
  coders
• Coding principle is same as that of ADPCM. it
  does not transmit error signal but a few
  characteristics of error signal.
• Types of LPC
• Single Pulse Excited (single pulse excitation is
  used at Rx, produces audible distortion)
• Multipulse excited (8 pulses per period are used,
  position and amplitude of an pulse is adjusted to
  reduce the distortion )
• Code excited (a codebook of Guassian excitation
  signals is maintained by both Tx and Rx. Besides
  other parameters, The transmitter transmits the
  index of code where it finds the best match)
• Residual excited (single pulse LPC is used for
  decoding at Tx itself and synthesized voice is
  generated. Now the difference (residue) of above
  two signals is coded and send with the original
  LPC codes)

8
Factors affecting choice of speech codec

• Cost and complexity
• DC power requirements
• Encoding delay
• Robustness to transmission errors
• Cell size
• Multiple access technique
• Modulation technique

9
Global System for Mobile(GSM)General Packet
Radio Service (GPRS)Enhanced Data rates for GSM
Evolution (EDGE)
10
Architecture of GSM
11
Frequency Channels in GSM

• Uplink frequency band 890 - 915 MHz
• Downlink frequency band 935 - 960 MHz
• 124 radio channels (of 200 kHz each) in each
  band. ??
• A radio channel is called ARFCN
• (absolute radio frequency channel number)
• Each channel has a TDMA structure with 8
  timeslots.
• ( gt upto 8 users per freq. channel ) i.e Each
  ARFCN
• carries 8 logical channels

960 Mhz

• A Data multiframe has 26 such frames and a
  control multiframe has 51 frames.

935 Mhz
12
GSM Frames

• 270.833 kbps data rate of the burst thus bit
  period 1/270.83 3.692 microsec
• slot duration 3.69156.25 576.92 microsec
• Frame duration 8576.92 4.615msec
• Control Multiframe duration 514.615 235
  milisec

13
260 bits in 20ms 13kbps
8 blocks of 57 bits
456 bits in 20ms, 22.8k
fragmenting
156.25 bits in 576.92 microsec
270.833 kbps
Downlink
In a burst, first payload block is taken from
current and another from previous 20 ms frame. it
minimizes the effect of time varying channel.
Delay
Uplink
So the MS does not have to Transmit and Receive
at the same time instance!
14
Channel Types

• Traffic (TCH)
• Full rate
• Speech 13kbps
• Data 9.6 kbps
• Data 4.8 kbps
• Half rate
• Speech 6.5 kbps
• Data 4.8 kbps
• Data 2.4 kbps

• O/P rate of channel coder is always 22.8kbps.
  lower data rate indicates more redundant bits
  thus more reliability
• In half rate two users alternately share the same
  logical channel. The frame format is same as of
  full rate.

15

• Control Channels (CCH) (34 ARFCNs are designated
  for control)
• Broadcast Channels (TS0 slot of an ARFCN)
• BCCH (2,3,4,5 frames carry information like n/w
  identity, CH availability, Congestion etc)
• FCCH (0,10,20,40 have special data bursts which
  allows user to detect exact frequency)
• SCH (1,11,22,41 contains base station identity
  number and timing synchronization signal)
• Common control channel (TS0 slots not used by BCH
  are used)
• PCH (notify MS an incoming call or incoming SMS)
• RACH (reverse channel used by MS to initiate a
  call, slotted ALOHA is used)
• AGCH (Ack for success of RACH, SDCCH channel
  number)
• Dedicated control Channel (duplex channels)
• SDCCH (any TCH can be used with stealing bits ON,
  Authentication before TCH allotment, SMS
  delivery)
• SACCH (13th and 26th frame of TCH to exchange
  technical data like power control, handoff
  request etc)
• FACCH (carries urgent message using stealing bits
  of TCH)

16
GPRS

• If GSM is used for Internet traffic
• High cost as the user has to pay for entire
  connection duration.
• Low data rate as at the most one TCH can be
  used.
• GPRS is packet switching technology
• Efficient for data traffic
• Always on access
• Billing is for per packet transmitted/ Received
• User can choose a class of following parameters
  (a) service precedence (b) reliability (c) delay
  (d) throughput

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GPRS Architecture
Other GPRS PLMN
GGSN
Gp
Gn
SGSN
Gb
Gf
BSC
Gr
Gs
Gi
BTS
Gc
GGSN
PDN
EIR
D
BTS
HLR
MS
MSC/VLR
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Important terms

• Attach (initiated by MS)
• Detach (either MS or network can initiate)
• TLLI CKSN
• PDP context
• Tunnel mechanism
• Location management (idle, ready, standby)
• Routing

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Location management and routing
BSC
BTS
BSC
BTS
SGSN
SGSN
MS
Inter-PLMN GPRS Backbone
Gn
Intra-PLMN GPRS Backbone
Intra-PLMN GPRS Backbone
Gp
PLMN1
Border Gateway
Gn
Border Gateway
Gn
GGSN
Packet Data Network(PDN) Eg.Internet,Intranet
SGSN
Gi
GGSN
Host
Router
LAN
20
RLC layer

• An LLC block is fragmented into RLC blocks.
• Each RLC block is encoded to get 456 bit radio
  block.
• Number of RLC depends upon selected coding
  scheme.
• A radio block is transmitted in four consecutive
  slots of either a single ARFCN (4 PDCHs ) or 4
  consecutive slot of a single PDCH.

21
MAC layer

• In uplink users are required to contend for
  reservation using slotted ALOHA. Depending upon
  QoS, successful users get one or more PDCH.
• In downlink, all packets are queued and depending
  upon QoS class these are delivered to MS.
• MS is explicitly instructed to tx/Rx on a
  particular PDCH.
• Besides forward error correction, selective
  reject ARQ is used.

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Network Layer (IP or X.25)
Relay SNDCP GTP
GTP
LLC TCP/UDP
TCP/UDP
Relay RLC BSSP
BSSGP IP
IP
MAC Network
Service
Network Data Link Service
Service
Data Link Layer
PLL RFL
Phy Layer
Phy Layer Phy Layer
Phy layer
Gm
Gb
BSS
SGSN
GGSN
Gi
RLC Radio link control BSSGPBSS
GPRS Application protocol PLL Physical link
layer GTP GPRS tunneling
protocol RFL Physical RF layer
TCP Transmission control protocol MACMedium
access control UDP user datagram
protocol
IP Internet Protocol
Transmission Plane
23
GPRS Air Inteface
Time Slot Number
0
1
2
4
3
5
6
7
0
1
2
3
4
0
1
2
4
3
5
6
7
0
1
2
3
4
F1
Uplink
F2
F3
F4
Carrier Frequency
0
1
2
4
3
5
6
7
0
1
2
3
4
0
1
2
4
3
5
6
7
0
1
2
3
4
F1
Downlink
F2
F3
F4
Voice User1
GPRS User1
GPRS User3
Voice User2
GPRS User2
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EDGE

• Uses GPRS architecture.
• Besides GMSK it allows 8-point PSK.
• 9 different schemes are supported.
• Incremental Redundancy (type-II hybrid ARQ) is
  used MCS9/6/3 or MCS7/5/2.