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CDMA Technology OverviewCDMA Technology Overview
February, 2001 - Page 2-*
CDMA Forward Traffic Channels
Used for the transmission of user and signaling information to a specific mobile station during a call
Maximum number of traffic channels: 64 minus one Pilot channel, one Sync channel, and 1 to 7 Paging channels
This leaves each CDMA frequency with at least 55 traffic channels
Unused paging channels can provide up to 6 additional channels
Realistic loading will typically be about 17 subscribers when using the 13 kb vocoder (22 when using the 8 kb vocoder)
Forward Traffic Channel
PILOT: WALSH CODE 0
The Pilot is a “structural beacon” which does not contain a character stream. It is a timing source used in system acquisition and as a measurement device during handoffs
SYNC: WALSH CODE 32
This carries a data stream of system identification and parameter information used by mobiles during system acquisition
PAGING: WALSH CODES 1 up to 7
There can be from one to seven paging channels as determined by capacity needs. They carry pages, system parameters information, and call setup orders
TRAFFIC: any remaining WALSH codes
The traffic channels are assigned to individual users to carry call traffic. All remaining Walsh codes are available, subject to overall capacity limited by noise
Pilot Walsh 0
its CDMA RF carrier Frequency
the unique Short Code PN Offset of the sector
the unique Walsh Code of the user
CDMA
Frequency
CDMA uses a superior Variable Rate Vocoder
full rate during speech
increased capacity
more natural sound
Voice, signaling, and user secondary data may be mixed in CDMA frames
Codebook
Pitch
Filter
Formant
Filter
bits
Voice
Signaling
Secondary
February, 2001 - Page 2-*
Converting Bits into Symbols
The bits in a 20 ms traffic frame may include one or more of the following
voice information (from the vocoder)
signaling information
secondary traffic information
When Forward Error Correction algorithms are applied to these information bits, the resulting 0s and 1s are called symbols
bits and symbols are related in a complex many-to-many fashion
the information in one bit is distributed among many symbols, and one symbol carries some of the information of many bits
all forward traffic frames contain 384 symbols
all reverse traffic frames contain 576 symbols
Bits
Symbols
there are 64 such patterns called “Walsh codes”
in the forward link, just one of these patterns is assigned to each user’s stream of symbols (code channel)
each ‘0’ symbol is replaced by the selected pattern (Walsh code)
each ‘1’ symbol is replaced by the logical negation of the selected pattern
in the reverse link, all the 64 patterns (but not their logical negations) are used in every code channel
each group of six symbols is interpreted as a binary value in the 0-63 range and replaced by the corresponding Walsh code
Symbols
Chips
Reverting the Process
To revert the process, first the symbols are recovered as follows
In the forward direction, the mobile station correlates the received signal with the selected Walsh code pattern (integrating the power over 64 chips)
In the reverse direction, the BTS matches the received signal with each possible Walsh code and selects the pattern that produces the highest degree of correlation
When all the symbols for a 20 millisecond frame have been recovered, the Viterbi decoder is used to guess the block of bits (frame) that most probably corresponds to this block of symbols
Then the CRC of this frame is calculated to determine if the guess was successful; if not, the frame is discarded
Symbols
Chips
Despreading
(integraton)
Bits
Viterbi
Decoder
Output is 20 ms frames at fixed rates:
Full Rate, 1/2 Rate , 1/4 Rate , 1/8 Rate, & Blank
CRC is added to all the frames for the 13 kb vocoder, but only to the Full and 1/2 rate frames for the 8 kb vocoder
CRC is not added to the lower rate frames in the 8 kb vocoder but that is ok because they consist mostly of background noise and have a higher processing gain
Current vocoder rates are 13 kb, 8 kb, and 8 k EVRC (Enhanced Variable Rate Coder)
To the
Speech coding algorithms (digital compression) are necessary to increase cellular system capacity
Coding must also ensure reasonable fidelity, that is, a minimum level of quality as perceived by the user
Coding can be performed in a variety of ways (for example, waveform, time or frequency domain)
Vocoders transmit parameters which control “reproduction” of voice instead of the explicit, point-by-point waveform description
A-to-D
C
O
N
V
E
R
T
E
R
Transmission
Rate
Total
Reserved
Information
CRC
Rate
Set
Is a means of error detection/correction
Results in 2 code symbols (or more, depending on the “R” constant) output for each bit input
Symbol repetition maintains a constant 19.2 Ksps output to be fed into the block interleaver
Also allows for reduction in transmit power
Reduces overall noise and increases capacity
Convolutional
Encoding
+
+
+
+
10110
0
0
0
1
0
11
01
00
011
111
000
1101
1110
1000
11010
11100
Rate 1/2, k=9 Convolutional Encoding
Symbols generated as the information bits transit through the encoder, are related to all the bits currently in the register
Each information bit contributes to multiple generated symbols
This pattern of inter-relationships helps detect and correct errors
The length of shift register plus 1 is called the “constraint length” of the convolutional encoder (K=9 in this case)
The longer the register, the better this scheme can correct bursty errors
Reduces power required to achieve same accuracy as without coding
Here, two symbols are generated for every bit input (Rate 1/2)
Code Symbol
Symbol repetition provides a constant rate to the block interleaver
Lower rates symbols are sent at reduced power levels
The energy per bit across all rates is identical when integrated
Overall signal power requirement (thus noise) is reduced
E =E /2
Rate Set 2 (13 kbps Vocoder)
Symbol repetition maintains a constant 28.8 ksps output to puncturing section
Symbol puncturing deletes 2 of every 6 inputs based on a six-bit pattern
Unrepeated symbols for 28.8 ksps frames are also deleted
Convolutional decoder in mobile station will correct these purposeful errors
Puncturing provides a constant 19.2 Ksps input to interleaver just like in rate set 1
This allows all other functions to remain exactly the same
PCM Voice
Combats the effects of fast fading
Separates repeated symbols at 4800 bps and below
Improves survival rate of symbol data
Spreads the effect of bursty interference
19.2 ksps
From Coding
9600 bps Block Interleaver (Input Array)
The 384 modulation symbols in a frame are input into a 24 by 16 block interleaver array (read down by columns, from left to right)

9600 bps Block Interleaver (Output Array)
This 24 by 16 array (read down by columns, from left to right) indicates the order in which the symbols are output from the block interleaver
The effect of bursty errors during transmission is minimized (the 2k contiguous symbols containing the information to restore one data bit have been separated)
Assume that a burst of noise damages all these bits
CDMA Technology Overview
9600 bps De-Interleaving
Notice how the effect of the burst of noise is spread over the transmitted block
Putting it All Together: CDMA Code Channels
The three spreading codes are used in different ways to create the forward and reverse links
A forward channel exists by having a specific Walsh Code assigned to the user, and a specific PN offset for the sector
A reverse channel exists because the mobile uses a specific offset of the Long PN sequence
WALSH CODE: Individual User
SHORT PN OFFSET: Sector
Transmitted constantly by the base station
The same PN sequences are shared by all base stations
Each base station is differentiated by a phase offset
Provides tracking of
within one CDMA channel frequency
Acquisition by mobile stations is enhanced by
Short duration of Pilot PN sequence
Uncoded nature of pilot signal
Facilitates mobile station-directed handoffs
Key factor in performing soft handoffs
Pilot Channel Generation
The Walsh function zero spreading sequence is applied to the Pilot
The use of short PN sequence offsets allows for up to 512 distinct Pilots per CDMA channel
The PN offset index value (0-511 inclusive) for a given pilot PN sequence is multiplied by 64 to determine the actual offset
Example: 15 (offset index) x 64 = 960 PN chips
Result: The start of the pilot PN sequence will be delayed
960 chips x 813.8 nanoseconds per chip = 781.25 µs
The quadrature spreading and baseband filtering (not shown), which are performed as with all the other forward and reverse code channels, will be discussed later
Gain
Control
Baseband
Filter
Baseband
Filter
CDMA “Short” and “Long” PN Codes
CDMA uses three PN code sequences: two “short” and one “long”
The two short PN codes (called “I” and “Q”) are used for quadrature spreading to differentiate between CDMA partitions (sectors/cells) in the forward direction
The two short codes are generated by 15-bit PN code generators. The generated strings are 215 -1 bits long plus one zero inserted following the longest string of generated zeroes (32,768); and their cycle period is 26.666... milliseconds (or 75 times every 2 seconds).
The long PN code is used for spreading and data scrambling/randomization, and to differentiate among mobile stations in the reverse direction.
The long code is generated by a 42-bit PN code generator. The generated string is 242 -1 with no zero inserted (about 4.4 trillion) bits long; and its cycle period is approximately 41 days, 10 hours, 12 minutes and 19.4 seconds.
The three CDMA PN codes are synchronized to the beginning of system time (January 6, 1980 at 00:00:00 hours)
Pilot Channel Acquisition
The mobile station starts generating the I and Q PN short sequences by itself and correlating them with the received composite signal at every possible offset
In less that 15 seconds (typically 2 to 4 seconds) all possibilities (32,768) are checked
The mobile station remembers the offsets for which it gets the best correlation (where the Ec/I0 is the best)
The mobile station locks on the best pilot (at the offset that results in the best Ec/I0), and identifies the pattern defining the start of the short sequences (a ‘1’ that follows fifteen consecutive ‘0’s)
Now the mobile station is ready to start de-correlating with Walsh code 32 to extract the Sync Channel (next section)
00...01
00...01
00...01
00...01
00...01
00...01
00...01
Used during system acquisition stage
The bit rate is 1200 bps
The Sync channel has a frame duration of 26 2/3 ms
this frame duration matches the period of repetition of the PN Short Sequences
this simplifies the acquisition of the Sync Channel once the Pilot Channel has been acquired
The Mobile Station re-synchronizes at the end of every call
The Pilot channel carries no data, therefore it has no frames.
The Sync channel uses 26 2/3 ms frames.
All other forward and reverse code channels use 20 ms frames.
(Acquired Pilot)
Sync Channel
no need for length info
Each frame includes one or more overhead bits in addition to the “payload” of information bits
these overhead bits define the structure of the frame
Logical unit of information
must include length info
A message is broken into small pieces that can fit in the payload portion of successive frames
one frame overhead bit could be used to identify the initial segment of a message
FRAME
MESSAGE
Sync Channel Generation
There are 32 bits (1200 bps x 0.02666... second) in one Sync Channel frame
The Rate 1/2 convolutional encoder doubles the bit rate, and the resulting 0s and 1s are now called “code symbols”
there are 64 code symbols in a Sync Channel frame
The repetition process doubles the rate again, and each repetition of a code symbol is now called a “modulation symbol”
there are 128 modulation symbols in a Sync Channel frame
Four copies of Walsh code #32 are used to spread each modulation symbol, resulting in a x256 rate increase; the resulting 0s and 1s are now called “chips”
there are 32,768 chips in a Sync Channel frame (1024 chips per original bit)
Gain
Control
Baseband
Filter
Baseband
Filter
1 3 2 4 1 3 2 4
33 35 34 36 33 35 34 36
17 19 18 20 17 19 18 20
49 51 50 52 49 51 50 52
9 11 10 12 9 11 10 12
41 43 42 44 41 43 42 44
25 27 26 28 25 27 26 28
57 59 58 60 57 59 58 60
5 7 6 8 5 7 6 8
37 39 38 40 37 39 38 40
21 23 22 24 21 23 22 24
53 55 54 56 53 55 54 56
13 15 14 16 13 15 14 16
45 47 46 48 45 47 46 48
29 31 30 32 29 31 30 32
61 63 62 64 61 63 62 64
Let’s assume that a burst of noise affects these symbols
Sync Channel Message (8 x MSG_LENGTH)
MSG_TYPE (‘00000001’)
Message Type (MSG_TYPE) – Identifies this message and determines its structure (set to the fixed value of ‘00000001’)
Protocol Revision Level (P_REV) – Shall be set to ‘00000001’
Minimum Protocol Revision Level (MIN_P_REV) – 8-bit unsigned integer identifying the minimum protocol revision level required to operate on this system. Only personal stations that support revision numbers greater than or equal to this field can access the system
System ID (SID) – 16-bit unsigned integer identifying the system
Network ID (NID) – 16-bit unsigned integer identifying the network within the system (defined by the owner of the SID)
Pilot PN Sequence Offset Index (PILOT_PN) – Set to the pilot PN offset for the base station (in units of 64 chips), assigned by the network planner
Long Code State (LC_STATE) – Provides the mobile station with the base station long code state at the time given by the SYS_TIME field, generated dynamically
System Time (SYS_TIME) – GPS system-wide time as 320 ms after the end of the last superframe containing any part of this message, minus the pilot PN offset, in units of 80 ms, generated dynamically
Sync Channel Message Parameters (cont)
Leap Seconds (LP_SEC) – Number of leap seconds that have occurred since the start of system time (January 6, 1980 at 00:00:00 hours) as given in the SYS_TIME field, generated dynamically
Local Time Offset (LTM_OFF) – Two’s complement offset of local time from system time in units of 30 minutes, generated dynamically
Current local = SYS_TIME – LP_SEC + LTM_OFF
Daylight savings time indicator (DAYLT) – Determined by the network planner
1 if daylight savings in effect in this base station
0 otherwise
Paging Channel Data Rate (PRAT) – The data rate of the paging channel for this system, determined by the network planner
00 if 9600 bps
01 if 4800 bps
CDMA Frequency Assignment (CDMA_FREQ) – The CDMA channel number, in the specified CDMA band class, corresponding to the frequency assignment for the CDMA Channel containing a Primary Paging Channel, determined by the network planner
Paging Channels
Up to seven paging channels can be supported on a single CDMA frequency assignment
Channel 1 (Walsh function 1) is the Primary Paging Channel
Additional Paging Channels use Walsh functions 2 through 7
Unused paging channels can be used as Forward Traffic Channels
Two rates are supported: 9600 and 4800 bps (PRAT parameter in the Sync Channel Message)
A single 9600 bps Paging Channel can support about 180 pages per second
Used by the base station to
transmit system overhead information
and mobile station-specific messages.
Paging Channel Generation
There are 192 [96] bits (9600 [4800] bps x 0.020 second) in one Paging Channel frame
The Rate 1/2 convolutional encoder doubles the bit rate, resulting” 384 [192] code symbols in a Paging Channel frame
If the 4800 bps rate is used, the repetition process doubles the rate again, so that, at either rate, 384 modulation symbols per Paging Channel frame result
384 modulation symbols per frame times 50 frames per second = 19.2 Ksps
One copy of Walsh code #1 (or #2, ... or #7) is used to spread each modulation symbol. This results in a x64 rate increase to 1.2288 Mcps
that is, 24,576 chips per Paging Channel frame, or 128 [256] chips per original bit at 9600 [4800] bps
Gain
Control
Baseband
Filter
Baseband
Filter
(1) First new capsule in slot, Synchronized Capsule
(2) Unsynchronized Capsules
(3) Synchronized Capsules
8 x MSG_LENGTH
2048 slots
(1)
(2)
(3)
Slot Channel 0
Slot Channel “n”
Slot Channel 2047
Lesson Review
What reference channel is used for acquisition, timing, and as a phase reference for coherent demodulation?
Pilot channel
True
The frame duration of what channel matches the period of repetition for of the short PN sequences?
Sync channel
The sync channel is identified by what Walsh code function?
32
The pilot channel is identified by what Walsh code function?
0
True
To transmit system overhead information and mobile station specific messages.
What Walsh functions are reserved for the paging channels?
1 through 7
Unused paging channels can be used as what type of channel?
Traffic channels
The effect of bursty errors are minimized by what function?
Block interleaving and de-interleaving
·
·
Group 3 Facsimile (9600 bps) at Rate Set 1 and (14400 bps) at Rate Set 2
Traffic
Processing
Data
Service
Option
IWF
BTS
elements that can be independently directed
·
components
One must be a “searcher” that scans and estimates signal
strength at each pilot PN sequence offset
Correlator 1
Correlator 2
Correlator 3