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Discriminating Among Forward Code Channels

Pilot. Sync. FW Traffic (for user #2). FW Traffic (for user #1). Paging. FW Traffic (for user #3). Discriminating Among Forward Code Channels. W0. I PN. 1.2288 Mcps. All 0's. Q PN. Forward IS-95B Channel Structure. Pilot Channel (Walsh Code 0).

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Discriminating Among Forward Code Channels

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  1. Pilot Sync FW Traffic (for user #2) FW Traffic (for user #1) Paging FW Traffic (for user #3) Discriminating Among ForwardCode Channels

  2. W0 I PN 1.2288 Mcps All 0's Q PN • Forward IS-95B Channel Structure • Pilot Channel (Walsh Code 0) - The Pilot is “structural beacon” which does not contain a character stream - Allows Mobile to Acquire the System - Reference Signal for System Acquiring, Timing, Coherent Modulation - Provides Mobile with Signal Strength Comparison during handoffs - Transmitted Constantly - Non-Modulated Spread Spectrum Signal (Transmit Short PN Code) - Has Unique PN Offset(512) for each Cell or Sector - Approximately 20% of radiated BTS power is in the pilot

  3. I PN W32 1.2288 Mcps Convolutional Encoder and Repetition 19.2 kbps Block Interleaver 1200 bps Q PN • Forward IS-95B Channel Structure • Sync Channel (Walsh Code 32) - Used by Mobile to Synchronize with System - Carries a data stream of system identification and Parameter information used by MS during system acquisition - Pilot PN Offset - System Time - Long PN Code - System ID - Network ID - Paging Channel Data Rate - Tx at 1200 bps

  4. I PN W1 R = 1/2 1.2288 Mcps 19.2 ksps Convolutional Encoder and Repetition Block Interleaver 9600 bps 4800 bps 19.2 ksps 1.2288 Mcps Long PN Code Generator Paging Channel Address Mask Decimator Q PN • Forward IS-95B Channel Structure • Paging Channel (Walsh Code 1 up to 7) - Used by Base Station to : - Page Mobile - Transmit Overhead Information - MS Control - Assign Mobile to Traffic Channel - Provides Mobile with: - System parameter Message - Neighbor List Message - Access Parameter Message - CDMA Channel List Message - Tx at 9600 or 4800 bps

  5. Forward IS-95B Channel Structure • Traffic Channel (any remaining Walsh codes) • Used to: • - Pass voice, commands, and requests from the Base Station to the Mobile - Tx up to 9600bps on Rate set 1 and up to 14400bps on Rate set 2

  6. Reverse IS-95B Channel Structure • Access Channel - Used by Mobiles not yet in a call to transmit : • - Registration Requests - Call Setup Requests - Page Responses - Order Responses - other Signaling information - Be Paired to Paging Channel (Each Paging Channel can have up 32 access channels) - Tx at 4800 bps, 20ms frame length I PN R = 1/3 1.2288 Mcps 28.8 kbps 28.8 kbps Convolutional Encoder and Repetition 307.2 Kbps Block Interleaver Walsh Cover 4800 bps 1.2288 Mcps Long PN Code Generator Access Channel Address Mask Q PN

  7. Reverse IS-95B Channel Structure • Traffic Channel - Be used by individual users during their actual calls to transmit traffic to the BTS - Be really just a user-specific public or private Long Code Mask - there are many reverse Traffic channels as there are CDMA phones in the world

  8. Long Code Register (@ 1.2288 MCPS) AND Public Long Code Mask (STATIC) 1 1 0 0 0 1 1 0 0 0 P E R M U T E D E S N = User Long Code Sequence (@1.2288 MCPS) S U M Modulo-2 Addition The Long PN Sequence • Each mobile station uses a unique User Long Code Sequence generated by applying a mask, based on its 32-bit ESN and 10 bits from the ysytem, to the 42-bit Long Code Generator which was synchronized with the CDMA system during the mobile station initialization. • Generated at 1.2288 Mcps, this sequence requires 41 days, 10 hours, 12 minutes and 19.4 seconds to complete. • Portions of the User Long Codes generated by different mobile stations for the duration of a call are not exactly orthogonal but are sufficiently different to permit reliable decoding on the reverse link.

  9. IS-95 CDMA Channels

  10. Pilot Sync FW Traffic (for user #2) FW Traffic (for user #1) Paging FW Traffic (for user #3) Discriminating Among ForwardCode Channels

  11. W0 I PN 1.2288 Mcps All 0's Q PN • Forward IS-95B Channel Structure • Pilot Channel (Walsh Code 0) - The Pilot is “structural beacon” which does not contain a character stream - Allows Mobile to Acquire the System - Reference Signal for System Acquiring, Timing, Coherent Modulation - Provides Mobile with Signal Strength Comparison during handoffs - Transmitted Constantly - Non-Modulated Spread Spectrum Signal (Transmit Short PN Code) - Has Unique PN Offset(512) for each Cell or Sector - Approximately 20% of radiated BTS power is in the pilot

  12. I PN W32 1.2288 Mcps Convolutional Encoder and Repetition 19.2 kbps Block Interleaver 1200 bps Q PN • Forward IS-95B Channel Structure • Sync Channel (Walsh Code 32) - Used by Mobile to Synchronize with System - Carries a data stream of system identification and Parameter information used by MS during system acquisition - Pilot PN Offset - System Time - Long PN Code - System ID - Network ID - Paging Channel Data Rate - Tx at 1200 bps

  13. I PN W1 R = 1/2 1.2288 Mcps 19.2 ksps Convolutional Encoder and Repetition Block Interleaver 9600 bps 4800 bps 19.2 ksps 1.2288 Mcps Long PN Code Generator Paging Channel Address Mask Decimator Q PN • Forward IS-95B Channel Structure • Paging Channel (Walsh Code 1 up to 7) - Used by Base Station to : - Page Mobile - Transmit Overhead Information - MS Control - Assign Mobile to Traffic Channel - Provides Mobile with: - System parameter Message - Neighbor List Message - Access Parameter Message - CDMA Channel List Message - Tx at 9600 or 4800 bps

  14. Forward IS-95B Channel Structure • Traffic Channel (any remaining Walsh codes) • Used to: • - Pass voice, commands, and requests from the Base Station to the Mobile - Tx up to 9600bps on Rate set 1 and up to 14400bps on Rate set 2

  15. Reverse IS-95B Channel Structure • Access Channel - Used by Mobiles not yet in a call to transmit : • - Registration Requests - Call Setup Requests - Page Responses - Order Responses - other Signaling information - Be Paired to Paging Channel (Each Paging Channel can have up 32 access channels) - Tx at 4800 bps, 20ms frame length I PN R = 1/3 1.2288 Mcps 28.8 kbps 28.8 kbps Convolutional Encoder and Repetition 307.2 Kbps Block Interleaver Walsh Cover 4800 bps 1.2288 Mcps Long PN Code Generator Access Channel Address Mask Q PN

  16. Reverse IS-95B Channel Structure • Traffic Channel - Be used by individual users during their actual calls to transmit traffic to the BTS - Be really just a user-specific public or private Long Code Mask - there are many reverse Traffic channels as there are CDMA phones in the world

  17. Pilot sets • The term pilot refers to a pilot channel identified by a pilot sequence offset. • A pilot is associated with the forward traffic channels in the same forward CDMA link. • Each pilot is assigned a different offset of the same short PN code. • In a particular position of MS, it may detect many pilot carriers from various cells. • Depending upon the received strength of these pilots are categorized as • Active set • Candidate set • Neighbor set • Remaining set

  18. Pilot Sets • Active set: It contains those pilots whose paging or traffic channels are actually being monitored or used. • If MS is in idle condition, it can have only one pilot set. The active pilot will be the one whose Ec/Io is highest among the candidate set. • If the MS is using traffic channel (conversation), then it can have up to six pilots. • Simply, the active set contains the currently serving pilots of Cells. • Candidate set: This set contains the pilots that are not currently in the active set. • However, these pilots have been received with sufficient signal strength to indicate that the associated forward traffic channels could be successfully demodulated. • Maximum size of the candidate set is six pilots. • The candidate pilot can become an active pilot any time.

  19. Pilot Sets Contd…. • Neighbor set: • This set consists of pilots that are not currently in the active or the candidate set but they are likely candidates for Handoff being in close vicinity. • The neighbor list is sent to the mobile in the system parameter message on the paging channel. • The maximum size of the neighbor set is 20. • The neighbor list should be updated according to the geographical locations of cells and the corresponding H/O between these cells. • Remaining set: • This set contains all possible pilots in the current system, excluding pilots in the active, candidate, or neighbor sets.

  20. Search Windows • Search window is centered on the earliest arriving usable signal (direct path). • We have three search windows • SRCH_WIN_A: size for active and candidate set • SRCH_WIN_N: size for neighboring sets. • SRCH_WIN_R: size for remaining sets.

  21. Search Windows • While searching for a pilot, the mobile is not limited to the exact offset of the short PN code. • The short PN offsets associated with various multipath components arrive a few chips later relative to the direct path component. • The mobile uses the search windows to accommodate such multipath components and add them constructively. • Search window sizes are defined in number of short PN chips. (1 chip = 244.14 meters) • Search window defines size of sets to include pilot on the basis of distance of BTS or shifted version of a signal due to multi-path effect.

  22. SRCH_WIN_A • Mobiles uses to track the active and candidate set pilots. • It defines Handoff region. • It should be large enough to capture all usable multi-path signal components of a base station at the same time it should be as small as possible in order to maximize searcher performance.

  23. Path A = 1 Km 4.1 chips Path B = 4 Km 16.4 chips Distance traveled between two path is (16.4 – 4.1 = 12.3 chips) Search window size 12.3 χ 2 = 24.6 chips SRCH_WIN_A Contd…

  24. SRCH_WIN_A Contd… • The above fig. shows multipath situation. • The direct path (path A) travels 1 km to the mobile, while the multipath (path B) effectively travels 4 km before reaching the mobile. • Since one chip corresponds to a propagation distance of 244.14m, the direct path travels a distance of 4.1 chips. • And the multipath travels a distance of 16.4 chips. • Therefore, the difference in distance traveled between the two paths is 16.4chips – 4.1 chips = 12.3 chips • Note that the direct path (path A) arrives the earliest and is thus at the center of the search window, while the multipath (path B) arrives 12.3 chips later. • In order for the search window to simultaneously capture these two paths, the window must be at least (2 X 12.3) chips, or 24.6 chips wide. • In general, an RF engineer must set SRCH_WIN_A according to his or her knowledge of multipath conditions within the cell.

  25. SRCH_WIN_N • This is the search window that the mobile uses to monitor the neighbor set pilots. • The size of this window is typically larger than that of SRCH_WIN_A. • The window needs to be large enough not only to capture all usable multipath of the serving base stations signal, but also to capture the potential multipath of neighbor’s signals. • The maximum size of this search window is limited by the distance between two neighboring base stations ie maximum size of SRCH_WIN_N is given by distance between two neighbor cell in number of chips.

  26. SRCH_WIN_R • SRCH_WIN_R monitors all the remaining set of PNs. • A typical requirement for the size of this window is that it is at least as large as SRCH_WIN_N.

  27. Pilot Search • Different pilot signals can arrive at the mobile at different times, and a multipath component of one pilot may arrive a few chips later than its direct-path component. • Therefore, search windows are provided to search for pilots that are in the active, candidate, neighbor, and remaining windows. • The parameter SRCH_WIN_A defines the search-window width used to search for pilots in the active and candidate sets. • The parameter SRCH_WIN_N defines the search-window width used to search for pilots in the neighbor sets. • The parameter SRCH_WIN_R defines the search-window width used to search for pilots in the remaining sets. • The mobile should center the search window for each pilot in the active and candidate sets around the earliest arriving usable multipath component of the pilot. • For example, if SRCH_WIN_A is defined to be 40 chips, then the mobile searches 20 chips around the earliest arriving multipath component of the pilot.

  28. Handoff • Handoff is a process in which a mobile station changes its serving BTS or moves to a new traffic channel. • In cellular communication, H/O is must to continue a call in progress even though the subscriber moves from the coverage area of one cell to another and so on. • H/o should occur as fast as possible and this operation must be successful. • Handoff has two scheme • Hard handoff (break before make scheme) • Soft handoff (make before break scheme) • Inter-sector or softer Handoff • Inter-cell or soft Handoff • Soft softer Handoff

  29. Soft Handoff Inter-sector or softer Inter-cell or Soft Soft-softer

  30. Soft handoff Contd…. • Soft handoff is also known as “make before break scheme” handoff. • The various types of soft handoff are as follows. • Softer handoff: • If H/O is occurring two sectors of the same BTS (Cell), then such type of H/O is called softer H/O. • Frequently occurred in macro cell (BTS having multiple sectors). • Soft H/O: • Soft H/O process occurring between different sectors of different cell (BTS). • Soft/softer H/O: • The mobile communicates with two sectors of one cell and one sector of another cell ie combination of soft and softer The mobile communicates with two sectors of one cell and one sector of another cell .

  31. Benefits of Soft Handoff • Less call drop because mobile set continuously monitor multiple pilots. Hence the quality of service is increased. • Soft handoff reduces transmission power of forward and reverse traffic channels. • Less transmission power from mobile results in longer battery life and reduces the overall interference hence the capacity also increased.

  32. Handoff parameters • Pilot detection threshold (T_ADD) • T_ADD defines a threshold value above which the pilot can be considered as active or candidate set. • T_ADD must be large enough to quickly add useful pilots and high enough to avoid false alarm due to noise. • Comparison threshold (T_COMP) • T_COMP also has effect similar to T_ADD. • Pilot drop threshold (T_DROP) • T_DROP defines a boundary below which the pilot signal is considered weak. • Drop timer threshold (T_TDROP) • As soon as pilot falls below T_DROP, T_TDROP counter starts and after finished counting the active or candidate pilot is moved to neighbor set.

  33. The following example shows a soft hand off process of a MS between two cells A and B.

  34. Hand off Contd… • Served by ‘A’ only. • Active set contains only pilot A. • The mobile measures pilot B Ec / I0 and finds it to be greater than T_ADD. • The mobile sends a pilot strength measurement message and moves pilot B from the neighbor set to the candidate set. • The mobile receives a handoff direction message from cell A. • The message directs the mobile to start communicating on a new traffic channel with cell B • The message contains the PN offset of cell B and the Walsh code of the newly assigned traffic channel.

  35. Hand off Contd… 3. The mobile moves pilot B from the candidate set to the active set. Now the active set contains two pilots. 4. The mobile detects that pilot A has now dropped below T_DROP. The mobile starts the drop timer. 5. The drop timer reaches T_TDROP. The mobile sends a pilot strength measurement message. 6. The mobile receives a handoff direction message. The message contains only the PN offset of cell B. The PN offset of cell A is not included in the message. 7. The mobile moves pilot A from the active set to the neighbor set, and it sends a handoff completion message.

  36. CDMA Power Control • Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users. • The power of each individual user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.

  37. CDMA Power Control • CDMA is an interference-limited system based on the number of users, the interference comes mainly from nearby users • Each user is a noise source on the shared channel, this creates a practical limit to how many users a system will handle

  38. CDMA Power Control • The above fig shows the near far effect due to absence of power control. • If there is no power control, both users would transmit a fixed amount of power pt. • Because of the difference in distance, the received power from user 2, or pr2, would be much larger than the received power from user 1, or pr1.

  39. CDMA Power Control • User 2 has a much higher SNR and thus enjoys great voice quality, but user 1’s SNR is barely making the required SNR. • This inequity is known as the classic near-far problem in a spread-spectrum multiple access system. • Moreover, absence of power control also reduces the capacity of the system as shown in the following example.

  40. Power control contd…. • Let us suppose that the minimum SNR required to establish reliable communication link be 1/10. • In the above example, the respective distances of users 1 and 2 are such that the received power from user 2 is 10 times greater than that from user 1. • ie,received power from user 1 = 1 unit. • received power from user 2 = 10 unit. • Thus, the SNR of user 2 is (10), which is very higher than the required SNR (1/10). Hence it enjoys greater voice quality. • Where as, the SNR of user 1 is (1/10), which is the minimum required value. Hence user 1 is barely making a communication link. • Such inequity effect is called classical near far problem. • Moreover, the capacity is also limited to only 2 users with this scenario. Because any user that tries to come into the system either does not get enough SNR (1/10) to establish reliable link or it has to kick out the existing users. • In any case the capacity of the system is limited.

  41. Power Control contd…. • The goal is to keep each MS at the absolute minimum power level necessary to ensure acceptable service quality • MS with excessive transmit power increase interference to other Mobile stations. • Ideally the power received at the base station from each mobile station should be the same(minimum signal to interference)

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