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Wireless networks - Lecture 20: EDGE. The main topics covered in this chapter include: walsh codes; IS-95 reverse link; EDGE introduction; modulation and coding schemes; link adaptation and incremental redundancy; capacity planning; dynamic abis pool;...
Wireless Networks Lecture 20 EDGE Dr Ghalib A Shah Outlines Last Lecture Review Walsh Codes IS-95 Reverse Link EDGE Introduction Modulation and Coding Schemes Link Adaptation and Incremental Redundancy Capacity Planning Dynamic Abis pool Last Lecture IS-136 CDMA/IS-95 Advantages ► Drwabacks ► Pilot Channel Sync Channel Paging Traffic IS-95 Reverse Channels ► ► Self-jamming, near-far problem, soft handoff IS-95 Forward Channels ► ► ► ► Frequency diversity, multipath resistance, privacy, graceful degradation Access Channels Traffic Next Lecture Forward Link Channel Parameters Channel Sync Paging Traffic rate Set 1 Traffic Rate Set 2 Data rate (bps) 1200 4800 9600 1200 2400 4800 9600 1800 3600 7200 14400 Code repetition 2 8 Modulation symbol rate (sps) 4800 1920 19200 19200 19200 19200 19200 19200 19200 19200 19200 PN Chips / modulation symbol 256 64 64 64 64 64 64 64 64 64 64 PN Chips / bit 1024 256 128 1024 512 256 128 682.67 341.33 170.67 85.33 Walsh Codes 2x2 Walsh Matrix 1 -1 User (1, 1) and user (1, -1) x Walsh matrix 1 1 -1 -1 1 -1 -1 -1 -1 IS-95 Reverse Link Consists of upto 94 logical channels each occupying same bandwidth of 1228 KHz It supports 32 access channels and 62 traffic channels Access channel is used to initiate a call, to respond to paging channel and for location update In reverse, convolutional encoder has a rate of 1/3, thus trippling the effective rate to a max of 28.8 kbps IS-95 CDMA Reverse Channel Uses OQPSK for power efficiency and QPSK demodulation is easy 869-894 MHz range No spreading of the data using orthogonal codes ► Data coming out of the block interleaver are grouped in units of bits that serves as an index to select a row of the 64x64 Walsh matrix and that row is substituted for the input ► Thus data rate is expanded by a factor of 64/6 to 307.2 kbps Enhanced Data rates for GSM Evolution GPRS data rates still fall short compared to that promised by 3G The delay in deployment of 3G technology led to the emergence of EDGE Phase (Release’99 & 2002 deployment) supports best effort packet data at speeds up to about 384 kbps Phase (Release’2000 & 2003 deployment) will add Voice over IP capability GPRS Architecture Similar to GPRS but some changes for higher data rates Important change is modulation scheme EDGE Functionality Other GPRS PLMN GGSN SGSN BSC BTS MS GGSN BTS EIR HLR MSC/VLR GMSK is used in GPRS, only one bit per symbol is used In EDGE, Octogonal PSK (8-PSK) is used which enables a threefold higher data rate of 59.2 kbps per radio time slot ► Achieved by transmitting bits per symbol GMSK has constant amplitude modulation while 8-PSK has variations in amplitude This changes the radio frequency characteristics requiring changes in BS 10 minor changes in hardware and software in existing systems, leads to major changes in network performance Radio network Planning ► Coding Scheme: nine modulation and coding schemes (MCS) that provide different throughput as shown in table 11 12 Payload Format MCS3 Fa mily A 37 octets 37 octets 37 octets 37 octets MCS6 MCS9 MCS3 34+3 octets Fa mily A padding 34+3 octets MCS6 34 octets 34 octets 34 octets 34 octets MCS8 MCS2 Fa mily B 28 octets 28 octets 28 octets 28 octets MCS5 MCS7 MCS1 Fa mily C 22 octets 22 octets MCS4 13 ► Based on this coding, a data rate of x 59.2 = 473kbps can be achieved ► Though GMSK is more robust but 8-PSK gives more throughput ► However increased data rate comes at the price of decreased sensitivity of the system This has impact on coverage and in turn network planning ► Another advantage in EDGE is that switching between different coding schemes takes place easily i.e data block can be sent with better protection on failure ► not possible in GPRS to switch to different coding scheme on reception failure, retransmission uses the same protection as for its initial transmission 14 Link Adaptation and Incremental Redundancy Link Adaptation (LA) ► As propagation condition changes, quality of signal changes MCS changes all the time ► LA is used for maximizing the throughput per channel by changing the coding scheme ► LA algorithms are based on bit error probability (BEP) measurements Incremental redundancy ► Improves the throughput and is done by automatically adapting the transmitted redundancy to the channel conditions ► Achieved through ARQ and FEC 15 Incremental Redundancy (IR) Send redundancy only if necessary Generalized Type-II ARQ ► Finer granularity of code rate Example Data Parity Transmitter 1st attempt Rate 1 2nd attempt Rate 1/2 3rd attempt Rate 1/3 Receiver 16 State Diagram for IR Initial data transmission Data Block ARQ Block in error Transmit parity or data subblock Block in error Error Detection Error Detection No error No error Accept data block Deliver to upper layer 17 Capacity Planning in EDGE Similar to GPRS but high throughput per radio time slot changes some aspects of planning The reuse pattern defines the number of cells in a cluster using different frequencies A frequency reuse of 3/9 means that each f is used only once in three sites/cluster, wherein each site is three sectored Reuse for control and reuse for traffic channels are independent of each other The actual reuse employed - for traffic or control - is operator controlled and limited only by the available spectrum Typically, 4/12 is used for control and 1/3 for traffic However, other combinations are also possible subject to performance requirements, environment and spectrum availability Higher the f reuse, higher the throughput and less delay Time slot capacities have dynamic range depending on users 18 1/3 Frequency Re-use (EDGE Compact) • 3 x 200 kHz carrier, reused in every site •