2005/08/01 暈頭轉向還沒解掉Bug

今天真是忙碌
為了Security的Bug解到現在還是沒有進展
老大已經發信 如果再解不出來
要我去發個code review meeting了
天ㄚ 真是害..
晚上跟同事討論明天要交的作業
算是小有心得 還不錯
只是週四要考試 還有兩份作業
工作又很忙碌 god.....

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Dual Network Homework1
Review Exercises I
1.Describe some differences between th CDMA physical channels used for
forward-direction transmissions and physical channels used in the reverse
direction.
Answer:
1)There are 64 Walsh codes available for the forward link(base station
[BS] to mobile station [MS]),providing 64 physical channels.On the reverse
link, channels are identified by long pseudorandom noise(PN) code
sequences.(textbook p141)
2)Uplink/Downlink frequence differs 25MHz.


2.What is the purpose of the timming offset in the pseudo-noise sequences of
the base station radio modulators?
Answer:
1)For Cross-Correaltion issue,in IS-95 networks, all base stations use GPS.
Each base station is identified by a finite timming offset of its PN
binary sequence in the forward direction.(textbook p121)
2)BSs are separated by different PN offsets,PN offset is based on the same
short code with different timing offset(75 cycles in 2 seconds)(note 2-46)

3.How do CDMA base stations use signals received from the global positioning
system?Why is it important that all base stations operate in synchronism?
Answer:
1)By using the global positioning system(GPS),a mobile station processes
the pilot channel to find the strongest multipath signal components.The
processed pilot signal provides an accurate estimation of time delay,phase
,and magnitude of the multipath components. These components are tracked
in the presence of fast fading,and coherent reception with combining is
used.The chip rate on the pilot channel and on all frequency carriers is
locked to precise system time,from GPS.
2)For CDMA use Orthogonal spread code,all signals must synchroized.
Once the mobile station identifies the strongest pilot offset by
processing the multipath components from the pilot channel correlator,it
examines the signal on its synch channel, which is locked to the
pseudorandom binary sequence signal on th e pilot channel.Since the synch
channel is time aligned with its base station's pilot channel, the mobile
station finds the information pertinent to this particular base station.
(textbook p81)

4.Why does the CDMA system employ closed-loop power control in reverse-
direction transmissions?
Answer:
Fading sources in multipath require a much faster power control than the
open loop power control.The additional power adjustment required to
compensate for fading losses are handled by the rreverse link closed loop
power control mechanism, which has response time of 1.25 ms for 1-dB steps
and a dynamic range of 48 dB(covered in three frames).The quicker response
time gives the closed loop power control mechanism the ability to override
the open loop power control mechanism in parctical applications.
(textbook p246)


5.When terminal T is in a soft handoff condition communicating simultaneously
with base station A and base station B, describe the nature of the four
physical channels in use(T to A, T to B,A to T, and B to T).Describe how
these physical channels are selected and how the network elements learn
their identity?
Answer:
1)T to A : use access channel.
T to B : use access channel.
A to T : use pilot , sync, paging channels.
B to T : use pilot , sync, paging channels.
2)use different Walsh code to learn their identity.



6.How does variable-bit-rate speech transmission help the performance of the
CDMA system?
Answer:
Variable-bit-rate coding can provide a given level speech quality at an
average bit rate that is substantially less than the bit rage that would
be required by an equivalent quality fixed-rate codec.(textbook p87)


7.The highest bit rate of the QCELP coder is 9600b/s.In what situations does
the system transmit speech at lower bit trates?
Answer:
While silence and background noise are coded at the lower rates.
(textbook p96)

8.Why is it useful for a CDMA terminal to acquire"neighbor list" informations?
How does a terminal use the information acquired in NEIGHBOR LIST and
NEIGHBOR LIST UPDATE messages?
Answer:
1)When is soft-handoff, the informations of "neighbor list" can shoter the
next setup time.
2)The NLUM is sent by the base station.It contains the latest compositve
neighbor list for the pilots in active set.The neighbor set is initialized
to contain the pilots specified in the most recently received neighbor
list message.This happens when the mobile is firste assigned a forward
traffic channel.The mobile maintains an age counter,AGE,for each pilot in
the neighbor set.If a pilot moves from the active set or candidate set to
the neighbor set, its counteris initialized to zero.However, if a pilot
moves from the remaining set to neighbor set, its counter is set to the
maximum age value.(textbook p231,239)

9.What is an advantage of sending an ABBREVIATED ALERT message on a paging
channel ralteive to sending an ALERT message on a fowrard traffic channel?
Answer:
Sending an ABBREVIATED ALERT message on a paging channel will ignore
the additonal processing of control message and data message.Because
paging channel just output control message.
But traffic channel output content data message,sending ALERT message on
forward traffic channel cause additional processing of control and data
message.

10.What is an advantage of open-loop power control over closed-loop power
control? Why is open-loop power control unalbe to maintain a narrow range
of received power levels at a base sation?
Answer:
In the open loop power control, the ase station is not involved.
The moible determines the initial power transmitted on the access channel
and traffic channel through open loop power control.A large dynamic range
of 80 dB is allowed to provide an ability to guard againset deep fads.
The reverse open loop power control(ROPC)provides a rough estimation of
transmit power base on the total received power on the forward link.
The response of ROPC is rapid,because no ase station transceiver(BTS)
timing alignment is required.The ROPC occurs every 20 ms.