yatambue mahesabu yanayotumika mwisho wa habari hii kwa wale wataalamu wa Vectors wanajua
Verizon wameanza kuuza iPhone 4 Marekani. ijue network ya Verizon utofauti wake na AT&T
Kampuni ya Mawasiliano ya simu za Mkononi nchini Marekani Verizon imeanza leo vita kwa kuingia na Mkataba na Apple Corp. kuuza simu za mkononi aina ya Apple iPhone. Iphone zilikuwa zinauzwa nchini Marekani kwa kupitia Kampuni la mawasiliano la AT&T pekee ambapo Apple Corp Company iliingia nalo mkataba mwaka2007 wakati ulimwengu ulivyoshuhudia madadiliko ya kiteknolojia ya simu za mkononi ambapo kwa mara ya kwanza Apple Corp company iliudhihirishia ulimwengu kuwa simu ya mkononi yenye ukubwa wa display 3.5" inawezekana ikiwa na uwezo wa kutunza battery PAMOJA na UBORA WA HALI YA JUU. iPHONE INAAMINIKA KUWA SIMU YA GHARAMA NA BORA KULIKO ZOTE DUNIANI. iPhone chini ya AT&T inatumia GMS za simu kawaida lakini mtandao wa simu wa Verizon ukiwa unatumia CDMA umeweza kuishawishi Apple Company kuingia nayo mkataba pia na kuruhusu wateja wake kutumia iPhone. Leo ni siku ya kwanza kwa Verizoni kuuza simu hii na imeshafanikiwa kupokea milioni za order toka kwa wateja wake. ijue CDMA network
CDMA2000, also known as 1x and 1xRTT, is the core CDMA2000 wireless air interface standard. The designation "1x", meaning 1 times Radio Transmission Technology, indicates the same RF bandwidth as IS-95: a duplex pair of 1.25 MHz radio channels. 1xRTT almost doubles the capacity of IS-95 by adding 64 more traffic channels to the forward link, orthogonal to (in quadraturewith) the original set of 64. The 1X standard supports packet data speeds of up to 153 kbps with real world data transmission averaging 60–100 kbps in most commercial applications.
IMT-2000 also made changes to the data link layer for the greater use of data services, including medium and link access control protocols and QoS. The IS-95 data link layer only provided "best effort delivery" for data and circuit switched channel for voice (i.e., a voice frame once every 20 ms).
One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called Multiple Access. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrumsignaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.
Kampuni ya Mawasiliano ya simu za Mkononi nchini Marekani Verizon imeanza leo vita kwa kuingia na Mkataba na Apple Corp. kuuza simu za mkononi aina ya Apple iPhone. Iphone zilikuwa zinauzwa nchini Marekani kwa kupitia Kampuni la mawasiliano la AT&T pekee ambapo Apple Corp Company iliingia nalo mkataba mwaka2007 wakati ulimwengu ulivyoshuhudia madadiliko ya kiteknolojia ya simu za mkononi ambapo kwa mara ya kwanza Apple Corp company iliudhihirishia ulimwengu kuwa simu ya mkononi yenye ukubwa wa display 3.5" inawezekana ikiwa na uwezo wa kutunza battery PAMOJA na UBORA WA HALI YA JUU. iPHONE INAAMINIKA KUWA SIMU YA GHARAMA NA BORA KULIKO ZOTE DUNIANI. iPhone chini ya AT&T inatumia GMS za simu kawaida lakini mtandao wa simu wa Verizon ukiwa unatumia CDMA umeweza kuishawishi Apple Company kuingia nayo mkataba pia na kuruhusu wateja wake kutumia iPhone. Leo ni siku ya kwanza kwa Verizoni kuuza simu hii na imeshafanikiwa kupokea milioni za order toka kwa wateja wake. ijue CDMA network
An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate.
CDMA is a spread spectrum multiple access[1] technique. A spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. Spreading code is a pseudorandom code that has a narrow Ambiguity function, unlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is combined via bitwise XOR (exclusive OR) with the faster code. The figure shows how spread spectrum signal is generated. The data signal with pulse duration of Tb is XOR’ed with the code signal with pulse duration of Tc. (Note: bandwidth is proportional to 1 / T where T = bit time) Therefore, the bandwidth of the data signal is 1 / Tb and the bandwidth of the spread spectrum signal is 1 / Tc. Since Tc is much smaller than Tb, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal. The ratio Tb / Tc is called spreading factor or processing gain and determines to a certain extent the upper limit of the total number of users supported simultaneously by a base station.[2]
Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. The best performance will occur when there is good separation between the signal of a desired user and the signals of other users. The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal. If the desired user's code has nothing in common with the signal the correlation should be as close to zero as possible (thus eliminating the signal); this is referred to as cross correlation. If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference.[3]
In general, CDMA belongs to two basic categories: synchronous (orthogonal codes) and asynchronous (pseudorandom codes).
Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string 1011 is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking their dot product, by summing the products of their respective components. If the dot product is zero, the two vectors are said to be orthogonal to each other (note: if u = (a, b) and v = (c, d), the dot product u·v = ac + bd). Some properties of the dot product aid understanding of how W-CDMA works. If vectors a and b are orthogonal, then 
and:
and:
Each user in synchronous CDMA uses a code orthogonal to the others' codes to modulate their signal. An example of four mutually orthogonal digital signals is shown in the figure. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS-95 64 bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes are orthogonal to one another, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each user's signal can be encoded and decoded.
Example
Start with a set of vectors that are mutually orthogonal. (Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.) An example of orthogonal functions is shown in the picture on the left. These vectors will be assigned to individual users and are called the code, chip code, or chipping code. In the interest of brevity, the rest of this example uses codes, v, with only 2 bits.
Each user is associated with a different code, say v. A 1 bit is represented by transmitting a positive code, v, and a 0 bit is represented by a negative code, –v. For example, if v = (1, –1) and the data that the user wishes to transmit is (1, 0, 1, 1), then the transmitted symbols would be (1, –1, 1, 1) ⊗ v = (v0, v1, –v0, –v1, v0, v1, v0, v1) = (1, –1, –1, 1, 1, –1, 1, –1), where ⊗ is the Kronecker product. For the purposes of this article, we call this constructed vector thetransmitted vector.
Each sender has a different, unique vector v chosen from that set, but the construction method of the transmitted vector is identical.
Now, due to physical properties of interference, if two signals at a point are in phase, they add to give twice the amplitude of each signal, but if they are out of phase, they subtract and give a signal that is the difference of the amplitudes. Digitally, this behaviour can be modelled by the addition of the transmission vectors, component by component.
If sender0 has code (1, –1) and data (1, 0, 1, 1), and sender1 has code (1, 1) and data (0, 0, 1, 1), and both senders transmit simultaneously, then this table describes the coding steps:
| Step | Encode sender0 | Encode sender1 |
| 0 | code0 = (1, –1), data0 = (1, 0, 1, 1) | code1 = (1, 1), data1 = (0, 0, 1, 1) |
| 1 | encode0 = 2(1, 0, 1, 1) – (1, 1, 1, 1) = (1, –1, 1, 1) | encode1 = 2(0, 0, 1, 1) – (1, 1, 1, 1) = (–1, –1, 1, 1) |
| 2 | signal0 = encode0 ⊗ code0 = (1, –1, 1, 1) ⊗ (1, –1) = (1, –1, –1, 1, 1, –1, 1, –1) | signal1 = encode1 ⊗ code1 = (–1, –1, 1, 1) ⊗ (1, 1) = (–1, –1, –1, –1, 1, 1, 1, 1) |
Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal:
- (1, –1, –1, 1, 1, –1, 1, –1) + (–1, –1, –1, –1, 1, 1, 1, 1) = (0, –2, –2, 0, 2, 0, 2, 0)
This raw signal is called an interference pattern. The receiver then extracts an intelligible signal for any known sender by combining the sender's code with the interference pattern, the receiver combines it with the codes of the senders. The following table explains how this works and shows that the signals do not interfere with one another:
| Step | Decode sender0 | Decode sender1 |
| 0 | code0 = (1, –1), signal = (0, –2, –2, 0, 2, 0, 2, 0) | code1 = (1, 1), signal = (0, –2, –2, 0, 2, 0, 2, 0) |
| 1 | decode0 = pattern.vector0 | decode1 = pattern.vector1 |
| 2 | decode0 = ((0, –2), (–2, 0), (2, 0), (2, 0)).(1, –1) | decode1 = ((0, –2), (–2, 0), (2, 0), (2, 0)).(1, 1) |
| 3 | decode0 = ((0 + 2), (–2 + 0), (2 + 0), (2 + 0)) | decode1 = ((0 – 2), (–2 + 0), (2 + 0), (2 + 0)) |
| 4 | data0=(2, –2, 2, 2), meaning (1, 0, 1, 1) | data1=(–2, –2, 2, 2), meaning (0, 0, 1, 1) |
Further, after decoding, all values greater than 0 are interpreted as 1 while all values less than zero are interpreted as 0. For example, after decoding, data0 is (2, –2, 2, 2), but the receiver interprets this as (1, 0, 1, 1). Values of exactly 0 means that the sender did not transmit any data, as in the following example:
Assume signal0 = (1, –1, –1, 1, 1, –1, 1, –1) is transmitted alone. The following table shows the decode at the receiver:
| Step | Decode sender0 | Decode sender1 |
| 0 | code0 = (1, –1), signal = (1, –1, –1, 1, 1, –1, 1, –1) | code1 = (1, 1), signal = (1, –1, –1, 1, 1, –1, 1, –1) |
| 1 | decode0 = pattern.vector0 | decode1 = pattern.vector1 |
| 2 | decode0 = ((1, –1), (–1, 1), (1, –1), (1, –1)).(1, –1) | decode1 = ((1, –1), (–1, 1), (1, –1), (1, –1)).(1, 1) |
| 3 | decode0 = ((1 + 1), (–1 – 1),(1 + 1), (1 + 1)) | decode1 = ((1 – 1), (–1 + 1),(1 – 1), (1 – 1)) |
| 4 | data0 = (2, –2, 2, 2), meaning (1, 0, 1, 1) | data1 = (0, 0, 0, 0), meaning no data |
When the receiver attempts to decode the signal using sender1's code, the data is all zeros, therefore the cross correlation is equal to zero and it is clear that sender1 did not transmit any data.
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